From 6e97acdeef811c2cd9b7df02e00058dfd9409a53 Mon Sep 17 00:00:00 2001
From: justschen <justchen@microsoft.com>
Date: Thu, 9 Jul 2026 21:30:17 -0700
Subject: [PATCH 1/3] chat: add turn timing metadata

- Carries authoritative start and end timestamps through turn lifecycle actions so clients can render consistent timing information.
- Persists turn start times and derived durations in state while preserving compatibility with older completed turns.
- Keeps reducer behavior aligned across TypeScript, Rust, Kotlin, Swift, and Go, including invalid timestamp handling.
- Regenerates schemas and client wire types and extends shared conformance fixtures.

References microsoft/vscode#325061

(Commit message generated by Copilot)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 clients/go/ahp/reducers.go                    | 31 ++++++--
 clients/go/ahptypes/actions.generated.go      |  8 +++
 clients/go/ahptypes/state.generated.go        |  6 ++
 clients/go/examples/reducers_demo/main.go     | 12 ++--
 .../microsoft/agenthostprotocol/Reducers.kt   | 70 ++++++++++++-------
 .../generated/Actions.generated.kt            | 16 +++++
 .../generated/State.generated.kt              | 12 ++++
 .../agenthostprotocol/ReducersTest.kt         |  8 +--
 clients/rust/Cargo.lock                       | 25 +++++++
 clients/rust/Cargo.toml                       |  1 +
 clients/rust/crates/ahp-types/src/actions.rs  |  8 +++
 clients/rust/crates/ahp-types/src/state.rs    |  8 +++
 clients/rust/crates/ahp/Cargo.toml            |  1 +
 clients/rust/crates/ahp/src/reducers.rs       | 53 +++++++++++---
 .../Generated/Actions.generated.swift         | 20 ++++++
 .../Generated/State.generated.swift           | 12 ++++
 .../Sources/AgentHostProtocol/Reducers.swift  | 34 +++++++--
 .../NativeReducerTests.swift                  |  1 +
 .../ReducersTests.swift                       |  9 ++-
 docs/.changes/20260709-turn-timing.json       |  4 ++
 schema/actions.schema.json                    | 37 +++++++++-
 schema/commands.schema.json                   | 37 +++++++++-
 schema/errors.schema.json                     | 37 +++++++++-
 schema/notifications.schema.json              | 13 ++++
 schema/state.schema.json                      | 13 ++++
 types/channels-chat/actions.ts                |  8 +++
 types/channels-chat/reducer.ts                | 65 +++++++++++++++--
 types/channels-chat/state.ts                  |  6 ++
 types/reducers.test.ts                        |  2 +-
 .../reducers/005-session-turnstarted.json     |  4 +-
 ...messageid-removes-from-queuedmessages.json |  4 +-
 ...messageid-removes-last-queued-message.json |  4 +-
 ...eid-removes-matching-steering-message.json |  4 +-
 ...ageid-does-not-touch-pending-messages.json |  4 +-
 .../010-session-delta-appends-content.json    |  2 +
 ...sion-delta-with-wrong-turnid-is-no-op.json |  2 +
 ...on-responsepart-adds-to-responseparts.json |  2 +
 ...4-session-turncomplete-finalizes-turn.json |  6 +-
 ...-session-turncancelled-finalizes-turn.json |  6 +-
 ...ssion-error-finalizes-turn-with-error.json |  4 ++
 ...-force-cancels-in-progress-tool-calls.json |  6 +-
 ...rncomplete-with-wrong-turnid-is-no-op.json |  5 +-
 ...-start-delta-ready-confirmed-complete.json |  2 +
 ...h-auto-confirm-transitions-to-running.json |  2 +
 ...-call-denied-transitions-to-cancelled.json |  2 +
 ...-result-confirmation-pending-approved.json |  2 +
 ...d-cancelled-with-result-denied-reason.json |  2 +
 ...nding-confirmation-defaults-confirmed.json |  2 +
 ...ions-for-unknown-toolcallid-are-no-op.json |  2 +
 ...ing-tool-back-to-pending-confirmation.json |  2 +
 ...-approved-transitions-back-to-running.json |  2 +
 ...ation-denied-transitions-to-cancelled.json |  2 +
 ...-non-streaming-non-running-tool-calls.json |  2 +
 ...on-usage-updates-usage-on-active-turn.json |  2 +
 ...n-reasoning-appends-reasoning-content.json |  2 +
 ...7-session-truncated-drops-active-turn.json |  1 +
 ...ps-active-turn-even-when-clearing-all.json |  1 +
 ...w-with-tool-calls-and-re-confirmation.json |  6 +-
 .../075-turnstarted-clears-isread.json        |  4 +-
 ...olcall-contentchanged-updates-running.json |  2 +
 ...lcall-contentchanged-noop-non-running.json |  2 +
 ...call-contentchanged-replaces-existing.json |  2 +
 ...0-toolcalldelta-wrong-turnid-is-no-op.json |  2 +
 ...91-responsepart-wrong-turnid-is-no-op.json |  2 +
 ...2-toolcallstart-wrong-turnid-is-no-op.json |  2 +
 .../093-usage-wrong-turnid-is-no-op.json      |  2 +
 ...094-delta-nonexistent-partid-is-no-op.json |  2 +
 ...5-toolcalldelta-wrong-status-is-no-op.json |  2 +
 ...olcallconfirmed-wrong-status-is-no-op.json |  2 +
 ...oolcallcomplete-wrong-status-is-no-op.json |  2 +
 ...resultconfirmed-wrong-status-is-no-op.json |  2 +
 ...dturn-force-cancels-running-tool-call.json |  6 +-
 ...ta-targeting-toolcall-partid-is-no-op.json |  2 +
 ...olcalldelta-without-invocationmessage.json |  2 +
 ...ning-targeting-non-reasoning-is-no-op.json |  2 +
 .../103-delta-skips-parts-without-id.json     |  2 +
 ...on-input-full-draft-and-complete-flow.json |  2 +
 ...on-input-requested-with-drafts-status.json |  2 +
 ...nput-turn-end-cleans-turn-scoped-only.json |  6 +-
 ...session-input-upsert-and-clear-answer.json |  2 +
 ...t-completion-and-truncation-filtering.json |  1 +
 ...confirmation-sets-input-needed-status.json |  2 +
 ...confirmation-sets-input-needed-status.json |  2 +
 ...olcallready-with-confirmation-options.json |  2 +
 ...onfirmed-approved-with-selectedoption.json |  2 +
 ...lconfirmed-denied-with-selectedoption.json |  2 +
 ...edoption-carries-through-to-completed.json |  2 +
 ...n-carries-through-result-confirmation.json |  2 +
 ...doption-carries-through-result-denied.json |  2 +
 ...th-editedtoolinput-overrides-original.json |  2 +
 .../161-chat-turn-lifecycle-on-chat.json      |  6 +-
 ...ies-mcp-contributor-through-lifecycle.json |  2 +
 .../220-toolcall-actions-update-meta.json     |  2 +
 ...tion-records-declined-request-in-turn.json |  2 +
 ...n-no-op-unknown-id-with-open-requests.json |  2 +
 ...invalid-turn-start-timestamp-is-no-op.json | 31 ++++++++
 ...9-invalid-turn-end-timestamp-is-no-op.json | 49 +++++++++++++
 .../240-turn-duration-is-clamped-to-zero.json | 54 ++++++++++++++
 98 files changed, 779 insertions(+), 86 deletions(-)
 create mode 100644 docs/.changes/20260709-turn-timing.json
 create mode 100644 types/test-cases/reducers/238-invalid-turn-start-timestamp-is-no-op.json
 create mode 100644 types/test-cases/reducers/239-invalid-turn-end-timestamp-is-no-op.json
 create mode 100644 types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json

diff --git a/clients/go/ahp/reducers.go b/clients/go/ahp/reducers.go
index e7ffacb3..c3905761 100644
--- a/clients/go/ahp/reducers.go
+++ b/clients/go/ahp/reducers.go
@@ -181,7 +181,11 @@ func touchChatModified(state *ahptypes.ChatState) {
 
 // ─── Active-turn helpers ───────────────────────────────────────────────
 
-func endTurn(state *ahptypes.ChatState, turnID string, turnState ahptypes.TurnState, terminalStatus *ahptypes.SessionStatus, errInfo *ahptypes.ErrorInfo) ReduceOutcome {
+func endTurn(state *ahptypes.ChatState, turnID, endedAt string, turnState ahptypes.TurnState, terminalStatus *ahptypes.SessionStatus, errInfo *ahptypes.ErrorInfo) ReduceOutcome {
+	ended, err := time.Parse(time.RFC3339Nano, endedAt)
+	if err != nil {
+		return ReduceOutcomeNoOp
+	}
 	if state.ActiveTurn == nil || state.ActiveTurn.Id != turnID {
 		return ReduceOutcomeNoOp
 	}
@@ -220,8 +224,19 @@ func endTurn(state *ahptypes.ChatState, turnID string, turnState ahptypes.TurnSt
 		}})
 	}
 
+	var duration *int64
+	if started, err := time.Parse(time.RFC3339Nano, active.StartedAt); err == nil {
+		ms := ended.UnixMilli() - started.UnixMilli()
+		if ms < 0 {
+			ms = 0
+		}
+		duration = &ms
+	}
+	startedAt := active.StartedAt
 	turn := ahptypes.Turn{
 		Id:            active.Id,
+		StartedAt:     &startedAt,
+		Duration:      duration,
 		Message:       active.Message,
 		ResponseParts: parts,
 		Usage:         active.Usage,
@@ -231,7 +246,7 @@ func endTurn(state *ahptypes.ChatState, turnID string, turnState ahptypes.TurnSt
 
 	state.Turns = append(state.Turns, turn)
 	state.InputRequests = nil
-	touchChatModified(state)
+	state.ModifiedAt = endedAt
 	state.Status = summaryStatus(state, terminalStatus)
 	return ReduceOutcomeApplied
 }
@@ -464,13 +479,13 @@ func ApplyActionToChat(state *ahptypes.ChatState, action ahptypes.StateAction) R
 		state.ActiveTurn.ResponseParts = append(state.ActiveTurn.ResponseParts, a.Part)
 		return ReduceOutcomeApplied
 	case *ahptypes.ChatTurnCompleteAction:
-		return endTurn(state, a.TurnId, ahptypes.TurnStateComplete, nil, nil)
+		return endTurn(state, a.TurnId, a.EndedAt, ahptypes.TurnStateComplete, nil, nil)
 	case *ahptypes.ChatTurnCancelledAction:
-		return endTurn(state, a.TurnId, ahptypes.TurnStateCancelled, nil, nil)
+		return endTurn(state, a.TurnId, a.EndedAt, ahptypes.TurnStateCancelled, nil, nil)
 	case *ahptypes.ChatErrorAction:
 		errCopy := a.Error
 		errStatus := ahptypes.SessionStatusError
-		return endTurn(state, a.TurnId, ahptypes.TurnStateError, &errStatus, &errCopy)
+		return endTurn(state, a.TurnId, a.EndedAt, ahptypes.TurnStateError, &errStatus, &errCopy)
 	case *ahptypes.ChatActivityChangedAction:
 		state.Activity = a.Activity
 		return ReduceOutcomeApplied
@@ -922,13 +937,17 @@ func ApplyActionToSession(state *ahptypes.SessionState, action ahptypes.StateAct
 }
 
 func applyTurnStarted(state *ahptypes.ChatState, a *ahptypes.ChatTurnStartedAction) ReduceOutcome {
+	if _, err := time.Parse(time.RFC3339Nano, a.StartedAt); err != nil {
+		return ReduceOutcomeNoOp
+	}
 	state.ActiveTurn = &ahptypes.ActiveTurn{
 		Id:            a.TurnId,
+		StartedAt:     a.StartedAt,
 		Message:       a.Message,
 		ResponseParts: []ahptypes.ResponsePart{},
 	}
 	state.Status = summaryStatus(state, nil)
-	touchChatModified(state)
+	state.ModifiedAt = a.StartedAt
 	state.Status = withStatusFlag(state.Status, ahptypes.SessionStatusIsRead, false)
 
 	if a.QueuedMessageId != nil {
diff --git a/clients/go/ahptypes/actions.generated.go b/clients/go/ahptypes/actions.generated.go
index f5f61b18..ca866fb1 100644
--- a/clients/go/ahptypes/actions.generated.go
+++ b/clients/go/ahptypes/actions.generated.go
@@ -212,6 +212,8 @@ type ChatTurnStartedAction struct {
 	Type ActionType `json:"type"`
 	// Turn identifier
 	TurnId string `json:"turnId"`
+	// ISO 8601 timestamp when this turn started.
+	StartedAt string `json:"startedAt"`
 	// The new message
 	Message Message `json:"message"`
 	// If this turn was auto-started from a queued message, the ID of that message
@@ -460,6 +462,8 @@ type ChatTurnCompleteAction struct {
 	Type ActionType `json:"type"`
 	// Turn identifier
 	TurnId string `json:"turnId"`
+	// ISO 8601 timestamp when this turn completed.
+	EndedAt string `json:"endedAt"`
 	// Additional provider-specific metadata for this action.
 	//
 	// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -475,6 +479,8 @@ type ChatTurnCancelledAction struct {
 	Type ActionType `json:"type"`
 	// Turn identifier
 	TurnId string `json:"turnId"`
+	// ISO 8601 timestamp when this turn was cancelled.
+	EndedAt string `json:"endedAt"`
 	// Additional provider-specific metadata for this action.
 	//
 	// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -490,6 +496,8 @@ type ChatErrorAction struct {
 	Type ActionType `json:"type"`
 	// Turn identifier
 	TurnId string `json:"turnId"`
+	// ISO 8601 timestamp when this turn failed.
+	EndedAt string `json:"endedAt"`
 	// Error details
 	Error ErrorInfo `json:"error"`
 	// Additional provider-specific metadata for this action.
diff --git a/clients/go/ahptypes/state.generated.go b/clients/go/ahptypes/state.generated.go
index 09d345d4..6e57fc01 100644
--- a/clients/go/ahptypes/state.generated.go
+++ b/clients/go/ahptypes/state.generated.go
@@ -1148,6 +1148,10 @@ type SessionConfigState struct {
 type Turn struct {
 	// Turn identifier
 	Id string `json:"id"`
+	// ISO 8601 timestamp when this turn started.
+	StartedAt *string `json:"startedAt,omitempty"`
+	// Turn duration in milliseconds.
+	Duration *int64 `json:"duration,omitempty"`
 	// The message that initiated the turn
 	Message Message `json:"message"`
 	// All response content in stream order: text, tool calls, reasoning, and content refs.
@@ -1167,6 +1171,8 @@ type Turn struct {
 type ActiveTurn struct {
 	// Turn identifier
 	Id string `json:"id"`
+	// ISO 8601 timestamp when this turn started.
+	StartedAt string `json:"startedAt"`
 	// The message that initiated the turn
 	Message Message `json:"message"`
 	// All response content in stream order: text, tool calls, reasoning, and content refs.
diff --git a/clients/go/examples/reducers_demo/main.go b/clients/go/examples/reducers_demo/main.go
index 7e10b7f0..69c7b4e3 100644
--- a/clients/go/examples/reducers_demo/main.go
+++ b/clients/go/examples/reducers_demo/main.go
@@ -20,9 +20,10 @@ func main() {
 
 	actions := []ahptypes.StateAction{
 		{Value: &ahptypes.ChatTurnStartedAction{
-			Type:    ahptypes.ActionTypeChatTurnStarted,
-			TurnId:  "t1",
-			Message: ahptypes.Message{Text: "Hello!", Origin: ahptypes.MessageOrigin{Kind: ahptypes.MessageKindUser}},
+			Type:      ahptypes.ActionTypeChatTurnStarted,
+			TurnId:    "t1",
+			StartedAt: "2026-07-09T20:00:00.000Z",
+			Message:   ahptypes.Message{Text: "Hello!", Origin: ahptypes.MessageOrigin{Kind: ahptypes.MessageKindUser}},
 		}},
 		{Value: &ahptypes.ChatResponsePartAction{
 			Type:   ahptypes.ActionTypeChatResponsePart,
@@ -40,8 +41,9 @@ func main() {
 			Content: "there!",
 		}},
 		{Value: &ahptypes.ChatTurnCompleteAction{
-			Type:   ahptypes.ActionTypeChatTurnComplete,
-			TurnId: "t1",
+			Type:    ahptypes.ActionTypeChatTurnComplete,
+			TurnId:  "t1",
+			EndedAt: "2026-07-09T20:00:01.000Z",
 		}},
 	}
 
diff --git a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
index 5edb26e1..049277f5 100644
--- a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
+++ b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
@@ -82,6 +82,11 @@ public var currentTimestampProvider: () -> Long = { System.currentTimeMillis() }
 
 private fun nowIsoString(): String = Instant.ofEpochMilli(currentTimestampProvider()).toString()
 
+private fun parseTimestampMillis(value: String): Long? {
+    val millis = runCatching { Instant.parse(value).toEpochMilli() }.getOrNull() ?: return null
+    return millis.takeIf { it in -8_640_000_000_000_000L..8_640_000_000_000_000L }
+}
+
 // ─── Status Bitset Helpers ──────────────────────────────────────────────────
 
 /** Bitmask covering the mutually-exclusive activity bits (bits 0–4). */
@@ -332,10 +337,12 @@ private fun updateResponsePart(
 private fun endTurn(
     state: ChatState,
     turnId: String,
+    endedAt: String,
     turnState: TurnState,
     terminalStatus: SessionStatus? = null,
     error: ErrorInfo? = null,
 ): ChatState {
+    val ended = parseTimestampMillis(endedAt) ?: return state
     val active = state.activeTurn ?: return state
     if (active.id != turnId) return state
 
@@ -386,8 +393,12 @@ private fun endTurn(
         )
     }
 
+    val duration = parseTimestampMillis(active.startedAt)
+        ?.let { started -> maxOf(0L, ended - started) }
     val turn = Turn(
         id = active.id,
+        startedAt = active.startedAt,
+        duration = duration,
         message = active.message,
         responseParts = finalizedParts,
         usage = active.usage,
@@ -399,7 +410,7 @@ private fun endTurn(
         turns = state.turns + turn,
         activeTurn = null,
         inputRequests = null,
-        modifiedAt = nowIsoString(),
+        modifiedAt = endedAt,
     )
     return withoutTurn.copy(status = chatSummaryStatus(withoutTurn, terminalStatus))
 }
@@ -744,31 +755,36 @@ public fun chatReducer(state: ChatState, action: StateAction): ChatState = when
 
     is StateActionChatTurnStarted -> {
         val a = action.value
-        val withTurn = state.copy(
-            activeTurn = ActiveTurn(
-                id = a.turnId,
-                message = a.message,
-                responseParts = emptyList(),
-                usage = null,
-            ),
-        )
-        val withStatus = withTurn.copy(
-            status = withStatusFlag(chatSummaryStatus(withTurn), SessionStatus.IS_READ, false),
-            modifiedAt = nowIsoString(),
-        )
-        if (a.queuedMessageId == null) {
-            withStatus
+        if (parseTimestampMillis(a.startedAt) == null) {
+            state
         } else {
-            var next = withStatus
-            if (next.steeringMessage?.id == a.queuedMessageId) {
-                next = next.copy(steeringMessage = null)
-            }
-            val queued = next.queuedMessages
-            if (queued != null) {
-                val filtered = queued.filter { it.id != a.queuedMessageId }
-                next = next.copy(queuedMessages = filtered.ifEmpty { null })
+            val withTurn = state.copy(
+                activeTurn = ActiveTurn(
+                    id = a.turnId,
+                    startedAt = a.startedAt,
+                    message = a.message,
+                    responseParts = emptyList(),
+                    usage = null,
+                ),
+            )
+            val withStatus = withTurn.copy(
+                status = withStatusFlag(chatSummaryStatus(withTurn), SessionStatus.IS_READ, false),
+                modifiedAt = a.startedAt,
+            )
+            if (a.queuedMessageId == null) {
+                withStatus
+            } else {
+                var next = withStatus
+                if (next.steeringMessage?.id == a.queuedMessageId) {
+                    next = next.copy(steeringMessage = null)
+                }
+                val queued = next.queuedMessages
+                if (queued != null) {
+                    val filtered = queued.filter { it.id != a.queuedMessageId }
+                    next = next.copy(queuedMessages = filtered.ifEmpty { null })
+                }
+                next
             }
-            next
         }
     }
 
@@ -796,13 +812,13 @@ public fun chatReducer(state: ChatState, action: StateAction): ChatState = when
     }
 
     is StateActionChatTurnComplete ->
-        endTurn(state, action.value.turnId, TurnState.COMPLETE)
+        endTurn(state, action.value.turnId, action.value.endedAt, TurnState.COMPLETE)
 
     is StateActionChatTurnCancelled ->
-        endTurn(state, action.value.turnId, TurnState.CANCELLED)
+        endTurn(state, action.value.turnId, action.value.endedAt, TurnState.CANCELLED)
 
     is StateActionChatError ->
-        endTurn(state, action.value.turnId, TurnState.ERROR, SessionStatus.ERROR, action.value.error)
+        endTurn(state, action.value.turnId, action.value.endedAt, TurnState.ERROR, SessionStatus.ERROR, action.value.error)
 
     is StateActionChatActivityChanged ->
         state.copy(activity = action.value.activity)
diff --git a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt
index cd859497..657d9280 100644
--- a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt
+++ b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt
@@ -290,6 +290,10 @@ data class ChatTurnStartedAction(
      * Turn identifier
      */
     val turnId: String,
+    /**
+     * ISO 8601 timestamp when this turn started.
+     */
+    val startedAt: String,
     /**
      * The new message
      */
@@ -609,6 +613,10 @@ data class ChatTurnCompleteAction(
      * Turn identifier
      */
     val turnId: String,
+    /**
+     * ISO 8601 timestamp when this turn completed.
+     */
+    val endedAt: String,
     /**
      * Additional provider-specific metadata for this action.
      *
@@ -629,6 +637,10 @@ data class ChatTurnCancelledAction(
      * Turn identifier
      */
     val turnId: String,
+    /**
+     * ISO 8601 timestamp when this turn was cancelled.
+     */
+    val endedAt: String,
     /**
      * Additional provider-specific metadata for this action.
      *
@@ -649,6 +661,10 @@ data class ChatErrorAction(
      * Turn identifier
      */
     val turnId: String,
+    /**
+     * ISO 8601 timestamp when this turn failed.
+     */
+    val endedAt: String,
     /**
      * Error details
      */
diff --git a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/State.generated.kt b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/State.generated.kt
index babb1ec2..ab8982a9 100644
--- a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/State.generated.kt
+++ b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/State.generated.kt
@@ -1576,6 +1576,14 @@ data class Turn(
      * Turn identifier
      */
     val id: String,
+    /**
+     * ISO 8601 timestamp when this turn started.
+     */
+    val startedAt: String? = null,
+    /**
+     * Turn duration in milliseconds.
+     */
+    val duration: Long? = null,
     /**
      * The message that initiated the turn
      */
@@ -1607,6 +1615,10 @@ data class ActiveTurn(
      * Turn identifier
      */
     val id: String,
+    /**
+     * ISO 8601 timestamp when this turn started.
+     */
+    val startedAt: String,
     /**
      * The message that initiated the turn
      */
diff --git a/clients/kotlin/src/test/kotlin/com/microsoft/agenthostprotocol/ReducersTest.kt b/clients/kotlin/src/test/kotlin/com/microsoft/agenthostprotocol/ReducersTest.kt
index ba36c542..111a693e 100644
--- a/clients/kotlin/src/test/kotlin/com/microsoft/agenthostprotocol/ReducersTest.kt
+++ b/clients/kotlin/src/test/kotlin/com/microsoft/agenthostprotocol/ReducersTest.kt
@@ -209,18 +209,14 @@ class ReducersTest {
     }
 
     @Test
-    fun `currentTimestampProvider override flows through to reducer outputs`() {
-        currentTimestampProvider = { 12345L }
-
-        // The chat reducer stamps `modifiedAt` from the injected timestamp
-        // provider. (The session reducer no longer stamps a timestamp — the
-        // host owns the root-channel summary's `modifiedAt`.)
+    fun `turn start timestamp flows through to reducer outputs`() {
         val chatResult = chatReducer(
             newChat(),
             StateActionChatTurnStarted(
                 ChatTurnStartedAction(
                     type = ActionType.CHAT_TURN_STARTED,
                     turnId = "turn-1",
+                    startedAt = "1970-01-01T00:00:12.345Z",
                     message = userMessage("hello"),
                 ),
             ),
diff --git a/clients/rust/Cargo.lock b/clients/rust/Cargo.lock
index b3a0f2ab..7d8aaa1c 100644
--- a/clients/rust/Cargo.lock
+++ b/clients/rust/Cargo.lock
@@ -8,6 +8,7 @@ version = "0.5.2"
 dependencies = [
  "ahp-types",
  "ahp-ws",
+ "chrono",
  "serde",
  "serde_json",
  "thiserror",
@@ -45,6 +46,12 @@ version = "1.0.102"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f202df86484c868dbad7eaa557ef785d5c66295e41b460ef922eca0723b842c"
 
+[[package]]
+name = "autocfg"
+version = "1.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f2032f911046de80f0a198e0901378627c33f59ea0ac00e363d481118bd70a53"
+
 [[package]]
 name = "bitflags"
 version = "2.11.1"
@@ -82,6 +89,15 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 
+[[package]]
+name = "chrono"
+version = "0.4.45"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1aa79e62e7697b8e29b513a68abacf485adcd1fe8284a4316c5ae868e6633327"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "core-foundation"
 version = "0.10.1"
@@ -543,6 +559,15 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "num-traits"
+version = "0.2.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "071dfc062690e90b734c0b2273ce72ad0ffa95f0c74596bc250dcfd960262841"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "once_cell"
 version = "1.21.4"
diff --git a/clients/rust/Cargo.toml b/clients/rust/Cargo.toml
index 9c23c7b4..1fdd4ba6 100644
--- a/clients/rust/Cargo.toml
+++ b/clients/rust/Cargo.toml
@@ -22,5 +22,6 @@ tokio = { version = "1", features = ["sync", "rt", "macros", "time"] }
 tokio-util = "0.7"
 futures-util = "0.3"
 tracing = "0.1"
+chrono = { version = "0.4", default-features = false, features = ["std"] }
 ahp-types = { path = "crates/ahp-types", version = "0.5.2" }
 ahp = { path = "crates/ahp", version = "0.5.2" }
diff --git a/clients/rust/crates/ahp-types/src/actions.rs b/clients/rust/crates/ahp-types/src/actions.rs
index e871381c..605ac97a 100644
--- a/clients/rust/crates/ahp-types/src/actions.rs
+++ b/clients/rust/crates/ahp-types/src/actions.rs
@@ -321,6 +321,8 @@ pub struct SessionDefaultChatChangedAction {
 pub struct ChatTurnStartedAction {
     /// Turn identifier
     pub turn_id: String,
+    /// ISO 8601 timestamp when this turn started.
+    pub started_at: String,
     /// The new message
     pub message: Message,
     /// If this turn was auto-started from a queued message, the ID of that message
@@ -613,6 +615,8 @@ pub struct ChatToolCallContentChangedAction {
 pub struct ChatTurnCompleteAction {
     /// Turn identifier
     pub turn_id: String,
+    /// ISO 8601 timestamp when this turn completed.
+    pub ended_at: String,
     /// Additional provider-specific metadata for this action.
     ///
     /// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -630,6 +634,8 @@ pub struct ChatTurnCompleteAction {
 pub struct ChatTurnCancelledAction {
     /// Turn identifier
     pub turn_id: String,
+    /// ISO 8601 timestamp when this turn was cancelled.
+    pub ended_at: String,
     /// Additional provider-specific metadata for this action.
     ///
     /// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -647,6 +653,8 @@ pub struct ChatTurnCancelledAction {
 pub struct ChatErrorAction {
     /// Turn identifier
     pub turn_id: String,
+    /// ISO 8601 timestamp when this turn failed.
+    pub ended_at: String,
     /// Error details
     pub error: ErrorInfo,
     /// Additional provider-specific metadata for this action.
diff --git a/clients/rust/crates/ahp-types/src/state.rs b/clients/rust/crates/ahp-types/src/state.rs
index 6775f54e..6da14c19 100644
--- a/clients/rust/crates/ahp-types/src/state.rs
+++ b/clients/rust/crates/ahp-types/src/state.rs
@@ -1474,6 +1474,12 @@ pub struct SessionConfigState {
 pub struct Turn {
     /// Turn identifier
     pub id: String,
+    /// ISO 8601 timestamp when this turn started.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub started_at: Option<String>,
+    /// Turn duration in milliseconds.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub duration: Option<i64>,
     /// The message that initiated the turn
     pub message: Message,
     /// All response content in stream order: text, tool calls, reasoning, and content refs.
@@ -1497,6 +1503,8 @@ pub struct Turn {
 pub struct ActiveTurn {
     /// Turn identifier
     pub id: String,
+    /// ISO 8601 timestamp when this turn started.
+    pub started_at: String,
     /// The message that initiated the turn
     pub message: Message,
     /// All response content in stream order: text, tool calls, reasoning, and content refs.
diff --git a/clients/rust/crates/ahp/Cargo.toml b/clients/rust/crates/ahp/Cargo.toml
index 4608dc74..86aefd8e 100644
--- a/clients/rust/crates/ahp/Cargo.toml
+++ b/clients/rust/crates/ahp/Cargo.toml
@@ -23,6 +23,7 @@ serde_json = { workspace = true }
 thiserror = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }
+chrono = { workspace = true }
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["full"] }
diff --git a/clients/rust/crates/ahp/src/reducers.rs b/clients/rust/crates/ahp/src/reducers.rs
index 4f854ba9..42a59a9d 100644
--- a/clients/rust/crates/ahp/src/reducers.rs
+++ b/clients/rust/crates/ahp/src/reducers.rs
@@ -68,6 +68,7 @@ use ahp_types::state::{
     ToolCallResponsePart, ToolCallRunningState, ToolCallState, ToolCallStreamingState, Turn,
     TurnState,
 };
+use chrono::{DateTime, FixedOffset};
 
 /// What happened when an action was applied.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -275,13 +276,21 @@ fn touch_chat_modified(state: &mut ChatState) {
     state.modified_at = now_iso();
 }
 
+fn parse_timestamp(value: &str) -> Option<DateTime<FixedOffset>> {
+    DateTime::parse_from_rfc3339(value).ok()
+}
+
 fn end_turn(
     state: &mut ChatState,
     turn_id: &str,
+    ended_at: &str,
     turn_state: TurnState,
     terminal_status: Option<SessionStatus>,
     error: Option<ErrorInfo>,
 ) -> ReduceOutcome {
+    let Some(ended) = parse_timestamp(ended_at) else {
+        return ReduceOutcome::NoOp;
+    };
     let Some(active) = state.active_turn.as_ref() else {
         return ReduceOutcome::NoOp;
     };
@@ -344,8 +353,16 @@ fn end_turn(
         })
         .collect();
 
+    let duration = parse_timestamp(&active.started_at).and_then(|started| {
+        ended
+            .timestamp_millis()
+            .checked_sub(started.timestamp_millis())
+            .map(|duration| duration.max(0))
+    });
     let turn = Turn {
         id: active.id,
+        started_at: Some(active.started_at),
+        duration,
         message: active.message,
         response_parts,
         usage: active.usage,
@@ -355,7 +372,7 @@ fn end_turn(
 
     state.turns.push(turn);
     state.input_requests = None;
-    touch_chat_modified(state);
+    state.modified_at = ended_at.to_owned();
     state.status = summary_status(state, terminal_status);
     ReduceOutcome::Applied
 }
@@ -874,15 +891,26 @@ pub fn apply_action_to_chat(state: &mut ChatState, action: &StateAction) -> Redu
             active.response_parts.push(a.part.clone());
             ReduceOutcome::Applied
         }
-        StateAction::ChatTurnComplete(a) => {
-            end_turn(state, &a.turn_id, TurnState::Complete, None, None)
-        }
-        StateAction::ChatTurnCancelled(a) => {
-            end_turn(state, &a.turn_id, TurnState::Cancelled, None, None)
-        }
+        StateAction::ChatTurnComplete(a) => end_turn(
+            state,
+            &a.turn_id,
+            &a.ended_at,
+            TurnState::Complete,
+            None,
+            None,
+        ),
+        StateAction::ChatTurnCancelled(a) => end_turn(
+            state,
+            &a.turn_id,
+            &a.ended_at,
+            TurnState::Cancelled,
+            None,
+            None,
+        ),
         StateAction::ChatError(a) => end_turn(
             state,
             &a.turn_id,
+            &a.ended_at,
             TurnState::Error,
             Some(SessionStatus::Error),
             Some(a.error.clone()),
@@ -1095,14 +1123,18 @@ pub fn apply_action_to_chat(state: &mut ChatState, action: &StateAction) -> Redu
 }
 
 fn apply_turn_started(state: &mut ChatState, a: &ChatTurnStartedAction) -> ReduceOutcome {
+    if parse_timestamp(&a.started_at).is_none() {
+        return ReduceOutcome::NoOp;
+    }
     state.active_turn = Some(ActiveTurn {
         id: a.turn_id.clone(),
+        started_at: a.started_at.clone(),
         message: a.message.clone(),
         response_parts: Vec::new(),
         usage: None,
     });
     state.status = summary_status(state, None);
-    touch_chat_modified(state);
+    state.modified_at = a.started_at.clone();
     state.status = with_status_flag(state.status, SessionStatus::IsRead, false);
 
     if let Some(qmid) = &a.queued_message_id {
@@ -1767,6 +1799,7 @@ mod tests {
         let mut s = empty_chat("copilot:/s1/chat/1");
         let action = StateAction::ChatTurnStarted(ChatTurnStartedAction {
             turn_id: "t1".into(),
+            started_at: "2026-07-09T20:00:00.000Z".into(),
             message: user_message("hi"),
             queued_message_id: None,
             meta: None,
@@ -1784,6 +1817,7 @@ mod tests {
         let mut s = empty_chat("copilot:/s1/chat/1");
         s.active_turn = Some(ActiveTurn {
             id: "t1".into(),
+            started_at: "2026-07-09T20:00:00.000Z".into(),
             message: user_message("hi"),
             response_parts: vec![ResponsePart::Markdown(MarkdownResponsePart {
                 id: "p1".into(),
@@ -1809,6 +1843,7 @@ mod tests {
         let mut s = empty_chat("copilot:/s1/chat/1");
         s.active_turn = Some(ActiveTurn {
             id: "t1".into(),
+            started_at: "2026-07-09T20:00:00.000Z".into(),
             message: user_message("hi"),
             response_parts: Vec::new(),
             usage: None,
@@ -1816,6 +1851,7 @@ mod tests {
         s.status = SessionStatus::InProgress.bits();
         let a = StateAction::ChatTurnComplete(ahp_types::actions::ChatTurnCompleteAction {
             turn_id: "t1".into(),
+            ended_at: "2026-07-09T20:00:01.000Z".into(),
             meta: None,
         });
         assert_eq!(apply_action_to_chat(&mut s, &a), ReduceOutcome::Applied);
@@ -1885,6 +1921,7 @@ mod tests {
         // A chat-scoped action is out of scope for the session reducer.
         let turn = StateAction::ChatTurnComplete(ahp_types::actions::ChatTurnCompleteAction {
             turn_id: "t1".into(),
+            ended_at: "2026-07-09T20:00:01.000Z".into(),
             meta: None,
         });
         assert_eq!(
diff --git a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift
index edb1dcdc..516f7a97 100644
--- a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift
+++ b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift
@@ -246,6 +246,8 @@ public struct ChatTurnStartedAction: Codable, Sendable {
     public var type: ActionType
     /// Turn identifier
     public var turnId: String
+    /// ISO 8601 timestamp when this turn started.
+    public var startedAt: String
     /// The new message
     public var message: Message
     /// If this turn was auto-started from a queued message, the ID of that message
@@ -262,6 +264,7 @@ public struct ChatTurnStartedAction: Codable, Sendable {
     enum CodingKeys: String, CodingKey {
         case type
         case turnId
+        case startedAt
         case message
         case queuedMessageId
         case meta = "_meta"
@@ -270,12 +273,14 @@ public struct ChatTurnStartedAction: Codable, Sendable {
     public init(
         type: ActionType,
         turnId: String,
+        startedAt: String,
         message: Message,
         queuedMessageId: String? = nil,
         meta: [String: AnyCodable]? = nil
     ) {
         self.type = type
         self.turnId = turnId
+        self.startedAt = startedAt
         self.message = message
         self.queuedMessageId = queuedMessageId
         self.meta = meta
@@ -711,6 +716,8 @@ public struct ChatTurnCompleteAction: Codable, Sendable {
     public var type: ActionType
     /// Turn identifier
     public var turnId: String
+    /// ISO 8601 timestamp when this turn completed.
+    public var endedAt: String
     /// Additional provider-specific metadata for this action.
     ///
     /// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -723,16 +730,19 @@ public struct ChatTurnCompleteAction: Codable, Sendable {
     enum CodingKeys: String, CodingKey {
         case type
         case turnId
+        case endedAt
         case meta = "_meta"
     }
 
     public init(
         type: ActionType,
         turnId: String,
+        endedAt: String,
         meta: [String: AnyCodable]? = nil
     ) {
         self.type = type
         self.turnId = turnId
+        self.endedAt = endedAt
         self.meta = meta
     }
 }
@@ -741,6 +751,8 @@ public struct ChatTurnCancelledAction: Codable, Sendable {
     public var type: ActionType
     /// Turn identifier
     public var turnId: String
+    /// ISO 8601 timestamp when this turn was cancelled.
+    public var endedAt: String
     /// Additional provider-specific metadata for this action.
     ///
     /// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -753,16 +765,19 @@ public struct ChatTurnCancelledAction: Codable, Sendable {
     enum CodingKeys: String, CodingKey {
         case type
         case turnId
+        case endedAt
         case meta = "_meta"
     }
 
     public init(
         type: ActionType,
         turnId: String,
+        endedAt: String,
         meta: [String: AnyCodable]? = nil
     ) {
         self.type = type
         self.turnId = turnId
+        self.endedAt = endedAt
         self.meta = meta
     }
 }
@@ -771,6 +786,8 @@ public struct ChatErrorAction: Codable, Sendable {
     public var type: ActionType
     /// Turn identifier
     public var turnId: String
+    /// ISO 8601 timestamp when this turn failed.
+    public var endedAt: String
     /// Error details
     public var error: ErrorInfo
     /// Additional provider-specific metadata for this action.
@@ -785,6 +802,7 @@ public struct ChatErrorAction: Codable, Sendable {
     enum CodingKeys: String, CodingKey {
         case type
         case turnId
+        case endedAt
         case error
         case meta = "_meta"
     }
@@ -792,11 +810,13 @@ public struct ChatErrorAction: Codable, Sendable {
     public init(
         type: ActionType,
         turnId: String,
+        endedAt: String,
         error: ErrorInfo,
         meta: [String: AnyCodable]? = nil
     ) {
         self.type = type
         self.turnId = turnId
+        self.endedAt = endedAt
         self.error = error
         self.meta = meta
     }
diff --git a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/State.generated.swift b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/State.generated.swift
index 9a6990fc..12c63924 100644
--- a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/State.generated.swift
+++ b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/State.generated.swift
@@ -1412,6 +1412,10 @@ public struct SessionConfigState: Codable, Sendable {
 public struct Turn: Codable, Sendable {
     /// Turn identifier
     public var id: String
+    /// ISO 8601 timestamp when this turn started.
+    public var startedAt: String?
+    /// Turn duration in milliseconds.
+    public var duration: Int?
     /// The message that initiated the turn
     public var message: Message
     /// All response content in stream order: text, tool calls, reasoning, and content refs.
@@ -1428,6 +1432,8 @@ public struct Turn: Codable, Sendable {
 
     public init(
         id: String,
+        startedAt: String? = nil,
+        duration: Int? = nil,
         message: Message,
         responseParts: [ResponsePart],
         usage: UsageInfo? = nil,
@@ -1435,6 +1441,8 @@ public struct Turn: Codable, Sendable {
         error: ErrorInfo? = nil
     ) {
         self.id = id
+        self.startedAt = startedAt
+        self.duration = duration
         self.message = message
         self.responseParts = responseParts
         self.usage = usage
@@ -1446,6 +1454,8 @@ public struct Turn: Codable, Sendable {
 public struct ActiveTurn: Codable, Sendable {
     /// Turn identifier
     public var id: String
+    /// ISO 8601 timestamp when this turn started.
+    public var startedAt: String
     /// The message that initiated the turn
     public var message: Message
     /// All response content in stream order: text, tool calls, reasoning, and content refs.
@@ -1457,11 +1467,13 @@ public struct ActiveTurn: Codable, Sendable {
 
     public init(
         id: String,
+        startedAt: String,
         message: Message,
         responseParts: [ResponsePart],
         usage: UsageInfo? = nil
     ) {
         self.id = id
+        self.startedAt = startedAt
         self.message = message
         self.responseParts = responseParts
         self.usage = usage
diff --git a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
index f3c955ee..f01364a5 100644
--- a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
+++ b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
@@ -17,6 +17,21 @@ private let iso8601TimestampFormatter: ISO8601DateFormatter = {
     return formatter
 }()
 
+private let iso8601TimestampFormatterWithoutFractionalSeconds: ISO8601DateFormatter = {
+    let formatter = ISO8601DateFormatter()
+    formatter.formatOptions = [.withInternetDateTime]
+    return formatter
+}()
+
+private func parseTimestamp(_ value: String) -> Date? {
+    iso8601TimestampFormatter.date(from: value)
+        ?? iso8601TimestampFormatterWithoutFractionalSeconds.date(from: value)
+}
+
+private func timestampMilliseconds(_ date: Date) -> Int {
+    Int(floor(date.timeIntervalSince1970 * 1000))
+}
+
 // MARK: - Status Bitset Helpers
 
 /// Bitmask covering the mutually-exclusive activity bits (bits 0–4).
@@ -101,10 +116,12 @@ public func chatReducer(state: ChatState, action: StateAction) -> ChatState {
     // ── Turn Lifecycle ────────────────────────────────────────────────────
 
     case .chatTurnStarted(let a):
+        guard parseTimestamp(a.startedAt) != nil else { return state }
         var next = state
-        next.modifiedAt = currentTimestamp()
+        next.modifiedAt = a.startedAt
         next.activeTurn = ActiveTurn(
             id: a.turnId,
+            startedAt: a.startedAt,
             message: a.message,
             responseParts: [],
             usage: nil
@@ -138,13 +155,13 @@ public func chatReducer(state: ChatState, action: StateAction) -> ChatState {
         return next
 
     case .chatTurnComplete(let a):
-        return endTurn(state: state, turnId: a.turnId, turnState: .complete)
+        return endTurn(state: state, turnId: a.turnId, endedAt: a.endedAt, turnState: .complete)
 
     case .chatTurnCancelled(let a):
-        return endTurn(state: state, turnId: a.turnId, turnState: .cancelled)
+        return endTurn(state: state, turnId: a.turnId, endedAt: a.endedAt, turnState: .cancelled)
 
     case .chatError(let a):
-        return endTurn(state: state, turnId: a.turnId, turnState: .error, terminalStatus: .error, error: a.error)
+        return endTurn(state: state, turnId: a.turnId, endedAt: a.endedAt, turnState: .error, terminalStatus: .error, error: a.error)
 
     case .chatActivityChanged(let a):
         var next = state
@@ -887,10 +904,12 @@ private func upsertInputRequest(state: ChatState, request: ChatInputRequest) ->
 private func endTurn(
     state: ChatState,
     turnId: String,
+    endedAt: String,
     turnState: TurnState,
     terminalStatus: SessionStatus? = nil,
     error: ErrorInfo? = nil
 ) -> ChatState {
+    guard let ended = parseTimestamp(endedAt) else { return state }
     guard let activeTurn = state.activeTurn, activeTurn.id == turnId else {
         return state
     }
@@ -940,8 +959,13 @@ private func endTurn(
         }
     }
 
+    let endedMilliseconds = timestampMilliseconds(ended)
+    let duration = parseTimestamp(activeTurn.startedAt)
+        .map { max(0, endedMilliseconds - timestampMilliseconds($0)) }
     let turn = Turn(
         id: activeTurn.id,
+        startedAt: activeTurn.startedAt,
+        duration: duration,
         message: activeTurn.message,
         responseParts: responseParts,
         usage: activeTurn.usage,
@@ -954,7 +978,7 @@ private func endTurn(
     next.activeTurn = nil
     next.inputRequests = nil
     next.status = chatSummaryStatus(next, terminalStatus: terminalStatus)
-    next.modifiedAt = currentTimestamp()
+    next.modifiedAt = endedAt
     return next
 }
 
diff --git a/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/NativeReducerTests.swift b/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/NativeReducerTests.swift
index 5de270a4..09bae9ea 100644
--- a/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/NativeReducerTests.swift
+++ b/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/NativeReducerTests.swift
@@ -50,6 +50,7 @@ final class NativeReducerTests: XCTestCase {
             turns: [],
             activeTurn: ActiveTurn(
                 id: T,
+                startedAt: "2026-07-09T20:00:00.000Z",
                 message: Message(text: "Hello", origin: MessageOrigin(kind: .user)),
                 responseParts: [],
                 usage: nil
diff --git a/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift b/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift
index 88924b3e..4d910ee2 100644
--- a/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift
+++ b/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift
@@ -46,6 +46,7 @@ final class ReducersTests: XCTestCase {
             turns: [],
             activeTurn: ActiveTurn(
                 id: T,
+                startedAt: "2026-07-09T20:00:00.000Z",
                 message: Message(text: "Hello", origin: MessageOrigin(kind: .user)),
                 responseParts: [],
                 usage: nil
@@ -88,7 +89,8 @@ final class ReducersTests: XCTestCase {
 
     func testClientDispatchableReturnsTrue() {
         let action: StateAction = .chatTurnStarted(ChatTurnStartedAction(
-            type: .chatTurnStarted, turnId: T, message: Message(text: "Hello", origin: MessageOrigin(kind: .user))
+            type: .chatTurnStarted, turnId: T, startedAt: "2026-07-09T20:00:00.000Z",
+            message: Message(text: "Hello", origin: MessageOrigin(kind: .user))
         ))
         XCTAssertTrue(isClientDispatchable(action))
     }
@@ -111,10 +113,11 @@ final class ReducersTests: XCTestCase {
         let next = chatReducer(
             state: state,
             action: .chatTurnStarted(ChatTurnStartedAction(
-                type: .chatTurnStarted, turnId: T, message: Message(text: "Hello", origin: MessageOrigin(kind: .user))
+                type: .chatTurnStarted, turnId: T, startedAt: "2026-07-09T20:00:00.000Z",
+                message: Message(text: "Hello", origin: MessageOrigin(kind: .user))
             ))
         )
-        XCTAssertGreaterThan(next.modifiedAt, state.modifiedAt)
+        XCTAssertEqual(next.modifiedAt, "2026-07-09T20:00:00.000Z")
     }
 
     func testTitleChangedUpdatesTitle() {
diff --git a/docs/.changes/20260709-turn-timing.json b/docs/.changes/20260709-turn-timing.json
new file mode 100644
index 00000000..43d248e7
--- /dev/null
+++ b/docs/.changes/20260709-turn-timing.json
@@ -0,0 +1,4 @@
+{
+  "type": "added",
+  "message": "Turn lifecycle actions now carry timestamps, and completed turns expose their start time and duration."
+}
diff --git a/schema/actions.schema.json b/schema/actions.schema.json
index 038c7b4c..d8e84ef1 100644
--- a/schema/actions.schema.json
+++ b/schema/actions.schema.json
@@ -649,6 +649,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The new message"
@@ -666,6 +670,7 @@
       "required": [
         "type",
         "turnId",
+        "startedAt",
         "message"
       ]
     },
@@ -1089,6 +1094,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn completed."
+        },
         "_meta": {
           "type": "object",
           "additionalProperties": {},
@@ -1097,7 +1106,8 @@
       },
       "required": [
         "type",
-        "turnId"
+        "turnId",
+        "endedAt"
       ]
     },
     "ChatTurnCancelledAction": {
@@ -1111,6 +1121,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn was cancelled."
+        },
         "_meta": {
           "type": "object",
           "additionalProperties": {},
@@ -1119,7 +1133,8 @@
       },
       "required": [
         "type",
-        "turnId"
+        "turnId",
+        "endedAt"
       ]
     },
     "ChatErrorAction": {
@@ -1133,6 +1148,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn failed."
+        },
         "error": {
           "$ref": "#/$defs/ErrorInfo",
           "description": "Error details"
@@ -1146,6 +1165,7 @@
       "required": [
         "type",
         "turnId",
+        "endedAt",
         "error"
       ]
     },
@@ -4779,6 +4799,14 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
+        "duration": {
+          "type": "number",
+          "description": "Turn duration in milliseconds."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -4819,6 +4847,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -4837,6 +4869,7 @@
       },
       "required": [
         "id",
+        "startedAt",
         "message",
         "responseParts",
         "usage"
diff --git a/schema/commands.schema.json b/schema/commands.schema.json
index c1b658c8..4eeeb18f 100644
--- a/schema/commands.schema.json
+++ b/schema/commands.schema.json
@@ -4067,6 +4067,14 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
+        "duration": {
+          "type": "number",
+          "description": "Turn duration in milliseconds."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -4107,6 +4115,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -4125,6 +4137,7 @@
       },
       "required": [
         "id",
+        "startedAt",
         "message",
         "responseParts",
         "usage"
@@ -6482,6 +6495,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The new message"
@@ -6499,6 +6516,7 @@
       "required": [
         "type",
         "turnId",
+        "startedAt",
         "message"
       ]
     },
@@ -6922,6 +6940,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn completed."
+        },
         "_meta": {
           "type": "object",
           "additionalProperties": {},
@@ -6930,7 +6952,8 @@
       },
       "required": [
         "type",
-        "turnId"
+        "turnId",
+        "endedAt"
       ]
     },
     "ChatTurnCancelledAction": {
@@ -6944,6 +6967,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn was cancelled."
+        },
         "_meta": {
           "type": "object",
           "additionalProperties": {},
@@ -6952,7 +6979,8 @@
       },
       "required": [
         "type",
-        "turnId"
+        "turnId",
+        "endedAt"
       ]
     },
     "ChatErrorAction": {
@@ -6966,6 +6994,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn failed."
+        },
         "error": {
           "$ref": "#/$defs/ErrorInfo",
           "description": "Error details"
@@ -6979,6 +7011,7 @@
       "required": [
         "type",
         "turnId",
+        "endedAt",
         "error"
       ]
     },
diff --git a/schema/errors.schema.json b/schema/errors.schema.json
index f64961ff..36f341e0 100644
--- a/schema/errors.schema.json
+++ b/schema/errors.schema.json
@@ -2858,6 +2858,14 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
+        "duration": {
+          "type": "number",
+          "description": "Turn duration in milliseconds."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -2898,6 +2906,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -2916,6 +2928,7 @@
       },
       "required": [
         "id",
+        "startedAt",
         "message",
         "responseParts",
         "usage"
@@ -7393,6 +7406,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The new message"
@@ -7410,6 +7427,7 @@
       "required": [
         "type",
         "turnId",
+        "startedAt",
         "message"
       ]
     },
@@ -7745,6 +7763,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn completed."
+        },
         "_meta": {
           "type": "object",
           "additionalProperties": {},
@@ -7753,7 +7775,8 @@
       },
       "required": [
         "type",
-        "turnId"
+        "turnId",
+        "endedAt"
       ]
     },
     "ChatTurnCancelledAction": {
@@ -7767,6 +7790,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn was cancelled."
+        },
         "_meta": {
           "type": "object",
           "additionalProperties": {},
@@ -7775,7 +7802,8 @@
       },
       "required": [
         "type",
-        "turnId"
+        "turnId",
+        "endedAt"
       ]
     },
     "ChatErrorAction": {
@@ -7789,6 +7817,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "endedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn failed."
+        },
         "error": {
           "$ref": "#/$defs/ErrorInfo",
           "description": "Error details"
@@ -7802,6 +7834,7 @@
       "required": [
         "type",
         "turnId",
+        "endedAt",
         "error"
       ]
     },
diff --git a/schema/notifications.schema.json b/schema/notifications.schema.json
index 3c534515..6a1f0318 100644
--- a/schema/notifications.schema.json
+++ b/schema/notifications.schema.json
@@ -3018,6 +3018,14 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
+        "duration": {
+          "type": "number",
+          "description": "Turn duration in milliseconds."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -3058,6 +3066,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -3076,6 +3088,7 @@
       },
       "required": [
         "id",
+        "startedAt",
         "message",
         "responseParts",
         "usage"
diff --git a/schema/state.schema.json b/schema/state.schema.json
index 7a107af7..d9f1ac3c 100644
--- a/schema/state.schema.json
+++ b/schema/state.schema.json
@@ -2769,6 +2769,14 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
+        "duration": {
+          "type": "number",
+          "description": "Turn duration in milliseconds."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -2809,6 +2817,10 @@
           "type": "string",
           "description": "Turn identifier"
         },
+        "startedAt": {
+          "type": "string",
+          "description": "ISO 8601 timestamp when this turn started."
+        },
         "message": {
           "$ref": "#/$defs/Message",
           "description": "The message that initiated the turn"
@@ -2827,6 +2839,7 @@
       },
       "required": [
         "id",
+        "startedAt",
         "message",
         "responseParts",
         "usage"
diff --git a/types/channels-chat/actions.ts b/types/channels-chat/actions.ts
index 0fd6b8d4..4ad13f5c 100644
--- a/types/channels-chat/actions.ts
+++ b/types/channels-chat/actions.ts
@@ -65,6 +65,8 @@ export interface ChatTurnStartedAction {
   type: ActionType.ChatTurnStarted;
   /** Turn identifier */
   turnId: string;
+  /** ISO 8601 timestamp when this turn started. */
+  startedAt: string;
   /** The new message */
   message: Message;
   /** If this turn was auto-started from a queued message, the ID of that message */
@@ -343,6 +345,8 @@ export interface ChatTurnCompleteAction {
   type: ActionType.ChatTurnComplete;
   /** Turn identifier */
   turnId: string;
+  /** ISO 8601 timestamp when this turn completed. */
+  endedAt: string;
   /**
    * Additional provider-specific metadata for this action.
    *
@@ -366,6 +370,8 @@ export interface ChatTurnCancelledAction {
   type: ActionType.ChatTurnCancelled;
   /** Turn identifier */
   turnId: string;
+  /** ISO 8601 timestamp when this turn was cancelled. */
+  endedAt: string;
   /**
    * Additional provider-specific metadata for this action.
    *
@@ -388,6 +394,8 @@ export interface ChatErrorAction {
   type: ActionType.ChatError;
   /** Turn identifier */
   turnId: string;
+  /** ISO 8601 timestamp when this turn failed. */
+  endedAt: string;
   /** Error details */
   error: ErrorInfo;
   /**
diff --git a/types/channels-chat/reducer.ts b/types/channels-chat/reducer.ts
index 23764292..ab8dff9c 100644
--- a/types/channels-chat/reducer.ts
+++ b/types/channels-chat/reducer.ts
@@ -117,6 +117,7 @@ function endTurn(
   state: ChatState,
   turnId: string,
   turnState: TurnState,
+  endedAt: string,
   terminalStatus?: SessionStatus.Error,
   error?: { errorType: string; message: string; stack?: string },
 ): ChatState {
@@ -146,8 +147,15 @@ function endTurn(
     };
   });
 
+  const startedAt = Date.parse(active.startedAt);
+  const completedAt = Date.parse(endedAt);
+  const duration = Number.isFinite(startedAt) && Number.isFinite(completedAt)
+    ? Math.max(0, completedAt - startedAt)
+    : undefined;
   const turn: Turn = {
     id: active.id,
+    startedAt: active.startedAt,
+    duration,
     message: active.message,
     responseParts,
     usage: active.usage,
@@ -159,7 +167,7 @@ function endTurn(
     ...state,
     turns: [...state.turns, turn],
     activeTurn: undefined,
-    modifiedAt: new Date(Date.now()).toISOString(),
+    modifiedAt: endedAt,
   };
   delete next.inputRequests;
   return {
@@ -264,6 +272,36 @@ function updateResponsePart(
 
 // ─── Chat Reducer ────────────────────────────────────────────────────────────
 
+function isValidTimestamp(value: string): boolean {
+  const match = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.\d{1,9})?(?:Z|[+-](\d{2}):(\d{2}))$/.exec(value);
+  if (!match) {
+    return false;
+  }
+
+  const [, yearText, monthText, dayText, hourText, minuteText, secondText, offsetHourText, offsetMinuteText] = match;
+  const year = Number(yearText);
+  const month = Number(monthText);
+  const day = Number(dayText);
+  const hour = Number(hourText);
+  const minute = Number(minuteText);
+  const second = Number(secondText);
+  const offsetHour = offsetHourText === undefined ? 0 : Number(offsetHourText);
+  const offsetMinute = offsetMinuteText === undefined ? 0 : Number(offsetMinuteText);
+  const leapYear = year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
+  const daysInMonth = [31, leapYear ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+
+  return month >= 1
+    && month <= 12
+    && day >= 1
+    && day <= daysInMonth[month - 1]
+    && hour <= 23
+    && minute <= 59
+    && second <= 59
+    && offsetHour <= 23
+    && offsetMinute <= 59
+    && Number.isFinite(Date.parse(value));
+}
+
 /**
  * Pure reducer for chat state. Handles all {@link ChatAction} variants.
  */
@@ -272,10 +310,15 @@ export function chatReducer(state: ChatState, action: ChatAction, log?: (msg: st
     // ── Turn Lifecycle ────────────────────────────────────────────────────
 
     case ActionType.ChatTurnStarted: {
+      if (!isValidTimestamp(action.startedAt)) {
+        log?.(`Ignoring ChatTurnStarted with invalid startedAt: ${action.startedAt}`);
+        return state;
+      }
       let next: ChatState = {
         ...state,
         activeTurn: {
           id: action.turnId,
+          startedAt: action.startedAt,
           message: action.message,
           responseParts: [],
           usage: undefined,
@@ -284,7 +327,7 @@ export function chatReducer(state: ChatState, action: ChatAction, log?: (msg: st
       next = {
         ...next,
         status: withStatusFlag(summaryStatus(next), SessionStatus.IsRead, false),
-        modifiedAt: new Date(Date.now()).toISOString(),
+        modifiedAt: action.startedAt,
       };
 
       // If this turn was auto-started from a pending message, remove it
@@ -322,13 +365,25 @@ export function chatReducer(state: ChatState, action: ChatAction, log?: (msg: st
       };
 
     case ActionType.ChatTurnComplete:
-      return endTurn(state, action.turnId, TurnState.Complete);
+      if (!isValidTimestamp(action.endedAt)) {
+        log?.(`Ignoring ChatTurnComplete with invalid endedAt: ${action.endedAt}`);
+        return state;
+      }
+      return endTurn(state, action.turnId, TurnState.Complete, action.endedAt);
 
     case ActionType.ChatTurnCancelled:
-      return endTurn(state, action.turnId, TurnState.Cancelled);
+      if (!isValidTimestamp(action.endedAt)) {
+        log?.(`Ignoring ChatTurnCancelled with invalid endedAt: ${action.endedAt}`);
+        return state;
+      }
+      return endTurn(state, action.turnId, TurnState.Cancelled, action.endedAt);
 
     case ActionType.ChatError:
-      return endTurn(state, action.turnId, TurnState.Error, SessionStatus.Error, action.error);
+      if (!isValidTimestamp(action.endedAt)) {
+        log?.(`Ignoring ChatError with invalid endedAt: ${action.endedAt}`);
+        return state;
+      }
+      return endTurn(state, action.turnId, TurnState.Error, action.endedAt, SessionStatus.Error, action.error);
 
     case ActionType.ChatActivityChanged:
       return { ...state, activity: action.activity };
diff --git a/types/channels-chat/state.ts b/types/channels-chat/state.ts
index 00e6f689..9dff723f 100644
--- a/types/channels-chat/state.ts
+++ b/types/channels-chat/state.ts
@@ -504,6 +504,10 @@ export const enum MessageAttachmentKind {
 export interface Turn {
   /** Turn identifier */
   id: string;
+  /** ISO 8601 timestamp when this turn started. */
+  startedAt?: string;
+  /** Turn duration in milliseconds. */
+  duration?: number;
   /** The message that initiated the turn */
   message: Message;
   /**
@@ -529,6 +533,8 @@ export interface Turn {
 export interface ActiveTurn {
   /** Turn identifier */
   id: string;
+  /** ISO 8601 timestamp when this turn started. */
+  startedAt: string;
   /** The message that initiated the turn */
   message: Message;
   /**
diff --git a/types/reducers.test.ts b/types/reducers.test.ts
index f3a8c7de..0d04b739 100644
--- a/types/reducers.test.ts
+++ b/types/reducers.test.ts
@@ -211,7 +211,7 @@ describe('IS_CLIENT_DISPATCHABLE', () => {
 
 describe('isClientDispatchable', () => {
   it('returns true for client-dispatchable actions', () => {
-    const action = { type: ActionType.ChatTurnStarted, turnId: 't', message: { text: 'Hello', origin: { kind: MessageKind.User } } } as const;
+    const action = { type: ActionType.ChatTurnStarted, turnId: 't', startedAt: '2026-07-10T00:00:00.000Z', message: { text: 'Hello', origin: { kind: MessageKind.User } } } as const;
     assert.equal(isClientDispatchable(action), true);
   });
 
diff --git a/types/test-cases/reducers/005-session-turnstarted.json b/types/test-cases/reducers/005-session-turnstarted.json
index 3c3576ad..0fb745ce 100644
--- a/types/test-cases/reducers/005-session-turnstarted.json
+++ b/types/test-cases/reducers/005-session-turnstarted.json
@@ -12,6 +12,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -24,6 +25,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -36,6 +38,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:09.999Z"
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
   }
 }
diff --git a/types/test-cases/reducers/006-turnstarted-with-queuedmessageid-removes-from-queuedmessages.json b/types/test-cases/reducers/006-turnstarted-with-queuedmessageid-removes-from-queuedmessages.json
index 8636d1b2..1977a91a 100644
--- a/types/test-cases/reducers/006-turnstarted-with-queuedmessageid-removes-from-queuedmessages.json
+++ b/types/test-cases/reducers/006-turnstarted-with-queuedmessageid-removes-from-queuedmessages.json
@@ -32,6 +32,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "First",
         "origin": {
@@ -45,6 +46,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "First",
         "origin": {
@@ -68,6 +70,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:09.999Z"
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
   }
 }
diff --git a/types/test-cases/reducers/007-turnstarted-with-queuedmessageid-removes-last-queued-message.json b/types/test-cases/reducers/007-turnstarted-with-queuedmessageid-removes-last-queued-message.json
index 15b4ecf4..faad3e50 100644
--- a/types/test-cases/reducers/007-turnstarted-with-queuedmessageid-removes-last-queued-message.json
+++ b/types/test-cases/reducers/007-turnstarted-with-queuedmessageid-removes-last-queued-message.json
@@ -23,6 +23,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Only",
         "origin": {
@@ -36,6 +37,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Only",
         "origin": {
@@ -49,6 +51,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:09.999Z"
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
   }
 }
diff --git a/types/test-cases/reducers/008-turnstarted-with-queuedmessageid-removes-matching-steering-message.json b/types/test-cases/reducers/008-turnstarted-with-queuedmessageid-removes-matching-steering-message.json
index c955affc..022860aa 100644
--- a/types/test-cases/reducers/008-turnstarted-with-queuedmessageid-removes-matching-steering-message.json
+++ b/types/test-cases/reducers/008-turnstarted-with-queuedmessageid-removes-matching-steering-message.json
@@ -21,6 +21,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Steer",
         "origin": {
@@ -34,6 +35,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Steer",
         "origin": {
@@ -47,6 +49,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:09.999Z"
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
   }
 }
diff --git a/types/test-cases/reducers/009-turnstarted-without-queuedmessageid-does-not-touch-pending-messages.json b/types/test-cases/reducers/009-turnstarted-without-queuedmessageid-does-not-touch-pending-messages.json
index 4b1418e5..77bf2b7a 100644
--- a/types/test-cases/reducers/009-turnstarted-without-queuedmessageid-does-not-touch-pending-messages.json
+++ b/types/test-cases/reducers/009-turnstarted-without-queuedmessageid-does-not-touch-pending-messages.json
@@ -32,6 +32,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -44,6 +45,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -76,6 +78,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:09.999Z"
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
   }
 }
diff --git a/types/test-cases/reducers/010-session-delta-appends-content.json b/types/test-cases/reducers/010-session-delta-appends-content.json
index f4cefe7e..69d38431 100644
--- a/types/test-cases/reducers/010-session-delta-appends-content.json
+++ b/types/test-cases/reducers/010-session-delta-appends-content.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -46,6 +47,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/011-session-delta-with-wrong-turnid-is-no-op.json b/types/test-cases/reducers/011-session-delta-with-wrong-turnid-is-no-op.json
index c7fef529..8575b05e 100644
--- a/types/test-cases/reducers/011-session-delta-with-wrong-turnid-is-no-op.json
+++ b/types/test-cases/reducers/011-session-delta-with-wrong-turnid-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -37,6 +38,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/013-session-responsepart-adds-to-responseparts.json b/types/test-cases/reducers/013-session-responsepart-adds-to-responseparts.json
index 5ea45c46..0feba633 100644
--- a/types/test-cases/reducers/013-session-responsepart-adds-to-responseparts.json
+++ b/types/test-cases/reducers/013-session-responsepart-adds-to-responseparts.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -34,6 +35,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json b/types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json
index 2e4f14b9..a0cf3243 100644
--- a/types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json
+++ b/types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -37,13 +38,16 @@
     },
     {
       "type": "chat/turnComplete",
-      "turnId": "turn-1"
+      "turnId": "turn-1",
+      "endedAt": "1970-01-01T00:00:09.999Z"
     }
   ],
   "expected": {
     "turns": [
       {
         "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 8999,
         "message": {
           "text": "Hello",
           "origin": {
diff --git a/types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json b/types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json
index 61677b59..a5bb9a97 100644
--- a/types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json
+++ b/types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -22,13 +23,16 @@
   "actions": [
     {
       "type": "chat/turnCancelled",
-      "turnId": "turn-1"
+      "turnId": "turn-1",
+      "endedAt": "1970-01-01T00:00:09.999Z"
     }
   ],
   "expected": {
     "turns": [
       {
         "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 8999,
         "message": {
           "text": "Hello",
           "origin": {
diff --git a/types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json b/types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json
index 40c41bbd..5863a121 100644
--- a/types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json
+++ b/types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -23,6 +24,7 @@
     {
       "type": "chat/error",
       "turnId": "turn-1",
+      "endedAt": "1970-01-01T00:00:09.999Z",
       "error": {
         "errorType": "runtime",
         "message": "Something broke"
@@ -33,6 +35,8 @@
     "turns": [
       {
         "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 8999,
         "message": {
           "text": "Hello",
           "origin": {
diff --git a/types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json b/types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json
index 51b4a653..c1cec0e1 100644
--- a/types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json
+++ b/types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -29,13 +30,16 @@
     },
     {
       "type": "chat/turnComplete",
-      "turnId": "turn-1"
+      "turnId": "turn-1",
+      "endedAt": "1970-01-01T00:00:09.999Z"
     }
   ],
   "expected": {
     "turns": [
       {
         "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 8999,
         "message": {
           "text": "Hello",
           "origin": {
diff --git a/types/test-cases/reducers/018-turncomplete-with-wrong-turnid-is-no-op.json b/types/test-cases/reducers/018-turncomplete-with-wrong-turnid-is-no-op.json
index 42df2388..5d15fb14 100644
--- a/types/test-cases/reducers/018-turncomplete-with-wrong-turnid-is-no-op.json
+++ b/types/test-cases/reducers/018-turncomplete-with-wrong-turnid-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -22,13 +23,15 @@
   "actions": [
     {
       "type": "chat/turnComplete",
-      "turnId": "wrong-turn"
+      "turnId": "wrong-turn",
+      "endedAt": "1970-01-01T00:00:09.999Z"
     }
   ],
   "expected": {
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/019-tool-call-full-lifecycle-start-delta-ready-confirmed-complete.json b/types/test-cases/reducers/019-tool-call-full-lifecycle-start-delta-ready-confirmed-complete.json
index ca778718..09391579 100644
--- a/types/test-cases/reducers/019-tool-call-full-lifecycle-start-delta-ready-confirmed-complete.json
+++ b/types/test-cases/reducers/019-tool-call-full-lifecycle-start-delta-ready-confirmed-complete.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -63,6 +64,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/020-tool-call-ready-with-auto-confirm-transitions-to-running.json b/types/test-cases/reducers/020-tool-call-ready-with-auto-confirm-transitions-to-running.json
index b6b84c22..c8577e42 100644
--- a/types/test-cases/reducers/020-tool-call-ready-with-auto-confirm-transitions-to-running.json
+++ b/types/test-cases/reducers/020-tool-call-ready-with-auto-confirm-transitions-to-running.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -39,6 +40,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/021-tool-call-denied-transitions-to-cancelled.json b/types/test-cases/reducers/021-tool-call-denied-transitions-to-cancelled.json
index 99550ee7..69bdd0cd 100644
--- a/types/test-cases/reducers/021-tool-call-denied-transitions-to-cancelled.json
+++ b/types/test-cases/reducers/021-tool-call-denied-transitions-to-cancelled.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -45,6 +46,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/022-tool-call-result-confirmation-pending-approved.json b/types/test-cases/reducers/022-tool-call-result-confirmation-pending-approved.json
index 9ed72edb..d5b5c1f2 100644
--- a/types/test-cases/reducers/022-tool-call-result-confirmation-pending-approved.json
+++ b/types/test-cases/reducers/022-tool-call-result-confirmation-pending-approved.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -55,6 +56,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/023-tool-call-result-denied-cancelled-with-result-denied-reason.json b/types/test-cases/reducers/023-tool-call-result-denied-cancelled-with-result-denied-reason.json
index 40782570..563b8dd0 100644
--- a/types/test-cases/reducers/023-tool-call-result-denied-cancelled-with-result-denied-reason.json
+++ b/types/test-cases/reducers/023-tool-call-result-denied-cancelled-with-result-denied-reason.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -55,6 +56,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/024-tool-call-complete-from-pending-confirmation-defaults-confirmed.json b/types/test-cases/reducers/024-tool-call-complete-from-pending-confirmation-defaults-confirmed.json
index fc5c96d7..e5b9f540 100644
--- a/types/test-cases/reducers/024-tool-call-complete-from-pending-confirmation-defaults-confirmed.json
+++ b/types/test-cases/reducers/024-tool-call-complete-from-pending-confirmation-defaults-confirmed.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -47,6 +48,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/025-tool-call-actions-for-unknown-toolcallid-are-no-op.json b/types/test-cases/reducers/025-tool-call-actions-for-unknown-toolcallid-are-no-op.json
index b4c0d1d9..543212f6 100644
--- a/types/test-cases/reducers/025-tool-call-actions-for-unknown-toolcallid-are-no-op.json
+++ b/types/test-cases/reducers/025-tool-call-actions-for-unknown-toolcallid-are-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -31,6 +32,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/026-toolcallready-transitions-running-tool-back-to-pending-confirmation.json b/types/test-cases/reducers/026-toolcallready-transitions-running-tool-back-to-pending-confirmation.json
index 86458d85..94f7f058 100644
--- a/types/test-cases/reducers/026-toolcallready-transitions-running-tool-back-to-pending-confirmation.json
+++ b/types/test-cases/reducers/026-toolcallready-transitions-running-tool-back-to-pending-confirmation.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -49,6 +50,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/027-toolcallready-re-confirmation-approved-transitions-back-to-running.json b/types/test-cases/reducers/027-toolcallready-re-confirmation-approved-transitions-back-to-running.json
index 334292b2..880caf71 100644
--- a/types/test-cases/reducers/027-toolcallready-re-confirmation-approved-transitions-back-to-running.json
+++ b/types/test-cases/reducers/027-toolcallready-re-confirmation-approved-transitions-back-to-running.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -52,6 +53,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/028-toolcallready-re-confirmation-denied-transitions-to-cancelled.json b/types/test-cases/reducers/028-toolcallready-re-confirmation-denied-transitions-to-cancelled.json
index cdc46c27..414ae35d 100644
--- a/types/test-cases/reducers/028-toolcallready-re-confirmation-denied-transitions-to-cancelled.json
+++ b/types/test-cases/reducers/028-toolcallready-re-confirmation-denied-transitions-to-cancelled.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -52,6 +53,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/029-toolcallready-ignores-non-streaming-non-running-tool-calls.json b/types/test-cases/reducers/029-toolcallready-ignores-non-streaming-non-running-tool-calls.json
index 9c72391a..d7e4acd6 100644
--- a/types/test-cases/reducers/029-toolcallready-ignores-non-streaming-non-running-tool-calls.json
+++ b/types/test-cases/reducers/029-toolcallready-ignores-non-streaming-non-running-tool-calls.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -47,6 +48,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/031-session-usage-updates-usage-on-active-turn.json b/types/test-cases/reducers/031-session-usage-updates-usage-on-active-turn.json
index c9cd4ec4..634f8b2f 100644
--- a/types/test-cases/reducers/031-session-usage-updates-usage-on-active-turn.json
+++ b/types/test-cases/reducers/031-session-usage-updates-usage-on-active-turn.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -33,6 +34,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/032-session-reasoning-appends-reasoning-content.json b/types/test-cases/reducers/032-session-reasoning-appends-reasoning-content.json
index 40338a9c..a9052ee6 100644
--- a/types/test-cases/reducers/032-session-reasoning-appends-reasoning-content.json
+++ b/types/test-cases/reducers/032-session-reasoning-appends-reasoning-content.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -46,6 +47,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/067-session-truncated-drops-active-turn.json b/types/test-cases/reducers/067-session-truncated-drops-active-turn.json
index d5dde94f..63c00236 100644
--- a/types/test-cases/reducers/067-session-truncated-drops-active-turn.json
+++ b/types/test-cases/reducers/067-session-truncated-drops-active-turn.json
@@ -30,6 +30,7 @@
     ],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/068-session-truncated-drops-active-turn-even-when-clearing-all.json b/types/test-cases/reducers/068-session-truncated-drops-active-turn-even-when-clearing-all.json
index e9435f3c..2912d048 100644
--- a/types/test-cases/reducers/068-session-truncated-drops-active-turn-even-when-clearing-all.json
+++ b/types/test-cases/reducers/068-session-truncated-drops-active-turn-even-when-clearing-all.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json b/types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json
index 8225df98..8343b1d4 100644
--- a/types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json
+++ b/types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json
@@ -12,6 +12,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "t1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Fix the bug",
         "origin": {
@@ -137,13 +138,16 @@
     },
     {
       "type": "chat/turnComplete",
-      "turnId": "t1"
+      "turnId": "t1",
+      "endedAt": "1970-01-01T00:00:09.999Z"
     }
   ],
   "expected": {
     "turns": [
       {
         "id": "t1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 8999,
         "message": {
           "text": "Fix the bug",
           "origin": {
diff --git a/types/test-cases/reducers/075-turnstarted-clears-isread.json b/types/test-cases/reducers/075-turnstarted-clears-isread.json
index 80005421..f7791425 100644
--- a/types/test-cases/reducers/075-turnstarted-clears-isread.json
+++ b/types/test-cases/reducers/075-turnstarted-clears-isread.json
@@ -12,6 +12,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -24,6 +25,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -36,6 +38,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:09.999Z"
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
   }
 }
diff --git a/types/test-cases/reducers/084-toolcall-contentchanged-updates-running.json b/types/test-cases/reducers/084-toolcall-contentchanged-updates-running.json
index fdd86bb8..2cb78504 100644
--- a/types/test-cases/reducers/084-toolcall-contentchanged-updates-running.json
+++ b/types/test-cases/reducers/084-toolcall-contentchanged-updates-running.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -51,6 +52,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/085-toolcall-contentchanged-noop-non-running.json b/types/test-cases/reducers/085-toolcall-contentchanged-noop-non-running.json
index 84740852..18582ffe 100644
--- a/types/test-cases/reducers/085-toolcall-contentchanged-noop-non-running.json
+++ b/types/test-cases/reducers/085-toolcall-contentchanged-noop-non-running.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -53,6 +54,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/086-toolcall-contentchanged-replaces-existing.json b/types/test-cases/reducers/086-toolcall-contentchanged-replaces-existing.json
index 171fafa2..90507115 100644
--- a/types/test-cases/reducers/086-toolcall-contentchanged-replaces-existing.json
+++ b/types/test-cases/reducers/086-toolcall-contentchanged-replaces-existing.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -62,6 +63,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/090-toolcalldelta-wrong-turnid-is-no-op.json b/types/test-cases/reducers/090-toolcalldelta-wrong-turnid-is-no-op.json
index 08773d64..b640dc88 100644
--- a/types/test-cases/reducers/090-toolcalldelta-wrong-turnid-is-no-op.json
+++ b/types/test-cases/reducers/090-toolcalldelta-wrong-turnid-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -42,6 +43,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/091-responsepart-wrong-turnid-is-no-op.json b/types/test-cases/reducers/091-responsepart-wrong-turnid-is-no-op.json
index db7e3c87..457e5f84 100644
--- a/types/test-cases/reducers/091-responsepart-wrong-turnid-is-no-op.json
+++ b/types/test-cases/reducers/091-responsepart-wrong-turnid-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -34,6 +35,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/092-toolcallstart-wrong-turnid-is-no-op.json b/types/test-cases/reducers/092-toolcallstart-wrong-turnid-is-no-op.json
index 64d7f700..e6f35b90 100644
--- a/types/test-cases/reducers/092-toolcallstart-wrong-turnid-is-no-op.json
+++ b/types/test-cases/reducers/092-toolcallstart-wrong-turnid-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -32,6 +33,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/093-usage-wrong-turnid-is-no-op.json b/types/test-cases/reducers/093-usage-wrong-turnid-is-no-op.json
index cb26096f..fa12e805 100644
--- a/types/test-cases/reducers/093-usage-wrong-turnid-is-no-op.json
+++ b/types/test-cases/reducers/093-usage-wrong-turnid-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -33,6 +34,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/094-delta-nonexistent-partid-is-no-op.json b/types/test-cases/reducers/094-delta-nonexistent-partid-is-no-op.json
index 7c564a93..964e516b 100644
--- a/types/test-cases/reducers/094-delta-nonexistent-partid-is-no-op.json
+++ b/types/test-cases/reducers/094-delta-nonexistent-partid-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -37,6 +38,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/095-toolcalldelta-wrong-status-is-no-op.json b/types/test-cases/reducers/095-toolcalldelta-wrong-status-is-no-op.json
index cdbaab61..93a0910e 100644
--- a/types/test-cases/reducers/095-toolcalldelta-wrong-status-is-no-op.json
+++ b/types/test-cases/reducers/095-toolcalldelta-wrong-status-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -45,6 +46,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/096-toolcallconfirmed-wrong-status-is-no-op.json b/types/test-cases/reducers/096-toolcallconfirmed-wrong-status-is-no-op.json
index a4472603..f616ee6b 100644
--- a/types/test-cases/reducers/096-toolcallconfirmed-wrong-status-is-no-op.json
+++ b/types/test-cases/reducers/096-toolcallconfirmed-wrong-status-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -46,6 +47,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/097-toolcallcomplete-wrong-status-is-no-op.json b/types/test-cases/reducers/097-toolcallcomplete-wrong-status-is-no-op.json
index b647b7ba..f4bc585f 100644
--- a/types/test-cases/reducers/097-toolcallcomplete-wrong-status-is-no-op.json
+++ b/types/test-cases/reducers/097-toolcallcomplete-wrong-status-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -50,6 +51,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/098-toolcallresultconfirmed-wrong-status-is-no-op.json b/types/test-cases/reducers/098-toolcallresultconfirmed-wrong-status-is-no-op.json
index a85b5135..44f6c44d 100644
--- a/types/test-cases/reducers/098-toolcallresultconfirmed-wrong-status-is-no-op.json
+++ b/types/test-cases/reducers/098-toolcallresultconfirmed-wrong-status-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -45,6 +46,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json b/types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json
index cb64c47b..8e04a242 100644
--- a/types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json
+++ b/types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -36,13 +37,16 @@
   "actions": [
     {
       "type": "chat/turnComplete",
-      "turnId": "turn-1"
+      "turnId": "turn-1",
+      "endedAt": "1970-01-01T00:00:09.999Z"
     }
   ],
   "expected": {
     "turns": [
       {
         "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 8999,
         "message": {
           "text": "Hello",
           "origin": {
diff --git a/types/test-cases/reducers/100-delta-targeting-toolcall-partid-is-no-op.json b/types/test-cases/reducers/100-delta-targeting-toolcall-partid-is-no-op.json
index b96319e0..2eee6a8d 100644
--- a/types/test-cases/reducers/100-delta-targeting-toolcall-partid-is-no-op.json
+++ b/types/test-cases/reducers/100-delta-targeting-toolcall-partid-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -45,6 +46,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/101-toolcalldelta-without-invocationmessage.json b/types/test-cases/reducers/101-toolcalldelta-without-invocationmessage.json
index a0b381c7..f8c0e1f5 100644
--- a/types/test-cases/reducers/101-toolcalldelta-without-invocationmessage.json
+++ b/types/test-cases/reducers/101-toolcalldelta-without-invocationmessage.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -43,6 +44,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/102-reasoning-targeting-non-reasoning-is-no-op.json b/types/test-cases/reducers/102-reasoning-targeting-non-reasoning-is-no-op.json
index f2121787..a07517e0 100644
--- a/types/test-cases/reducers/102-reasoning-targeting-non-reasoning-is-no-op.json
+++ b/types/test-cases/reducers/102-reasoning-targeting-non-reasoning-is-no-op.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -37,6 +38,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/103-delta-skips-parts-without-id.json b/types/test-cases/reducers/103-delta-skips-parts-without-id.json
index c205564e..4e83b722 100644
--- a/types/test-cases/reducers/103-delta-skips-parts-without-id.json
+++ b/types/test-cases/reducers/103-delta-skips-parts-without-id.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -40,6 +41,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/105-session-input-full-draft-and-complete-flow.json b/types/test-cases/reducers/105-session-input-full-draft-and-complete-flow.json
index db553adc..88fdff59 100644
--- a/types/test-cases/reducers/105-session-input-full-draft-and-complete-flow.json
+++ b/types/test-cases/reducers/105-session-input-full-draft-and-complete-flow.json
@@ -12,6 +12,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -82,6 +83,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/106-session-input-requested-with-drafts-status.json b/types/test-cases/reducers/106-session-input-requested-with-drafts-status.json
index bd66b38c..102076bb 100644
--- a/types/test-cases/reducers/106-session-input-requested-with-drafts-status.json
+++ b/types/test-cases/reducers/106-session-input-requested-with-drafts-status.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -51,6 +52,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json b/types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json
index b7c5236c..62ba2446 100644
--- a/types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json
+++ b/types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -40,13 +41,16 @@
   "actions": [
     {
       "type": "chat/turnComplete",
-      "turnId": "turn-1"
+      "turnId": "turn-1",
+      "endedAt": "1970-01-01T00:00:09.999Z"
     }
   ],
   "expected": {
     "turns": [
       {
         "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 8999,
         "message": {
           "text": "Hello",
           "origin": {
diff --git a/types/test-cases/reducers/108-session-input-upsert-and-clear-answer.json b/types/test-cases/reducers/108-session-input-upsert-and-clear-answer.json
index 64523607..548373c6 100644
--- a/types/test-cases/reducers/108-session-input-upsert-and-clear-answer.json
+++ b/types/test-cases/reducers/108-session-input-upsert-and-clear-answer.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -64,6 +65,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/110-session-input-completion-and-truncation-filtering.json b/types/test-cases/reducers/110-session-input-completion-and-truncation-filtering.json
index 8987abf0..676d4c21 100644
--- a/types/test-cases/reducers/110-session-input-completion-and-truncation-filtering.json
+++ b/types/test-cases/reducers/110-session-input-completion-and-truncation-filtering.json
@@ -30,6 +30,7 @@
     ],
     "activeTurn": {
       "id": "active",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Active",
         "origin": {
diff --git a/types/test-cases/reducers/111-toolcall-pending-confirmation-sets-input-needed-status.json b/types/test-cases/reducers/111-toolcall-pending-confirmation-sets-input-needed-status.json
index 5c32932a..fbf03a73 100644
--- a/types/test-cases/reducers/111-toolcall-pending-confirmation-sets-input-needed-status.json
+++ b/types/test-cases/reducers/111-toolcall-pending-confirmation-sets-input-needed-status.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -39,6 +40,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/112-toolcall-pending-result-confirmation-sets-input-needed-status.json b/types/test-cases/reducers/112-toolcall-pending-result-confirmation-sets-input-needed-status.json
index 1d112bdd..0b8153ce 100644
--- a/types/test-cases/reducers/112-toolcall-pending-result-confirmation-sets-input-needed-status.json
+++ b/types/test-cases/reducers/112-toolcall-pending-result-confirmation-sets-input-needed-status.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -49,6 +50,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/127-toolcallready-with-confirmation-options.json b/types/test-cases/reducers/127-toolcallready-with-confirmation-options.json
index 3a58c403..0822e2fc 100644
--- a/types/test-cases/reducers/127-toolcallready-with-confirmation-options.json
+++ b/types/test-cases/reducers/127-toolcallready-with-confirmation-options.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -64,6 +65,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/128-toolcallconfirmed-approved-with-selectedoption.json b/types/test-cases/reducers/128-toolcallconfirmed-approved-with-selectedoption.json
index 09c87fde..17caaa65 100644
--- a/types/test-cases/reducers/128-toolcallconfirmed-approved-with-selectedoption.json
+++ b/types/test-cases/reducers/128-toolcallconfirmed-approved-with-selectedoption.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -66,6 +67,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/129-toolcallconfirmed-denied-with-selectedoption.json b/types/test-cases/reducers/129-toolcallconfirmed-denied-with-selectedoption.json
index 3e2254fc..0409698e 100644
--- a/types/test-cases/reducers/129-toolcallconfirmed-denied-with-selectedoption.json
+++ b/types/test-cases/reducers/129-toolcallconfirmed-denied-with-selectedoption.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -67,6 +68,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/130-selectedoption-carries-through-to-completed.json b/types/test-cases/reducers/130-selectedoption-carries-through-to-completed.json
index 8909bac5..31cfac14 100644
--- a/types/test-cases/reducers/130-selectedoption-carries-through-to-completed.json
+++ b/types/test-cases/reducers/130-selectedoption-carries-through-to-completed.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -69,6 +70,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/131-selectedoption-carries-through-result-confirmation.json b/types/test-cases/reducers/131-selectedoption-carries-through-result-confirmation.json
index 6d17774a..3a823536 100644
--- a/types/test-cases/reducers/131-selectedoption-carries-through-result-confirmation.json
+++ b/types/test-cases/reducers/131-selectedoption-carries-through-result-confirmation.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -70,6 +71,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/132-selectedoption-carries-through-result-denied.json b/types/test-cases/reducers/132-selectedoption-carries-through-result-denied.json
index 805add3b..871fff31 100644
--- a/types/test-cases/reducers/132-selectedoption-carries-through-result-denied.json
+++ b/types/test-cases/reducers/132-selectedoption-carries-through-result-denied.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -70,6 +71,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/158-toolcallconfirmed-approved-with-editedtoolinput-overrides-original.json b/types/test-cases/reducers/158-toolcallconfirmed-approved-with-editedtoolinput-overrides-original.json
index d24fd7de..4fd274f3 100644
--- a/types/test-cases/reducers/158-toolcallconfirmed-approved-with-editedtoolinput-overrides-original.json
+++ b/types/test-cases/reducers/158-toolcallconfirmed-approved-with-editedtoolinput-overrides-original.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -56,6 +57,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json b/types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json
index df65d14c..fa43ea16 100644
--- a/types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json
+++ b/types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json
@@ -15,6 +15,7 @@
     {
       "type": "chat/turnStarted",
       "turnId": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -39,13 +40,16 @@
     },
     {
       "type": "chat/turnComplete",
-      "turnId": "turn-1"
+      "turnId": "turn-1",
+      "endedAt": "1970-01-01T00:00:09.999Z"
     }
   ],
   "expected": {
     "turns": [
       {
         "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 8999,
         "message": {
           "text": "Hello",
           "origin": {
diff --git a/types/test-cases/reducers/163-toolcallstart-carries-mcp-contributor-through-lifecycle.json b/types/test-cases/reducers/163-toolcallstart-carries-mcp-contributor-through-lifecycle.json
index a897e5e7..a01886f2 100644
--- a/types/test-cases/reducers/163-toolcallstart-carries-mcp-contributor-through-lifecycle.json
+++ b/types/test-cases/reducers/163-toolcallstart-carries-mcp-contributor-through-lifecycle.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -53,6 +54,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/220-toolcall-actions-update-meta.json b/types/test-cases/reducers/220-toolcall-actions-update-meta.json
index e3b7e797..2eebdd14 100644
--- a/types/test-cases/reducers/220-toolcall-actions-update-meta.json
+++ b/types/test-cases/reducers/220-toolcall-actions-update-meta.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
@@ -216,6 +217,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hello",
         "origin": {
diff --git a/types/test-cases/reducers/235-session-input-completion-records-declined-request-in-turn.json b/types/test-cases/reducers/235-session-input-completion-records-declined-request-in-turn.json
index 968e563c..3fd17ebc 100644
--- a/types/test-cases/reducers/235-session-input-completion-records-declined-request-in-turn.json
+++ b/types/test-cases/reducers/235-session-input-completion-records-declined-request-in-turn.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Review",
         "origin": {
@@ -63,6 +64,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Review",
         "origin": {
diff --git a/types/test-cases/reducers/237-session-input-completion-no-op-unknown-id-with-open-requests.json b/types/test-cases/reducers/237-session-input-completion-no-op-unknown-id-with-open-requests.json
index 231852ff..77d568c8 100644
--- a/types/test-cases/reducers/237-session-input-completion-no-op-unknown-id-with-open-requests.json
+++ b/types/test-cases/reducers/237-session-input-completion-no-op-unknown-id-with-open-requests.json
@@ -5,6 +5,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hi",
         "origin": {
@@ -36,6 +37,7 @@
     "turns": [],
     "activeTurn": {
       "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
       "message": {
         "text": "Hi",
         "origin": {
diff --git a/types/test-cases/reducers/238-invalid-turn-start-timestamp-is-no-op.json b/types/test-cases/reducers/238-invalid-turn-start-timestamp-is-no-op.json
new file mode 100644
index 00000000..bc6f6d62
--- /dev/null
+++ b/types/test-cases/reducers/238-invalid-turn-start-timestamp-is-no-op.json
@@ -0,0 +1,31 @@
+{
+  "description": "chat/turnStarted with an invalid timestamp is a no-op",
+  "reducer": "chat",
+  "initial": {
+    "turns": [],
+    "resource": "copilot:/test-session",
+    "title": "Test Session",
+    "status": 1,
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
+  },
+  "actions": [
+    {
+      "type": "chat/turnStarted",
+      "turnId": "turn-1",
+      "startedAt": "July 9 2026 20:00:00",
+      "message": {
+        "text": "Hello",
+        "origin": {
+          "kind": "user"
+        }
+      }
+    }
+  ],
+  "expected": {
+    "turns": [],
+    "resource": "copilot:/test-session",
+    "title": "Test Session",
+    "status": 1,
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
+  }
+}
diff --git a/types/test-cases/reducers/239-invalid-turn-end-timestamp-is-no-op.json b/types/test-cases/reducers/239-invalid-turn-end-timestamp-is-no-op.json
new file mode 100644
index 00000000..78d664f9
--- /dev/null
+++ b/types/test-cases/reducers/239-invalid-turn-end-timestamp-is-no-op.json
@@ -0,0 +1,49 @@
+{
+  "description": "chat/turnComplete with an invalid timestamp is a no-op",
+  "reducer": "chat",
+  "initial": {
+    "turns": [],
+    "activeTurn": {
+      "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
+      "message": {
+        "text": "Hello",
+        "origin": {
+          "kind": "user"
+        }
+      },
+      "responseParts": [],
+      "usage": null
+    },
+    "resource": "copilot:/test-session",
+    "title": "Test Session",
+    "status": 8,
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
+  },
+  "actions": [
+    {
+      "type": "chat/turnComplete",
+      "turnId": "turn-1",
+      "endedAt": "2026-07-09T20:00:00.000"
+    }
+  ],
+  "expected": {
+    "turns": [],
+    "activeTurn": {
+      "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:01.000Z",
+      "message": {
+        "text": "Hello",
+        "origin": {
+          "kind": "user"
+        }
+      },
+      "responseParts": [],
+      "usage": null
+    },
+    "resource": "copilot:/test-session",
+    "title": "Test Session",
+    "status": 8,
+    "modifiedAt": "1970-01-01T00:00:01.000Z"
+  }
+}
diff --git a/types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json b/types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json
new file mode 100644
index 00000000..dbe67810
--- /dev/null
+++ b/types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json
@@ -0,0 +1,54 @@
+{
+  "description": "chat/turnComplete clamps a negative turn duration to zero",
+  "reducer": "chat",
+  "initial": {
+    "turns": [],
+    "activeTurn": {
+      "id": "turn-1",
+      "startedAt": "1970-01-01T00:00:10.000Z",
+      "message": {
+        "text": "Hello",
+        "origin": {
+          "kind": "user"
+        }
+      },
+      "responseParts": [],
+      "usage": null
+    },
+    "resource": "copilot:/test-session",
+    "title": "Test Session",
+    "status": 8,
+    "modifiedAt": "1970-01-01T00:00:10.000Z"
+  },
+  "actions": [
+    {
+      "type": "chat/turnComplete",
+      "turnId": "turn-1",
+      "endedAt": "1970-01-01T00:00:09.000Z"
+    }
+  ],
+  "expected": {
+    "turns": [
+      {
+        "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:10.000Z",
+        "duration": 0,
+        "message": {
+          "text": "Hello",
+          "origin": {
+            "kind": "user"
+          }
+        },
+        "responseParts": [],
+        "usage": null,
+        "state": "complete",
+        "error": null
+      }
+    ],
+    "activeTurn": null,
+    "resource": "copilot:/test-session",
+    "title": "Test Session",
+    "status": 1,
+    "modifiedAt": "1970-01-01T00:00:09.000Z"
+  }
+}

From bcd3fdd22bdd94441b0488dabdcdc3ad87802853 Mon Sep 17 00:00:00 2001
From: justschen <justchen@microsoft.com>
Date: Thu, 9 Jul 2026 21:38:23 -0700
Subject: [PATCH 2/3] swift: fix turn duration rounding

Normalize timestamp fractions to whole milliseconds before converting through Foundation so floating-point representation cannot undercount elapsed turn durations.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../Sources/AgentHostProtocol/Reducers.swift  | 26 ++++++++++++++-----
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
index f01364a5..b1eb3c43 100644
--- a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
+++ b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
@@ -28,8 +28,21 @@ private func parseTimestamp(_ value: String) -> Date? {
         ?? iso8601TimestampFormatterWithoutFractionalSeconds.date(from: value)
 }
 
-private func timestampMilliseconds(_ date: Date) -> Int {
-    Int(floor(date.timeIntervalSince1970 * 1000))
+private func parseTimestampMilliseconds(_ value: String) -> Int? {
+    guard parseTimestamp(value) != nil else { return nil }
+
+    var normalized = value
+    if let decimal = value.firstIndex(of: "."),
+       let timezone = value[decimal...].firstIndex(where: { $0 == "Z" || $0 == "+" || $0 == "-" }) {
+        let fractionStart = value.index(after: decimal)
+        let fraction = value[fractionStart..<timezone]
+        let milliseconds = String(fraction.prefix(3))
+            .padding(toLength: 3, withPad: "0", startingAt: 0)
+        normalized = String(value[..<fractionStart]) + milliseconds + String(value[timezone...])
+    }
+
+    guard let date = parseTimestamp(normalized) else { return nil }
+    return Int((date.timeIntervalSince1970 * 1000).rounded())
 }
 
 // MARK: - Status Bitset Helpers
@@ -116,7 +129,7 @@ public func chatReducer(state: ChatState, action: StateAction) -> ChatState {
     // ── Turn Lifecycle ────────────────────────────────────────────────────
 
     case .chatTurnStarted(let a):
-        guard parseTimestamp(a.startedAt) != nil else { return state }
+        guard parseTimestampMilliseconds(a.startedAt) != nil else { return state }
         var next = state
         next.modifiedAt = a.startedAt
         next.activeTurn = ActiveTurn(
@@ -909,7 +922,7 @@ private func endTurn(
     terminalStatus: SessionStatus? = nil,
     error: ErrorInfo? = nil
 ) -> ChatState {
-    guard let ended = parseTimestamp(endedAt) else { return state }
+    guard let endedMilliseconds = parseTimestampMilliseconds(endedAt) else { return state }
     guard let activeTurn = state.activeTurn, activeTurn.id == turnId else {
         return state
     }
@@ -959,9 +972,8 @@ private func endTurn(
         }
     }
 
-    let endedMilliseconds = timestampMilliseconds(ended)
-    let duration = parseTimestamp(activeTurn.startedAt)
-        .map { max(0, endedMilliseconds - timestampMilliseconds($0)) }
+    let duration = parseTimestampMilliseconds(activeTurn.startedAt)
+        .map { max(0, endedMilliseconds - $0) }
     let turn = Turn(
         id: activeTurn.id,
         startedAt: activeTurn.startedAt,

From af2a6d3468b3e62dc5e37958af2acfa07f362b64 Mon Sep 17 00:00:00 2001
From: justschen <justchen@microsoft.com>
Date: Fri, 10 Jul 2026 11:42:37 -0700
Subject: [PATCH 3/3] chat: use producer-measured turn duration

Replace terminal action timestamps with elapsed durations measured by the action producer. This avoids cross-clock subtraction in reducers, removes timestamp parsing and the Rust chrono dependency, and keeps modifiedAt on a consistent reducer-local clock.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 clients/go/ahp/reducers.go                    | 32 +++-----
 clients/go/ahptypes/actions.generated.go      | 21 ++++--
 clients/go/examples/reducers_demo/main.go     |  6 +-
 .../microsoft/agenthostprotocol/Reducers.kt   | 74 ++++++++-----------
 .../generated/Actions.generated.kt            | 21 ++++--
 .../agenthostprotocol/ReducersTest.kt         |  5 +-
 clients/rust/Cargo.lock                       | 25 -------
 clients/rust/Cargo.toml                       |  1 -
 clients/rust/crates/ahp-types/src/actions.rs  | 21 ++++--
 clients/rust/crates/ahp/Cargo.toml            |  1 -
 clients/rust/crates/ahp/src/reducers.rs       | 37 +++-------
 .../Generated/Actions.generated.swift         | 39 ++++++----
 .../Sources/AgentHostProtocol/Reducers.swift  | 48 +++---------
 .../ReducersTests.swift                       |  7 +-
 docs/.changes/20260709-turn-timing.json       |  2 +-
 schema/actions.schema.json                    | 24 +++---
 schema/commands.schema.json                   | 24 +++---
 schema/errors.schema.json                     | 24 +++---
 types/channels-chat/actions.ts                | 27 +++++--
 types/channels-chat/reducer.ts                | 67 +++--------------
 .../reducers/005-session-turnstarted.json     |  2 +-
 ...messageid-removes-from-queuedmessages.json |  2 +-
 ...messageid-removes-last-queued-message.json |  2 +-
 ...eid-removes-matching-steering-message.json |  2 +-
 ...ageid-does-not-touch-pending-messages.json |  2 +-
 ...4-session-turncomplete-finalizes-turn.json |  2 +-
 ...-session-turncancelled-finalizes-turn.json |  2 +-
 ...ssion-error-finalizes-turn-with-error.json |  2 +-
 ...-force-cancels-in-progress-tool-calls.json |  2 +-
 ...rncomplete-with-wrong-turnid-is-no-op.json |  2 +-
 ...w-with-tool-calls-and-re-confirmation.json |  2 +-
 .../075-turnstarted-clears-isread.json        |  2 +-
 ...dturn-force-cancels-running-tool-call.json |  2 +-
 ...nput-turn-end-cleans-turn-scoped-only.json |  2 +-
 .../161-chat-turn-lifecycle-on-chat.json      |  2 +-
 ...invalid-turn-start-timestamp-is-no-op.json | 31 --------
 ...9-invalid-turn-end-timestamp-is-no-op.json | 49 ------------
 .../240-turn-duration-is-clamped-to-zero.json |  4 +-
 38 files changed, 228 insertions(+), 392 deletions(-)
 delete mode 100644 types/test-cases/reducers/238-invalid-turn-start-timestamp-is-no-op.json
 delete mode 100644 types/test-cases/reducers/239-invalid-turn-end-timestamp-is-no-op.json

diff --git a/clients/go/ahp/reducers.go b/clients/go/ahp/reducers.go
index c3905761..a498d4c5 100644
--- a/clients/go/ahp/reducers.go
+++ b/clients/go/ahp/reducers.go
@@ -181,11 +181,7 @@ func touchChatModified(state *ahptypes.ChatState) {
 
 // ─── Active-turn helpers ───────────────────────────────────────────────
 
-func endTurn(state *ahptypes.ChatState, turnID, endedAt string, turnState ahptypes.TurnState, terminalStatus *ahptypes.SessionStatus, errInfo *ahptypes.ErrorInfo) ReduceOutcome {
-	ended, err := time.Parse(time.RFC3339Nano, endedAt)
-	if err != nil {
-		return ReduceOutcomeNoOp
-	}
+func endTurn(state *ahptypes.ChatState, turnID string, duration int64, turnState ahptypes.TurnState, terminalStatus *ahptypes.SessionStatus, errInfo *ahptypes.ErrorInfo) ReduceOutcome {
 	if state.ActiveTurn == nil || state.ActiveTurn.Id != turnID {
 		return ReduceOutcomeNoOp
 	}
@@ -224,19 +220,16 @@ func endTurn(state *ahptypes.ChatState, turnID, endedAt string, turnState ahptyp
 		}})
 	}
 
-	var duration *int64
-	if started, err := time.Parse(time.RFC3339Nano, active.StartedAt); err == nil {
-		ms := ended.UnixMilli() - started.UnixMilli()
-		if ms < 0 {
-			ms = 0
-		}
-		duration = &ms
+	// Defensive clamp: duration is producer-supplied and opaque to this
+	// reducer, but a negative value would be nonsensical to display.
+	if duration < 0 {
+		duration = 0
 	}
 	startedAt := active.StartedAt
 	turn := ahptypes.Turn{
 		Id:            active.Id,
 		StartedAt:     &startedAt,
-		Duration:      duration,
+		Duration:      &duration,
 		Message:       active.Message,
 		ResponseParts: parts,
 		Usage:         active.Usage,
@@ -246,7 +239,7 @@ func endTurn(state *ahptypes.ChatState, turnID, endedAt string, turnState ahptyp
 
 	state.Turns = append(state.Turns, turn)
 	state.InputRequests = nil
-	state.ModifiedAt = endedAt
+	state.ModifiedAt = nowISOString()
 	state.Status = summaryStatus(state, terminalStatus)
 	return ReduceOutcomeApplied
 }
@@ -479,13 +472,13 @@ func ApplyActionToChat(state *ahptypes.ChatState, action ahptypes.StateAction) R
 		state.ActiveTurn.ResponseParts = append(state.ActiveTurn.ResponseParts, a.Part)
 		return ReduceOutcomeApplied
 	case *ahptypes.ChatTurnCompleteAction:
-		return endTurn(state, a.TurnId, a.EndedAt, ahptypes.TurnStateComplete, nil, nil)
+		return endTurn(state, a.TurnId, a.Duration, ahptypes.TurnStateComplete, nil, nil)
 	case *ahptypes.ChatTurnCancelledAction:
-		return endTurn(state, a.TurnId, a.EndedAt, ahptypes.TurnStateCancelled, nil, nil)
+		return endTurn(state, a.TurnId, a.Duration, ahptypes.TurnStateCancelled, nil, nil)
 	case *ahptypes.ChatErrorAction:
 		errCopy := a.Error
 		errStatus := ahptypes.SessionStatusError
-		return endTurn(state, a.TurnId, a.EndedAt, ahptypes.TurnStateError, &errStatus, &errCopy)
+		return endTurn(state, a.TurnId, a.Duration, ahptypes.TurnStateError, &errStatus, &errCopy)
 	case *ahptypes.ChatActivityChangedAction:
 		state.Activity = a.Activity
 		return ReduceOutcomeApplied
@@ -937,9 +930,6 @@ func ApplyActionToSession(state *ahptypes.SessionState, action ahptypes.StateAct
 }
 
 func applyTurnStarted(state *ahptypes.ChatState, a *ahptypes.ChatTurnStartedAction) ReduceOutcome {
-	if _, err := time.Parse(time.RFC3339Nano, a.StartedAt); err != nil {
-		return ReduceOutcomeNoOp
-	}
 	state.ActiveTurn = &ahptypes.ActiveTurn{
 		Id:            a.TurnId,
 		StartedAt:     a.StartedAt,
@@ -947,7 +937,7 @@ func applyTurnStarted(state *ahptypes.ChatState, a *ahptypes.ChatTurnStartedActi
 		ResponseParts: []ahptypes.ResponsePart{},
 	}
 	state.Status = summaryStatus(state, nil)
-	state.ModifiedAt = a.StartedAt
+	state.ModifiedAt = nowISOString()
 	state.Status = withStatusFlag(state.Status, ahptypes.SessionStatusIsRead, false)
 
 	if a.QueuedMessageId != nil {
diff --git a/clients/go/ahptypes/actions.generated.go b/clients/go/ahptypes/actions.generated.go
index ca866fb1..495c9c3c 100644
--- a/clients/go/ahptypes/actions.generated.go
+++ b/clients/go/ahptypes/actions.generated.go
@@ -462,8 +462,11 @@ type ChatTurnCompleteAction struct {
 	Type ActionType `json:"type"`
 	// Turn identifier
 	TurnId string `json:"turnId"`
-	// ISO 8601 timestamp when this turn completed.
-	EndedAt string `json:"endedAt"`
+	// Elapsed turn duration in milliseconds, measured by the producer's own
+	// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+	// client clocks may differ — and MUST treat it as opaque, producer-supplied
+	// data.
+	Duration int64 `json:"duration"`
 	// Additional provider-specific metadata for this action.
 	//
 	// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -479,8 +482,11 @@ type ChatTurnCancelledAction struct {
 	Type ActionType `json:"type"`
 	// Turn identifier
 	TurnId string `json:"turnId"`
-	// ISO 8601 timestamp when this turn was cancelled.
-	EndedAt string `json:"endedAt"`
+	// Elapsed turn duration in milliseconds, measured by the producer's own
+	// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+	// client clocks may differ — and MUST treat it as opaque, producer-supplied
+	// data.
+	Duration int64 `json:"duration"`
 	// Additional provider-specific metadata for this action.
 	//
 	// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -496,8 +502,11 @@ type ChatErrorAction struct {
 	Type ActionType `json:"type"`
 	// Turn identifier
 	TurnId string `json:"turnId"`
-	// ISO 8601 timestamp when this turn failed.
-	EndedAt string `json:"endedAt"`
+	// Elapsed turn duration in milliseconds, measured by the producer's own
+	// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+	// client clocks may differ — and MUST treat it as opaque, producer-supplied
+	// data.
+	Duration int64 `json:"duration"`
 	// Error details
 	Error ErrorInfo `json:"error"`
 	// Additional provider-specific metadata for this action.
diff --git a/clients/go/examples/reducers_demo/main.go b/clients/go/examples/reducers_demo/main.go
index 69c7b4e3..139d8c63 100644
--- a/clients/go/examples/reducers_demo/main.go
+++ b/clients/go/examples/reducers_demo/main.go
@@ -41,9 +41,9 @@ func main() {
 			Content: "there!",
 		}},
 		{Value: &ahptypes.ChatTurnCompleteAction{
-			Type:    ahptypes.ActionTypeChatTurnComplete,
-			TurnId:  "t1",
-			EndedAt: "2026-07-09T20:00:01.000Z",
+			Type:     ahptypes.ActionTypeChatTurnComplete,
+			TurnId:   "t1",
+			Duration: 1000,
 		}},
 	}
 
diff --git a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
index 049277f5..dbe1f83c 100644
--- a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
+++ b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/Reducers.kt
@@ -82,11 +82,6 @@ public var currentTimestampProvider: () -> Long = { System.currentTimeMillis() }
 
 private fun nowIsoString(): String = Instant.ofEpochMilli(currentTimestampProvider()).toString()
 
-private fun parseTimestampMillis(value: String): Long? {
-    val millis = runCatching { Instant.parse(value).toEpochMilli() }.getOrNull() ?: return null
-    return millis.takeIf { it in -8_640_000_000_000_000L..8_640_000_000_000_000L }
-}
-
 // ─── Status Bitset Helpers ──────────────────────────────────────────────────
 
 /** Bitmask covering the mutually-exclusive activity bits (bits 0–4). */
@@ -337,12 +332,11 @@ private fun updateResponsePart(
 private fun endTurn(
     state: ChatState,
     turnId: String,
-    endedAt: String,
+    duration: Long,
     turnState: TurnState,
     terminalStatus: SessionStatus? = null,
     error: ErrorInfo? = null,
 ): ChatState {
-    val ended = parseTimestampMillis(endedAt) ?: return state
     val active = state.activeTurn ?: return state
     if (active.id != turnId) return state
 
@@ -393,12 +387,12 @@ private fun endTurn(
         )
     }
 
-    val duration = parseTimestampMillis(active.startedAt)
-        ?.let { started -> maxOf(0L, ended - started) }
+    // Defensive clamp: `duration` is producer-supplied and opaque to this
+    // reducer, but a negative value would be nonsensical to display.
     val turn = Turn(
         id = active.id,
         startedAt = active.startedAt,
-        duration = duration,
+        duration = maxOf(0L, duration),
         message = active.message,
         responseParts = finalizedParts,
         usage = active.usage,
@@ -410,7 +404,7 @@ private fun endTurn(
         turns = state.turns + turn,
         activeTurn = null,
         inputRequests = null,
-        modifiedAt = endedAt,
+        modifiedAt = nowIsoString(),
     )
     return withoutTurn.copy(status = chatSummaryStatus(withoutTurn, terminalStatus))
 }
@@ -755,36 +749,32 @@ public fun chatReducer(state: ChatState, action: StateAction): ChatState = when
 
     is StateActionChatTurnStarted -> {
         val a = action.value
-        if (parseTimestampMillis(a.startedAt) == null) {
-            state
+        val withTurn = state.copy(
+            activeTurn = ActiveTurn(
+                id = a.turnId,
+                startedAt = a.startedAt,
+                message = a.message,
+                responseParts = emptyList(),
+                usage = null,
+            ),
+        )
+        val withStatus = withTurn.copy(
+            status = withStatusFlag(chatSummaryStatus(withTurn), SessionStatus.IS_READ, false),
+            modifiedAt = nowIsoString(),
+        )
+        if (a.queuedMessageId == null) {
+            withStatus
         } else {
-            val withTurn = state.copy(
-                activeTurn = ActiveTurn(
-                    id = a.turnId,
-                    startedAt = a.startedAt,
-                    message = a.message,
-                    responseParts = emptyList(),
-                    usage = null,
-                ),
-            )
-            val withStatus = withTurn.copy(
-                status = withStatusFlag(chatSummaryStatus(withTurn), SessionStatus.IS_READ, false),
-                modifiedAt = a.startedAt,
-            )
-            if (a.queuedMessageId == null) {
-                withStatus
-            } else {
-                var next = withStatus
-                if (next.steeringMessage?.id == a.queuedMessageId) {
-                    next = next.copy(steeringMessage = null)
-                }
-                val queued = next.queuedMessages
-                if (queued != null) {
-                    val filtered = queued.filter { it.id != a.queuedMessageId }
-                    next = next.copy(queuedMessages = filtered.ifEmpty { null })
-                }
-                next
+            var next = withStatus
+            if (next.steeringMessage?.id == a.queuedMessageId) {
+                next = next.copy(steeringMessage = null)
+            }
+            val queued = next.queuedMessages
+            if (queued != null) {
+                val filtered = queued.filter { it.id != a.queuedMessageId }
+                next = next.copy(queuedMessages = filtered.ifEmpty { null })
             }
+            next
         }
     }
 
@@ -812,13 +802,13 @@ public fun chatReducer(state: ChatState, action: StateAction): ChatState = when
     }
 
     is StateActionChatTurnComplete ->
-        endTurn(state, action.value.turnId, action.value.endedAt, TurnState.COMPLETE)
+        endTurn(state, action.value.turnId, action.value.duration, TurnState.COMPLETE)
 
     is StateActionChatTurnCancelled ->
-        endTurn(state, action.value.turnId, action.value.endedAt, TurnState.CANCELLED)
+        endTurn(state, action.value.turnId, action.value.duration, TurnState.CANCELLED)
 
     is StateActionChatError ->
-        endTurn(state, action.value.turnId, action.value.endedAt, TurnState.ERROR, SessionStatus.ERROR, action.value.error)
+        endTurn(state, action.value.turnId, action.value.duration, TurnState.ERROR, SessionStatus.ERROR, action.value.error)
 
     is StateActionChatActivityChanged ->
         state.copy(activity = action.value.activity)
diff --git a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt
index 657d9280..46101893 100644
--- a/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt
+++ b/clients/kotlin/src/main/kotlin/com/microsoft/agenthostprotocol/generated/Actions.generated.kt
@@ -614,9 +614,12 @@ data class ChatTurnCompleteAction(
      */
     val turnId: String,
     /**
-     * ISO 8601 timestamp when this turn completed.
+     * Elapsed turn duration in milliseconds, measured by the producer's own
+     * clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+     * client clocks may differ — and MUST treat it as opaque, producer-supplied
+     * data.
      */
-    val endedAt: String,
+    val duration: Long,
     /**
      * Additional provider-specific metadata for this action.
      *
@@ -638,9 +641,12 @@ data class ChatTurnCancelledAction(
      */
     val turnId: String,
     /**
-     * ISO 8601 timestamp when this turn was cancelled.
+     * Elapsed turn duration in milliseconds, measured by the producer's own
+     * clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+     * client clocks may differ — and MUST treat it as opaque, producer-supplied
+     * data.
      */
-    val endedAt: String,
+    val duration: Long,
     /**
      * Additional provider-specific metadata for this action.
      *
@@ -662,9 +668,12 @@ data class ChatErrorAction(
      */
     val turnId: String,
     /**
-     * ISO 8601 timestamp when this turn failed.
+     * Elapsed turn duration in milliseconds, measured by the producer's own
+     * clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+     * client clocks may differ — and MUST treat it as opaque, producer-supplied
+     * data.
      */
-    val endedAt: String,
+    val duration: Long,
     /**
      * Error details
      */
diff --git a/clients/kotlin/src/test/kotlin/com/microsoft/agenthostprotocol/ReducersTest.kt b/clients/kotlin/src/test/kotlin/com/microsoft/agenthostprotocol/ReducersTest.kt
index 111a693e..633193b6 100644
--- a/clients/kotlin/src/test/kotlin/com/microsoft/agenthostprotocol/ReducersTest.kt
+++ b/clients/kotlin/src/test/kotlin/com/microsoft/agenthostprotocol/ReducersTest.kt
@@ -209,7 +209,7 @@ class ReducersTest {
     }
 
     @Test
-    fun `turn start timestamp flows through to reducer outputs`() {
+    fun `turn start stores producer timestamp while modifiedAt uses local clock`() {
         val chatResult = chatReducer(
             newChat(),
             StateActionChatTurnStarted(
@@ -221,7 +221,8 @@ class ReducersTest {
                 ),
             ),
         )
-        assertEquals("1970-01-01T00:00:12.345Z", chatResult.modifiedAt)
+        assertEquals("1970-01-01T00:00:09.999Z", chatResult.modifiedAt)
+        assertEquals("1970-01-01T00:00:12.345Z", chatResult.activeTurn?.startedAt)
     }
 
     @Test
diff --git a/clients/rust/Cargo.lock b/clients/rust/Cargo.lock
index 7d8aaa1c..b3a0f2ab 100644
--- a/clients/rust/Cargo.lock
+++ b/clients/rust/Cargo.lock
@@ -8,7 +8,6 @@ version = "0.5.2"
 dependencies = [
  "ahp-types",
  "ahp-ws",
- "chrono",
  "serde",
  "serde_json",
  "thiserror",
@@ -46,12 +45,6 @@ version = "1.0.102"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f202df86484c868dbad7eaa557ef785d5c66295e41b460ef922eca0723b842c"
 
-[[package]]
-name = "autocfg"
-version = "1.5.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f2032f911046de80f0a198e0901378627c33f59ea0ac00e363d481118bd70a53"
-
 [[package]]
 name = "bitflags"
 version = "2.11.1"
@@ -89,15 +82,6 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 
-[[package]]
-name = "chrono"
-version = "0.4.45"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1aa79e62e7697b8e29b513a68abacf485adcd1fe8284a4316c5ae868e6633327"
-dependencies = [
- "num-traits",
-]
-
 [[package]]
 name = "core-foundation"
 version = "0.10.1"
@@ -559,15 +543,6 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
-[[package]]
-name = "num-traits"
-version = "0.2.19"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "071dfc062690e90b734c0b2273ce72ad0ffa95f0c74596bc250dcfd960262841"
-dependencies = [
- "autocfg",
-]
-
 [[package]]
 name = "once_cell"
 version = "1.21.4"
diff --git a/clients/rust/Cargo.toml b/clients/rust/Cargo.toml
index 1fdd4ba6..9c23c7b4 100644
--- a/clients/rust/Cargo.toml
+++ b/clients/rust/Cargo.toml
@@ -22,6 +22,5 @@ tokio = { version = "1", features = ["sync", "rt", "macros", "time"] }
 tokio-util = "0.7"
 futures-util = "0.3"
 tracing = "0.1"
-chrono = { version = "0.4", default-features = false, features = ["std"] }
 ahp-types = { path = "crates/ahp-types", version = "0.5.2" }
 ahp = { path = "crates/ahp", version = "0.5.2" }
diff --git a/clients/rust/crates/ahp-types/src/actions.rs b/clients/rust/crates/ahp-types/src/actions.rs
index 605ac97a..cd5b3e25 100644
--- a/clients/rust/crates/ahp-types/src/actions.rs
+++ b/clients/rust/crates/ahp-types/src/actions.rs
@@ -615,8 +615,11 @@ pub struct ChatToolCallContentChangedAction {
 pub struct ChatTurnCompleteAction {
     /// Turn identifier
     pub turn_id: String,
-    /// ISO 8601 timestamp when this turn completed.
-    pub ended_at: String,
+    /// Elapsed turn duration in milliseconds, measured by the producer's own
+    /// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+    /// client clocks may differ — and MUST treat it as opaque, producer-supplied
+    /// data.
+    pub duration: i64,
     /// Additional provider-specific metadata for this action.
     ///
     /// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -634,8 +637,11 @@ pub struct ChatTurnCompleteAction {
 pub struct ChatTurnCancelledAction {
     /// Turn identifier
     pub turn_id: String,
-    /// ISO 8601 timestamp when this turn was cancelled.
-    pub ended_at: String,
+    /// Elapsed turn duration in milliseconds, measured by the producer's own
+    /// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+    /// client clocks may differ — and MUST treat it as opaque, producer-supplied
+    /// data.
+    pub duration: i64,
     /// Additional provider-specific metadata for this action.
     ///
     /// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -653,8 +659,11 @@ pub struct ChatTurnCancelledAction {
 pub struct ChatErrorAction {
     /// Turn identifier
     pub turn_id: String,
-    /// ISO 8601 timestamp when this turn failed.
-    pub ended_at: String,
+    /// Elapsed turn duration in milliseconds, measured by the producer's own
+    /// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+    /// client clocks may differ — and MUST treat it as opaque, producer-supplied
+    /// data.
+    pub duration: i64,
     /// Error details
     pub error: ErrorInfo,
     /// Additional provider-specific metadata for this action.
diff --git a/clients/rust/crates/ahp/Cargo.toml b/clients/rust/crates/ahp/Cargo.toml
index 86aefd8e..4608dc74 100644
--- a/clients/rust/crates/ahp/Cargo.toml
+++ b/clients/rust/crates/ahp/Cargo.toml
@@ -23,7 +23,6 @@ serde_json = { workspace = true }
 thiserror = { workspace = true }
 tokio = { workspace = true }
 tracing = { workspace = true }
-chrono = { workspace = true }
 
 [dev-dependencies]
 tokio = { workspace = true, features = ["full"] }
diff --git a/clients/rust/crates/ahp/src/reducers.rs b/clients/rust/crates/ahp/src/reducers.rs
index 42a59a9d..fdf36a9a 100644
--- a/clients/rust/crates/ahp/src/reducers.rs
+++ b/clients/rust/crates/ahp/src/reducers.rs
@@ -68,7 +68,6 @@ use ahp_types::state::{
     ToolCallResponsePart, ToolCallRunningState, ToolCallState, ToolCallStreamingState, Turn,
     TurnState,
 };
-use chrono::{DateTime, FixedOffset};
 
 /// What happened when an action was applied.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -276,21 +275,14 @@ fn touch_chat_modified(state: &mut ChatState) {
     state.modified_at = now_iso();
 }
 
-fn parse_timestamp(value: &str) -> Option<DateTime<FixedOffset>> {
-    DateTime::parse_from_rfc3339(value).ok()
-}
-
 fn end_turn(
     state: &mut ChatState,
     turn_id: &str,
-    ended_at: &str,
+    duration: i64,
     turn_state: TurnState,
     terminal_status: Option<SessionStatus>,
     error: Option<ErrorInfo>,
 ) -> ReduceOutcome {
-    let Some(ended) = parse_timestamp(ended_at) else {
-        return ReduceOutcome::NoOp;
-    };
     let Some(active) = state.active_turn.as_ref() else {
         return ReduceOutcome::NoOp;
     };
@@ -353,16 +345,12 @@ fn end_turn(
         })
         .collect();
 
-    let duration = parse_timestamp(&active.started_at).and_then(|started| {
-        ended
-            .timestamp_millis()
-            .checked_sub(started.timestamp_millis())
-            .map(|duration| duration.max(0))
-    });
+    // Defensive clamp: `duration` is producer-supplied and opaque to this
+    // reducer, but a negative value would be nonsensical to display.
     let turn = Turn {
         id: active.id,
         started_at: Some(active.started_at),
-        duration,
+        duration: Some(duration.max(0)),
         message: active.message,
         response_parts,
         usage: active.usage,
@@ -372,7 +360,7 @@ fn end_turn(
 
     state.turns.push(turn);
     state.input_requests = None;
-    state.modified_at = ended_at.to_owned();
+    state.modified_at = now_iso();
     state.status = summary_status(state, terminal_status);
     ReduceOutcome::Applied
 }
@@ -894,7 +882,7 @@ pub fn apply_action_to_chat(state: &mut ChatState, action: &StateAction) -> Redu
         StateAction::ChatTurnComplete(a) => end_turn(
             state,
             &a.turn_id,
-            &a.ended_at,
+            a.duration,
             TurnState::Complete,
             None,
             None,
@@ -902,7 +890,7 @@ pub fn apply_action_to_chat(state: &mut ChatState, action: &StateAction) -> Redu
         StateAction::ChatTurnCancelled(a) => end_turn(
             state,
             &a.turn_id,
-            &a.ended_at,
+            a.duration,
             TurnState::Cancelled,
             None,
             None,
@@ -910,7 +898,7 @@ pub fn apply_action_to_chat(state: &mut ChatState, action: &StateAction) -> Redu
         StateAction::ChatError(a) => end_turn(
             state,
             &a.turn_id,
-            &a.ended_at,
+            a.duration,
             TurnState::Error,
             Some(SessionStatus::Error),
             Some(a.error.clone()),
@@ -1123,9 +1111,6 @@ pub fn apply_action_to_chat(state: &mut ChatState, action: &StateAction) -> Redu
 }
 
 fn apply_turn_started(state: &mut ChatState, a: &ChatTurnStartedAction) -> ReduceOutcome {
-    if parse_timestamp(&a.started_at).is_none() {
-        return ReduceOutcome::NoOp;
-    }
     state.active_turn = Some(ActiveTurn {
         id: a.turn_id.clone(),
         started_at: a.started_at.clone(),
@@ -1134,7 +1119,7 @@ fn apply_turn_started(state: &mut ChatState, a: &ChatTurnStartedAction) -> Reduc
         usage: None,
     });
     state.status = summary_status(state, None);
-    state.modified_at = a.started_at.clone();
+    touch_chat_modified(state);
     state.status = with_status_flag(state.status, SessionStatus::IsRead, false);
 
     if let Some(qmid) = &a.queued_message_id {
@@ -1851,7 +1836,7 @@ mod tests {
         s.status = SessionStatus::InProgress.bits();
         let a = StateAction::ChatTurnComplete(ahp_types::actions::ChatTurnCompleteAction {
             turn_id: "t1".into(),
-            ended_at: "2026-07-09T20:00:01.000Z".into(),
+            duration: 1000,
             meta: None,
         });
         assert_eq!(apply_action_to_chat(&mut s, &a), ReduceOutcome::Applied);
@@ -1921,7 +1906,7 @@ mod tests {
         // A chat-scoped action is out of scope for the session reducer.
         let turn = StateAction::ChatTurnComplete(ahp_types::actions::ChatTurnCompleteAction {
             turn_id: "t1".into(),
-            ended_at: "2026-07-09T20:00:01.000Z".into(),
+            duration: 1000,
             meta: None,
         });
         assert_eq!(
diff --git a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift
index 516f7a97..be9c894b 100644
--- a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift
+++ b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Generated/Actions.generated.swift
@@ -716,8 +716,11 @@ public struct ChatTurnCompleteAction: Codable, Sendable {
     public var type: ActionType
     /// Turn identifier
     public var turnId: String
-    /// ISO 8601 timestamp when this turn completed.
-    public var endedAt: String
+    /// Elapsed turn duration in milliseconds, measured by the producer's own
+    /// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+    /// client clocks may differ — and MUST treat it as opaque, producer-supplied
+    /// data.
+    public var duration: Int
     /// Additional provider-specific metadata for this action.
     ///
     /// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -730,19 +733,19 @@ public struct ChatTurnCompleteAction: Codable, Sendable {
     enum CodingKeys: String, CodingKey {
         case type
         case turnId
-        case endedAt
+        case duration
         case meta = "_meta"
     }
 
     public init(
         type: ActionType,
         turnId: String,
-        endedAt: String,
+        duration: Int,
         meta: [String: AnyCodable]? = nil
     ) {
         self.type = type
         self.turnId = turnId
-        self.endedAt = endedAt
+        self.duration = duration
         self.meta = meta
     }
 }
@@ -751,8 +754,11 @@ public struct ChatTurnCancelledAction: Codable, Sendable {
     public var type: ActionType
     /// Turn identifier
     public var turnId: String
-    /// ISO 8601 timestamp when this turn was cancelled.
-    public var endedAt: String
+    /// Elapsed turn duration in milliseconds, measured by the producer's own
+    /// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+    /// client clocks may differ — and MUST treat it as opaque, producer-supplied
+    /// data.
+    public var duration: Int
     /// Additional provider-specific metadata for this action.
     ///
     /// Clients MAY look for well-known keys here to provide enhanced UI, and
@@ -765,19 +771,19 @@ public struct ChatTurnCancelledAction: Codable, Sendable {
     enum CodingKeys: String, CodingKey {
         case type
         case turnId
-        case endedAt
+        case duration
         case meta = "_meta"
     }
 
     public init(
         type: ActionType,
         turnId: String,
-        endedAt: String,
+        duration: Int,
         meta: [String: AnyCodable]? = nil
     ) {
         self.type = type
         self.turnId = turnId
-        self.endedAt = endedAt
+        self.duration = duration
         self.meta = meta
     }
 }
@@ -786,8 +792,11 @@ public struct ChatErrorAction: Codable, Sendable {
     public var type: ActionType
     /// Turn identifier
     public var turnId: String
-    /// ISO 8601 timestamp when this turn failed.
-    public var endedAt: String
+    /// Elapsed turn duration in milliseconds, measured by the producer's own
+    /// clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+    /// client clocks may differ — and MUST treat it as opaque, producer-supplied
+    /// data.
+    public var duration: Int
     /// Error details
     public var error: ErrorInfo
     /// Additional provider-specific metadata for this action.
@@ -802,7 +811,7 @@ public struct ChatErrorAction: Codable, Sendable {
     enum CodingKeys: String, CodingKey {
         case type
         case turnId
-        case endedAt
+        case duration
         case error
         case meta = "_meta"
     }
@@ -810,13 +819,13 @@ public struct ChatErrorAction: Codable, Sendable {
     public init(
         type: ActionType,
         turnId: String,
-        endedAt: String,
+        duration: Int,
         error: ErrorInfo,
         meta: [String: AnyCodable]? = nil
     ) {
         self.type = type
         self.turnId = turnId
-        self.endedAt = endedAt
+        self.duration = duration
         self.error = error
         self.meta = meta
     }
diff --git a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
index b1eb3c43..bc7a4028 100644
--- a/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
+++ b/clients/swift/AgentHostProtocol/Sources/AgentHostProtocol/Reducers.swift
@@ -17,34 +17,6 @@ private let iso8601TimestampFormatter: ISO8601DateFormatter = {
     return formatter
 }()
 
-private let iso8601TimestampFormatterWithoutFractionalSeconds: ISO8601DateFormatter = {
-    let formatter = ISO8601DateFormatter()
-    formatter.formatOptions = [.withInternetDateTime]
-    return formatter
-}()
-
-private func parseTimestamp(_ value: String) -> Date? {
-    iso8601TimestampFormatter.date(from: value)
-        ?? iso8601TimestampFormatterWithoutFractionalSeconds.date(from: value)
-}
-
-private func parseTimestampMilliseconds(_ value: String) -> Int? {
-    guard parseTimestamp(value) != nil else { return nil }
-
-    var normalized = value
-    if let decimal = value.firstIndex(of: "."),
-       let timezone = value[decimal...].firstIndex(where: { $0 == "Z" || $0 == "+" || $0 == "-" }) {
-        let fractionStart = value.index(after: decimal)
-        let fraction = value[fractionStart..<timezone]
-        let milliseconds = String(fraction.prefix(3))
-            .padding(toLength: 3, withPad: "0", startingAt: 0)
-        normalized = String(value[..<fractionStart]) + milliseconds + String(value[timezone...])
-    }
-
-    guard let date = parseTimestamp(normalized) else { return nil }
-    return Int((date.timeIntervalSince1970 * 1000).rounded())
-}
-
 // MARK: - Status Bitset Helpers
 
 /// Bitmask covering the mutually-exclusive activity bits (bits 0–4).
@@ -129,9 +101,8 @@ public func chatReducer(state: ChatState, action: StateAction) -> ChatState {
     // ── Turn Lifecycle ────────────────────────────────────────────────────
 
     case .chatTurnStarted(let a):
-        guard parseTimestampMilliseconds(a.startedAt) != nil else { return state }
         var next = state
-        next.modifiedAt = a.startedAt
+        next.modifiedAt = currentTimestamp()
         next.activeTurn = ActiveTurn(
             id: a.turnId,
             startedAt: a.startedAt,
@@ -168,13 +139,13 @@ public func chatReducer(state: ChatState, action: StateAction) -> ChatState {
         return next
 
     case .chatTurnComplete(let a):
-        return endTurn(state: state, turnId: a.turnId, endedAt: a.endedAt, turnState: .complete)
+        return endTurn(state: state, turnId: a.turnId, duration: a.duration, turnState: .complete)
 
     case .chatTurnCancelled(let a):
-        return endTurn(state: state, turnId: a.turnId, endedAt: a.endedAt, turnState: .cancelled)
+        return endTurn(state: state, turnId: a.turnId, duration: a.duration, turnState: .cancelled)
 
     case .chatError(let a):
-        return endTurn(state: state, turnId: a.turnId, endedAt: a.endedAt, turnState: .error, terminalStatus: .error, error: a.error)
+        return endTurn(state: state, turnId: a.turnId, duration: a.duration, turnState: .error, terminalStatus: .error, error: a.error)
 
     case .chatActivityChanged(let a):
         var next = state
@@ -917,12 +888,11 @@ private func upsertInputRequest(state: ChatState, request: ChatInputRequest) ->
 private func endTurn(
     state: ChatState,
     turnId: String,
-    endedAt: String,
+    duration: Int,
     turnState: TurnState,
     terminalStatus: SessionStatus? = nil,
     error: ErrorInfo? = nil
 ) -> ChatState {
-    guard let endedMilliseconds = parseTimestampMilliseconds(endedAt) else { return state }
     guard let activeTurn = state.activeTurn, activeTurn.id == turnId else {
         return state
     }
@@ -972,12 +942,12 @@ private func endTurn(
         }
     }
 
-    let duration = parseTimestampMilliseconds(activeTurn.startedAt)
-        .map { max(0, endedMilliseconds - $0) }
+    // Defensive clamp: `duration` is producer-supplied and opaque to this
+    // reducer, but a negative value would be nonsensical to display.
     let turn = Turn(
         id: activeTurn.id,
         startedAt: activeTurn.startedAt,
-        duration: duration,
+        duration: max(0, duration),
         message: activeTurn.message,
         responseParts: responseParts,
         usage: activeTurn.usage,
@@ -990,7 +960,7 @@ private func endTurn(
     next.activeTurn = nil
     next.inputRequests = nil
     next.status = chatSummaryStatus(next, terminalStatus: terminalStatus)
-    next.modifiedAt = endedAt
+    next.modifiedAt = currentTimestamp()
     return next
 }
 
diff --git a/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift b/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift
index 4d910ee2..b234db36 100644
--- a/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift
+++ b/clients/swift/AgentHostProtocol/Tests/AgentHostProtocolTests/ReducersTests.swift
@@ -103,6 +103,10 @@ final class ReducersTests: XCTestCase {
     // MARK: - Timestamp Behavior
 
     func testChatTurnStartedUpdatesModifiedAt() {
+        let originalProvider = currentTimestampProvider
+        currentTimestampProvider = { 9_999 }
+        defer { currentTimestampProvider = originalProvider }
+
         let state = ChatState(
             resource: C,
             title: "Test Chat",
@@ -117,7 +121,8 @@ final class ReducersTests: XCTestCase {
                 message: Message(text: "Hello", origin: MessageOrigin(kind: .user))
             ))
         )
-        XCTAssertEqual(next.modifiedAt, "2026-07-09T20:00:00.000Z")
+        XCTAssertEqual(next.modifiedAt, "1970-01-01T00:00:09.999Z")
+        XCTAssertEqual(next.activeTurn?.startedAt, "2026-07-09T20:00:00.000Z")
     }
 
     func testTitleChangedUpdatesTitle() {
diff --git a/docs/.changes/20260709-turn-timing.json b/docs/.changes/20260709-turn-timing.json
index 43d248e7..e8e37dea 100644
--- a/docs/.changes/20260709-turn-timing.json
+++ b/docs/.changes/20260709-turn-timing.json
@@ -1,4 +1,4 @@
 {
   "type": "added",
-  "message": "Turn lifecycle actions now carry timestamps, and completed turns expose their start time and duration."
+  "message": "`chat/turnStarted` carries a `startedAt` timestamp, `chat/turnComplete`/`chat/turnCancelled`/`chat/error` carry an elapsed `duration` (milliseconds, producer's own clock), and completed turns expose their start time and duration."
 }
diff --git a/schema/actions.schema.json b/schema/actions.schema.json
index d8e84ef1..26162e44 100644
--- a/schema/actions.schema.json
+++ b/schema/actions.schema.json
@@ -1094,9 +1094,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn completed."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "_meta": {
           "type": "object",
@@ -1107,7 +1107,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt"
+        "duration"
       ]
     },
     "ChatTurnCancelledAction": {
@@ -1121,9 +1121,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn was cancelled."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "_meta": {
           "type": "object",
@@ -1134,7 +1134,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt"
+        "duration"
       ]
     },
     "ChatErrorAction": {
@@ -1148,9 +1148,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn failed."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "error": {
           "$ref": "#/$defs/ErrorInfo",
@@ -1165,7 +1165,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt",
+        "duration",
         "error"
       ]
     },
diff --git a/schema/commands.schema.json b/schema/commands.schema.json
index 4eeeb18f..838c6cf5 100644
--- a/schema/commands.schema.json
+++ b/schema/commands.schema.json
@@ -6940,9 +6940,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn completed."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "_meta": {
           "type": "object",
@@ -6953,7 +6953,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt"
+        "duration"
       ]
     },
     "ChatTurnCancelledAction": {
@@ -6967,9 +6967,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn was cancelled."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "_meta": {
           "type": "object",
@@ -6980,7 +6980,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt"
+        "duration"
       ]
     },
     "ChatErrorAction": {
@@ -6994,9 +6994,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn failed."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "error": {
           "$ref": "#/$defs/ErrorInfo",
@@ -7011,7 +7011,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt",
+        "duration",
         "error"
       ]
     },
diff --git a/schema/errors.schema.json b/schema/errors.schema.json
index 36f341e0..7491372f 100644
--- a/schema/errors.schema.json
+++ b/schema/errors.schema.json
@@ -7763,9 +7763,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn completed."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "_meta": {
           "type": "object",
@@ -7776,7 +7776,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt"
+        "duration"
       ]
     },
     "ChatTurnCancelledAction": {
@@ -7790,9 +7790,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn was cancelled."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "_meta": {
           "type": "object",
@@ -7803,7 +7803,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt"
+        "duration"
       ]
     },
     "ChatErrorAction": {
@@ -7817,9 +7817,9 @@
           "type": "string",
           "description": "Turn identifier"
         },
-        "endedAt": {
-          "type": "string",
-          "description": "ISO 8601 timestamp when this turn failed."
+        "duration": {
+          "type": "number",
+          "description": "Elapsed turn duration in milliseconds, measured by the producer's own\nclock. Clients MUST NOT derive this by subtracting timestamps — cross-\nclient clocks may differ — and MUST treat it as opaque, producer-supplied\ndata."
         },
         "error": {
           "$ref": "#/$defs/ErrorInfo",
@@ -7834,7 +7834,7 @@
       "required": [
         "type",
         "turnId",
-        "endedAt",
+        "duration",
         "error"
       ]
     },
diff --git a/types/channels-chat/actions.ts b/types/channels-chat/actions.ts
index 4ad13f5c..c1f5e663 100644
--- a/types/channels-chat/actions.ts
+++ b/types/channels-chat/actions.ts
@@ -345,8 +345,13 @@ export interface ChatTurnCompleteAction {
   type: ActionType.ChatTurnComplete;
   /** Turn identifier */
   turnId: string;
-  /** ISO 8601 timestamp when this turn completed. */
-  endedAt: string;
+  /**
+   * Elapsed turn duration in milliseconds, measured by the producer's own
+   * clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+   * client clocks may differ — and MUST treat it as opaque, producer-supplied
+   * data.
+   */
+  duration: number;
   /**
    * Additional provider-specific metadata for this action.
    *
@@ -370,8 +375,13 @@ export interface ChatTurnCancelledAction {
   type: ActionType.ChatTurnCancelled;
   /** Turn identifier */
   turnId: string;
-  /** ISO 8601 timestamp when this turn was cancelled. */
-  endedAt: string;
+  /**
+   * Elapsed turn duration in milliseconds, measured by the producer's own
+   * clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+   * client clocks may differ — and MUST treat it as opaque, producer-supplied
+   * data.
+   */
+  duration: number;
   /**
    * Additional provider-specific metadata for this action.
    *
@@ -394,8 +404,13 @@ export interface ChatErrorAction {
   type: ActionType.ChatError;
   /** Turn identifier */
   turnId: string;
-  /** ISO 8601 timestamp when this turn failed. */
-  endedAt: string;
+  /**
+   * Elapsed turn duration in milliseconds, measured by the producer's own
+   * clock. Clients MUST NOT derive this by subtracting timestamps — cross-
+   * client clocks may differ — and MUST treat it as opaque, producer-supplied
+   * data.
+   */
+  duration: number;
   /** Error details */
   error: ErrorInfo;
   /**
diff --git a/types/channels-chat/reducer.ts b/types/channels-chat/reducer.ts
index ab8dff9c..1da23f9a 100644
--- a/types/channels-chat/reducer.ts
+++ b/types/channels-chat/reducer.ts
@@ -117,7 +117,7 @@ function endTurn(
   state: ChatState,
   turnId: string,
   turnState: TurnState,
-  endedAt: string,
+  duration: number,
   terminalStatus?: SessionStatus.Error,
   error?: { errorType: string; message: string; stack?: string },
 ): ChatState {
@@ -147,15 +147,12 @@ function endTurn(
     };
   });
 
-  const startedAt = Date.parse(active.startedAt);
-  const completedAt = Date.parse(endedAt);
-  const duration = Number.isFinite(startedAt) && Number.isFinite(completedAt)
-    ? Math.max(0, completedAt - startedAt)
-    : undefined;
   const turn: Turn = {
     id: active.id,
     startedAt: active.startedAt,
-    duration,
+    // Defensive clamp: the duration is producer-supplied and opaque to this
+    // reducer, but a negative value would be nonsensical to display.
+    duration: Math.max(0, duration),
     message: active.message,
     responseParts,
     usage: active.usage,
@@ -167,7 +164,7 @@ function endTurn(
     ...state,
     turns: [...state.turns, turn],
     activeTurn: undefined,
-    modifiedAt: endedAt,
+    modifiedAt: new Date(Date.now()).toISOString(),
   };
   delete next.inputRequests;
   return {
@@ -272,36 +269,6 @@ function updateResponsePart(
 
 // ─── Chat Reducer ────────────────────────────────────────────────────────────
 
-function isValidTimestamp(value: string): boolean {
-  const match = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.\d{1,9})?(?:Z|[+-](\d{2}):(\d{2}))$/.exec(value);
-  if (!match) {
-    return false;
-  }
-
-  const [, yearText, monthText, dayText, hourText, minuteText, secondText, offsetHourText, offsetMinuteText] = match;
-  const year = Number(yearText);
-  const month = Number(monthText);
-  const day = Number(dayText);
-  const hour = Number(hourText);
-  const minute = Number(minuteText);
-  const second = Number(secondText);
-  const offsetHour = offsetHourText === undefined ? 0 : Number(offsetHourText);
-  const offsetMinute = offsetMinuteText === undefined ? 0 : Number(offsetMinuteText);
-  const leapYear = year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
-  const daysInMonth = [31, leapYear ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
-
-  return month >= 1
-    && month <= 12
-    && day >= 1
-    && day <= daysInMonth[month - 1]
-    && hour <= 23
-    && minute <= 59
-    && second <= 59
-    && offsetHour <= 23
-    && offsetMinute <= 59
-    && Number.isFinite(Date.parse(value));
-}
-
 /**
  * Pure reducer for chat state. Handles all {@link ChatAction} variants.
  */
@@ -310,10 +277,6 @@ export function chatReducer(state: ChatState, action: ChatAction, log?: (msg: st
     // ── Turn Lifecycle ────────────────────────────────────────────────────
 
     case ActionType.ChatTurnStarted: {
-      if (!isValidTimestamp(action.startedAt)) {
-        log?.(`Ignoring ChatTurnStarted with invalid startedAt: ${action.startedAt}`);
-        return state;
-      }
       let next: ChatState = {
         ...state,
         activeTurn: {
@@ -327,7 +290,7 @@ export function chatReducer(state: ChatState, action: ChatAction, log?: (msg: st
       next = {
         ...next,
         status: withStatusFlag(summaryStatus(next), SessionStatus.IsRead, false),
-        modifiedAt: action.startedAt,
+        modifiedAt: new Date(Date.now()).toISOString(),
       };
 
       // If this turn was auto-started from a pending message, remove it
@@ -365,25 +328,13 @@ export function chatReducer(state: ChatState, action: ChatAction, log?: (msg: st
       };
 
     case ActionType.ChatTurnComplete:
-      if (!isValidTimestamp(action.endedAt)) {
-        log?.(`Ignoring ChatTurnComplete with invalid endedAt: ${action.endedAt}`);
-        return state;
-      }
-      return endTurn(state, action.turnId, TurnState.Complete, action.endedAt);
+      return endTurn(state, action.turnId, TurnState.Complete, action.duration);
 
     case ActionType.ChatTurnCancelled:
-      if (!isValidTimestamp(action.endedAt)) {
-        log?.(`Ignoring ChatTurnCancelled with invalid endedAt: ${action.endedAt}`);
-        return state;
-      }
-      return endTurn(state, action.turnId, TurnState.Cancelled, action.endedAt);
+      return endTurn(state, action.turnId, TurnState.Cancelled, action.duration);
 
     case ActionType.ChatError:
-      if (!isValidTimestamp(action.endedAt)) {
-        log?.(`Ignoring ChatError with invalid endedAt: ${action.endedAt}`);
-        return state;
-      }
-      return endTurn(state, action.turnId, TurnState.Error, action.endedAt, SessionStatus.Error, action.error);
+      return endTurn(state, action.turnId, TurnState.Error, action.duration, SessionStatus.Error, action.error);
 
     case ActionType.ChatActivityChanged:
       return { ...state, activity: action.activity };
diff --git a/types/test-cases/reducers/005-session-turnstarted.json b/types/test-cases/reducers/005-session-turnstarted.json
index 0fb745ce..9b8300dd 100644
--- a/types/test-cases/reducers/005-session-turnstarted.json
+++ b/types/test-cases/reducers/005-session-turnstarted.json
@@ -38,6 +38,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
+    "modifiedAt": "1970-01-01T00:00:09.999Z"
   }
 }
diff --git a/types/test-cases/reducers/006-turnstarted-with-queuedmessageid-removes-from-queuedmessages.json b/types/test-cases/reducers/006-turnstarted-with-queuedmessageid-removes-from-queuedmessages.json
index 1977a91a..a63e36ad 100644
--- a/types/test-cases/reducers/006-turnstarted-with-queuedmessageid-removes-from-queuedmessages.json
+++ b/types/test-cases/reducers/006-turnstarted-with-queuedmessageid-removes-from-queuedmessages.json
@@ -70,6 +70,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
+    "modifiedAt": "1970-01-01T00:00:09.999Z"
   }
 }
diff --git a/types/test-cases/reducers/007-turnstarted-with-queuedmessageid-removes-last-queued-message.json b/types/test-cases/reducers/007-turnstarted-with-queuedmessageid-removes-last-queued-message.json
index faad3e50..8916d48d 100644
--- a/types/test-cases/reducers/007-turnstarted-with-queuedmessageid-removes-last-queued-message.json
+++ b/types/test-cases/reducers/007-turnstarted-with-queuedmessageid-removes-last-queued-message.json
@@ -51,6 +51,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
+    "modifiedAt": "1970-01-01T00:00:09.999Z"
   }
 }
diff --git a/types/test-cases/reducers/008-turnstarted-with-queuedmessageid-removes-matching-steering-message.json b/types/test-cases/reducers/008-turnstarted-with-queuedmessageid-removes-matching-steering-message.json
index 022860aa..23d591ef 100644
--- a/types/test-cases/reducers/008-turnstarted-with-queuedmessageid-removes-matching-steering-message.json
+++ b/types/test-cases/reducers/008-turnstarted-with-queuedmessageid-removes-matching-steering-message.json
@@ -49,6 +49,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
+    "modifiedAt": "1970-01-01T00:00:09.999Z"
   }
 }
diff --git a/types/test-cases/reducers/009-turnstarted-without-queuedmessageid-does-not-touch-pending-messages.json b/types/test-cases/reducers/009-turnstarted-without-queuedmessageid-does-not-touch-pending-messages.json
index 77bf2b7a..3775d8a4 100644
--- a/types/test-cases/reducers/009-turnstarted-without-queuedmessageid-does-not-touch-pending-messages.json
+++ b/types/test-cases/reducers/009-turnstarted-without-queuedmessageid-does-not-touch-pending-messages.json
@@ -78,6 +78,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
+    "modifiedAt": "1970-01-01T00:00:09.999Z"
   }
 }
diff --git a/types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json b/types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json
index a0cf3243..b2769329 100644
--- a/types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json
+++ b/types/test-cases/reducers/014-session-turncomplete-finalizes-turn.json
@@ -39,7 +39,7 @@
     {
       "type": "chat/turnComplete",
       "turnId": "turn-1",
-      "endedAt": "1970-01-01T00:00:09.999Z"
+      "duration": 8999
     }
   ],
   "expected": {
diff --git a/types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json b/types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json
index a5bb9a97..618aefb3 100644
--- a/types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json
+++ b/types/test-cases/reducers/015-session-turncancelled-finalizes-turn.json
@@ -24,7 +24,7 @@
     {
       "type": "chat/turnCancelled",
       "turnId": "turn-1",
-      "endedAt": "1970-01-01T00:00:09.999Z"
+      "duration": 8999
     }
   ],
   "expected": {
diff --git a/types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json b/types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json
index 5863a121..6a690e3f 100644
--- a/types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json
+++ b/types/test-cases/reducers/016-session-error-finalizes-turn-with-error.json
@@ -24,7 +24,7 @@
     {
       "type": "chat/error",
       "turnId": "turn-1",
-      "endedAt": "1970-01-01T00:00:09.999Z",
+      "duration": 8999,
       "error": {
         "errorType": "runtime",
         "message": "Something broke"
diff --git a/types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json b/types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json
index c1cec0e1..b56073aa 100644
--- a/types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json
+++ b/types/test-cases/reducers/017-turncomplete-force-cancels-in-progress-tool-calls.json
@@ -31,7 +31,7 @@
     {
       "type": "chat/turnComplete",
       "turnId": "turn-1",
-      "endedAt": "1970-01-01T00:00:09.999Z"
+      "duration": 8999
     }
   ],
   "expected": {
diff --git a/types/test-cases/reducers/018-turncomplete-with-wrong-turnid-is-no-op.json b/types/test-cases/reducers/018-turncomplete-with-wrong-turnid-is-no-op.json
index 5d15fb14..c95e01e2 100644
--- a/types/test-cases/reducers/018-turncomplete-with-wrong-turnid-is-no-op.json
+++ b/types/test-cases/reducers/018-turncomplete-with-wrong-turnid-is-no-op.json
@@ -24,7 +24,7 @@
     {
       "type": "chat/turnComplete",
       "turnId": "wrong-turn",
-      "endedAt": "1970-01-01T00:00:09.999Z"
+      "duration": 8999
     }
   ],
   "expected": {
diff --git a/types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json b/types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json
index 8343b1d4..95a3cbef 100644
--- a/types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json
+++ b/types/test-cases/reducers/070-full-turn-flow-with-tool-calls-and-re-confirmation.json
@@ -139,7 +139,7 @@
     {
       "type": "chat/turnComplete",
       "turnId": "t1",
-      "endedAt": "1970-01-01T00:00:09.999Z"
+      "duration": 8999
     }
   ],
   "expected": {
diff --git a/types/test-cases/reducers/075-turnstarted-clears-isread.json b/types/test-cases/reducers/075-turnstarted-clears-isread.json
index f7791425..fd06d1d9 100644
--- a/types/test-cases/reducers/075-turnstarted-clears-isread.json
+++ b/types/test-cases/reducers/075-turnstarted-clears-isread.json
@@ -38,6 +38,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 8,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
+    "modifiedAt": "1970-01-01T00:00:09.999Z"
   }
 }
diff --git a/types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json b/types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json
index 8e04a242..937e2c6f 100644
--- a/types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json
+++ b/types/test-cases/reducers/099-endturn-force-cancels-running-tool-call.json
@@ -38,7 +38,7 @@
     {
       "type": "chat/turnComplete",
       "turnId": "turn-1",
-      "endedAt": "1970-01-01T00:00:09.999Z"
+      "duration": 8999
     }
   ],
   "expected": {
diff --git a/types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json b/types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json
index 62ba2446..64de36e7 100644
--- a/types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json
+++ b/types/test-cases/reducers/107-session-input-turn-end-cleans-turn-scoped-only.json
@@ -42,7 +42,7 @@
     {
       "type": "chat/turnComplete",
       "turnId": "turn-1",
-      "endedAt": "1970-01-01T00:00:09.999Z"
+      "duration": 8999
     }
   ],
   "expected": {
diff --git a/types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json b/types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json
index fa43ea16..4d04fe63 100644
--- a/types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json
+++ b/types/test-cases/reducers/161-chat-turn-lifecycle-on-chat.json
@@ -41,7 +41,7 @@
     {
       "type": "chat/turnComplete",
       "turnId": "turn-1",
-      "endedAt": "1970-01-01T00:00:09.999Z"
+      "duration": 8999
     }
   ],
   "expected": {
diff --git a/types/test-cases/reducers/238-invalid-turn-start-timestamp-is-no-op.json b/types/test-cases/reducers/238-invalid-turn-start-timestamp-is-no-op.json
deleted file mode 100644
index bc6f6d62..00000000
--- a/types/test-cases/reducers/238-invalid-turn-start-timestamp-is-no-op.json
+++ /dev/null
@@ -1,31 +0,0 @@
-{
-  "description": "chat/turnStarted with an invalid timestamp is a no-op",
-  "reducer": "chat",
-  "initial": {
-    "turns": [],
-    "resource": "copilot:/test-session",
-    "title": "Test Session",
-    "status": 1,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
-  },
-  "actions": [
-    {
-      "type": "chat/turnStarted",
-      "turnId": "turn-1",
-      "startedAt": "July 9 2026 20:00:00",
-      "message": {
-        "text": "Hello",
-        "origin": {
-          "kind": "user"
-        }
-      }
-    }
-  ],
-  "expected": {
-    "turns": [],
-    "resource": "copilot:/test-session",
-    "title": "Test Session",
-    "status": 1,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
-  }
-}
diff --git a/types/test-cases/reducers/239-invalid-turn-end-timestamp-is-no-op.json b/types/test-cases/reducers/239-invalid-turn-end-timestamp-is-no-op.json
deleted file mode 100644
index 78d664f9..00000000
--- a/types/test-cases/reducers/239-invalid-turn-end-timestamp-is-no-op.json
+++ /dev/null
@@ -1,49 +0,0 @@
-{
-  "description": "chat/turnComplete with an invalid timestamp is a no-op",
-  "reducer": "chat",
-  "initial": {
-    "turns": [],
-    "activeTurn": {
-      "id": "turn-1",
-      "startedAt": "1970-01-01T00:00:01.000Z",
-      "message": {
-        "text": "Hello",
-        "origin": {
-          "kind": "user"
-        }
-      },
-      "responseParts": [],
-      "usage": null
-    },
-    "resource": "copilot:/test-session",
-    "title": "Test Session",
-    "status": 8,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
-  },
-  "actions": [
-    {
-      "type": "chat/turnComplete",
-      "turnId": "turn-1",
-      "endedAt": "2026-07-09T20:00:00.000"
-    }
-  ],
-  "expected": {
-    "turns": [],
-    "activeTurn": {
-      "id": "turn-1",
-      "startedAt": "1970-01-01T00:00:01.000Z",
-      "message": {
-        "text": "Hello",
-        "origin": {
-          "kind": "user"
-        }
-      },
-      "responseParts": [],
-      "usage": null
-    },
-    "resource": "copilot:/test-session",
-    "title": "Test Session",
-    "status": 8,
-    "modifiedAt": "1970-01-01T00:00:01.000Z"
-  }
-}
diff --git a/types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json b/types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json
index dbe67810..ea33eebe 100644
--- a/types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json
+++ b/types/test-cases/reducers/240-turn-duration-is-clamped-to-zero.json
@@ -24,7 +24,7 @@
     {
       "type": "chat/turnComplete",
       "turnId": "turn-1",
-      "endedAt": "1970-01-01T00:00:09.000Z"
+      "duration": -1000
     }
   ],
   "expected": {
@@ -49,6 +49,6 @@
     "resource": "copilot:/test-session",
     "title": "Test Session",
     "status": 1,
-    "modifiedAt": "1970-01-01T00:00:09.000Z"
+    "modifiedAt": "1970-01-01T00:00:09.999Z"
   }
 }