diff --git a/cmd/agentcli/prestage.go b/cmd/agentcli/prestage.go
index 5f79d84..42bcb76 100644
--- a/cmd/agentcli/prestage.go
+++ b/cmd/agentcli/prestage.go
@@ -154,7 +154,7 @@ func runPreStage(cfg cliConfig, messages []oai.Message, stderr io.Writer) ([]oai
 		}
 	}
 	prepMessages = append(prepMessages, applyTranscriptHygiene(normalizedIn, cfg.debug)...)
-	req := oai.ChatCompletionsRequest{
+    req := oai.ChatCompletionsRequest{
 		Model:    prepModel,
 		Messages: prepMessages,
 	}
@@ -168,7 +168,14 @@ func runPreStage(cfg cliConfig, messages []oai.Message, stderr io.Writer) ([]oai
 	} else if effectiveTemp != nil {
 		req.Temperature = effectiveTemp
 	}
-	// Create a dedicated client honoring pre-stage timeout and normal retry policy
+    // Enforce prompt to fit context window for pre-stage as well
+    window := oai.ContextWindowForModel(prepModel)
+    promptBudget := oai.PromptTokenBudget(window, 0)
+    if oai.EstimateTokens(req.Messages) > promptBudget {
+        req.Messages = oai.TrimMessagesToFit(req.Messages, promptBudget)
+    }
+
+    // Create a dedicated client honoring pre-stage timeout and normal retry policy
 	httpClient := oai.NewClientWithRetry(prepBaseURL, prepAPIKey, cfg.prepHTTPTimeout, oai.RetryPolicy{MaxRetries: retries, Backoff: backoff})
 	dumpJSONIfDebug(stderr, "prep.request", req, cfg.debug)
 	// Tag context with audit stage so HTTP audit lines include stage: "prep"
diff --git a/cmd/agentcli/run_agent.go b/cmd/agentcli/run_agent.go
index 519823a..4560099 100644
--- a/cmd/agentcli/run_agent.go
+++ b/cmd/agentcli/run_agent.go
@@ -179,7 +179,7 @@ func runAgent(cfg cliConfig, stdout io.Writer, stderr io.Writer) int {
 		for {
 			// Apply transcript hygiene before sending to the API when -debug is off
 			hygienic := applyTranscriptHygiene(messages, cfg.debug)
-			req := oai.ChatCompletionsRequest{
+            req := oai.ChatCompletionsRequest{
 				Model:    cfg.model,
 				Messages: hygienic,
 			}
@@ -203,11 +203,19 @@ func runAgent(cfg cliConfig, stdout io.Writer, stderr io.Writer) int {
 				req.ToolChoice = "auto"
 			}
 
-			// Include MaxTokens only when a positive completionCap is set.
+            // Include MaxTokens only when a positive completionCap is set.
 			if completionCap > 0 {
 				req.MaxTokens = completionCap
 			}
 
+            // Enforce prompt to fit the context window, leaving room for completionCap
+            // Compute prompt budget and trim deterministically when needed
+            window := oai.ContextWindowForModel(cfg.model)
+            promptBudget := oai.PromptTokenBudget(window, req.MaxTokens)
+            if oai.EstimateTokens(req.Messages) > promptBudget {
+                req.Messages = oai.TrimMessagesToFit(req.Messages, promptBudget)
+            }
+
 			// Pre-flight validate message sequence to avoid API 400s for stray tool messages
 			if err := oai.ValidateMessageSequence(req.Messages); err != nil {
 				safeFprintf(stderr, "error: %v\n", err)
@@ -276,7 +284,7 @@ func runAgent(cfg cliConfig, stdout io.Writer, stderr io.Writer) int {
 				callCtx, cancel = context.WithTimeout(context.Background(), cfg.httpTimeout)
 			}
 
-			// Fallback: non-streaming request
+            // Fallback: non-streaming request
 			resp, err := httpClient.CreateChatCompletion(callCtx, req)
 			cancel()
 			if err != nil {
diff --git a/internal/oai/context_window.go b/internal/oai/context_window.go
index 6e2dfd6..a8ccfa3 100644
--- a/internal/oai/context_window.go
+++ b/internal/oai/context_window.go
@@ -45,3 +45,14 @@ func ClampCompletionCap(messages []Message, requestedCap int, window int) int {
 	}
 	return requestedCap
 }
+
+// PromptTokenBudget returns a safe token budget for the prompt given a
+// model context window and a desired completion cap. A small safety margin
+// of 32 tokens is reserved for reply/control tokens.
+func PromptTokenBudget(window int, completionCap int) int {
+    budget := window - completionCap - 32
+    if budget < 1 {
+        return 1
+    }
+    return budget
+}
diff --git a/internal/oai/trim.go b/internal/oai/trim.go
new file mode 100644
index 0000000..ac8f736
--- /dev/null
+++ b/internal/oai/trim.go
@@ -0,0 +1,182 @@
+package oai
+
+// TrimMessagesToFit reduces a transcript so its estimated tokens do not exceed
+// the provided limit. Policy:
+// - Pin the first system and developer messages when present.
+// - Drop oldest non-pinned messages first until within limit.
+// - If only pinned remain and still exceed limit, truncate their content
+//   proportionally but keep both messages.
+// - As a last resort, keep only the newest message, truncated to fit.
+func TrimMessagesToFit(in []Message, limit int) []Message {
+    if limit <= 0 || len(in) == 0 {
+        return []Message{}
+    }
+    estimate := func(msgs []Message) int { return EstimateTokens(msgs) }
+
+    // Fast path: already fits
+    if estimate(in) <= limit {
+        return in
+    }
+
+    out := append([]Message(nil), in...)
+
+    // Drop oldest non-pinned messages until within limit.
+    for len(out) > 0 && estimate(out) > limit {
+        // Find first indices of pinned roles in current slice
+        sysIdx, devIdx := -1, -1
+        for i := range out {
+            if sysIdx == -1 && out[i].Role == RoleSystem {
+                sysIdx = i
+            }
+            if devIdx == -1 && out[i].Role == RoleDeveloper {
+                devIdx = i
+            }
+            if sysIdx != -1 && devIdx != -1 {
+                break
+            }
+        }
+        // Remove first non-pinned from the front if any
+        removed := false
+        for j := 0; j < len(out); j++ {
+            if j != sysIdx && j != devIdx {
+                out = append(out[:j], out[j+1:]...)
+                removed = true
+                break
+            }
+        }
+        if !removed {
+            // Only pinned remain; proceed to truncation
+            break
+        }
+    }
+
+    if estimate(out) <= limit {
+        return out
+    }
+
+    // Truncation path: only pinned remain or still too large
+    // Identify pinned indices in current slice
+    sysIdx, devIdx := -1, -1
+    for i := range out {
+        if sysIdx == -1 && out[i].Role == RoleSystem {
+            sysIdx = i
+        }
+        if devIdx == -1 && out[i].Role == RoleDeveloper {
+            devIdx = i
+        }
+    }
+
+    // If no pinned present, keep newest single message truncated to fit
+    if sysIdx == -1 && devIdx == -1 {
+        last := out[len(out)-1]
+        return []Message{truncateMessageToBudget(last, limit)}
+    }
+
+    cur := estimate(out)
+    if cur <= limit {
+        return out
+    }
+
+    // Compute budgets
+    if sysIdx != -1 && devIdx != -1 {
+        sysTok := EstimateTokens([]Message{out[sysIdx]})
+        devTok := EstimateTokens([]Message{out[devIdx]})
+        totalPinned := sysTok + devTok
+        if totalPinned == 0 {
+            totalPinned = 1
+        }
+        nonPinned := cur - totalPinned
+        targetPinned := limit - nonPinned
+        if targetPinned < 2 { // ensure at least 1 per pinned
+            targetPinned = 2
+        }
+        // Allocate at least 1 token to each, distribute remainder proportionally
+        minPerPinned := 1
+        remaining := targetPinned - 2*minPerPinned
+        if remaining < 0 {
+            remaining = 0
+        }
+        var extraSys, extraDev int
+        if sysTok+devTok > 0 && remaining > 0 {
+            extraSys = (sysTok * remaining) / (sysTok + devTok)
+            extraDev = remaining - extraSys
+        } else {
+            extraSys, extraDev = 0, 0
+        }
+        targetSys := minPerPinned + extraSys
+        targetDev := minPerPinned + extraDev
+        out[sysIdx] = truncateMessageToBudget(out[sysIdx], targetSys)
+        out[devIdx] = truncateMessageToBudget(out[devIdx], targetDev)
+    } else if sysIdx != -1 { // only system pinned
+        // allocate entire limit minus non-system tokens
+        nonSys := cur - EstimateTokens([]Message{out[sysIdx]})
+        budget := limit - nonSys
+        if budget < 1 {
+            budget = 1
+        }
+        out[sysIdx] = truncateMessageToBudget(out[sysIdx], budget)
+    } else if devIdx != -1 { // only developer pinned
+        nonDev := cur - EstimateTokens([]Message{out[devIdx]})
+        budget := limit - nonDev
+        if budget < 1 {
+            budget = 1
+        }
+        out[devIdx] = truncateMessageToBudget(out[devIdx], budget)
+    }
+
+    // Final guard: if still above limit, drop oldest non-pinned if any; otherwise truncate newest to fit
+    for estimate(out) > limit {
+        removed := false
+        // Try to remove a non-pinned from the front
+        // Recompute pinned indices
+        sysIdx, devIdx = -1, -1
+        for i := range out {
+            if sysIdx == -1 && out[i].Role == RoleSystem {
+                sysIdx = i
+            }
+            if devIdx == -1 && out[i].Role == RoleDeveloper {
+                devIdx = i
+            }
+        }
+        for j := 0; j < len(out); j++ {
+            if j != sysIdx && j != devIdx {
+                out = append(out[:j], out[j+1:]...)
+                removed = true
+                break
+            }
+        }
+        if !removed {
+            // No non-pinned remain; keep newest one truncated
+            last := out[len(out)-1]
+            out = []Message{truncateMessageToBudget(last, limit)}
+            break
+        }
+    }
+
+    return out
+}
+
+// truncateMessageToBudget returns a copy of msg with content truncated such that
+// the single-message token estimate is <= budget (best-effort heuristic).
+func truncateMessageToBudget(msg Message, budget int) Message {
+    if budget <= 1 {
+        msg.Content = ""
+        return msg
+    }
+    // Binary search on content length, using EstimateTokens heuristic
+    lo, hi := 0, len(msg.Content)
+    best := 0
+    for lo <= hi {
+        mid := (lo + hi) / 2
+        test := msg
+        test.Content = truncate(msg.Content, mid)
+        if EstimateTokens([]Message{test}) <= budget {
+            best = mid
+            lo = mid + 1
+        } else {
+            hi = mid - 1
+        }
+    }
+    msg.Content = truncate(msg.Content, best)
+    return msg
+}
diff --git a/internal/oai/trim_test.go b/internal/oai/trim_test.go
new file mode 100644
index 0000000..aa210dd
--- /dev/null
+++ b/internal/oai/trim_test.go
@@ -0,0 +1,85 @@
+package oai
+
+import "testing"
+
+// helper to build a message with role and content
+func m(role, content string) Message { return Message{Role: role, Content: content} }
+
+func TestTrimMessagesToFit_PreservesSystemAndDeveloper(t *testing.T) {
+    sys := m(RoleSystem, repeat("S", 4000))      // ~1000 tokens
+    dev := m(RoleDeveloper, repeat("D", 4000))   // ~1000 tokens
+    u1 := m(RoleUser, repeat("u", 4000))        // ~1000 tokens
+    a1 := m(RoleAssistant, repeat("a", 4000))   // ~1000 tokens
+    u2 := m(RoleUser, repeat("u", 4000))        // ~1000 tokens
+    in := []Message{sys, dev, u1, a1, u2}
+
+    // Limit so that we cannot keep all messages; must drop from the front (u1,a1)
+    limit := EstimateTokens(in) - 1500
+    out := TrimMessagesToFit(in, limit)
+
+    if EstimateTokens(out) > limit {
+        t.Fatalf("trim did not reduce to limit: got=%d limit=%d", EstimateTokens(out), limit)
+    }
+    if len(out) >= 2 {
+        if out[0].Role != RoleSystem {
+            t.Fatalf("first message should be system; got %q", out[0].Role)
+        }
+        if out[1].Role != RoleDeveloper {
+            t.Fatalf("second message should be developer; got %q", out[1].Role)
+        }
+    } else {
+        t.Fatalf("expected to preserve at least system and developer; got %d", len(out))
+    }
+}
+
+func TestTrimMessagesToFit_DropsOldestNonPinned(t *testing.T) {
+    sys := m(RoleSystem, "policy")
+    // 5 alternating user/assistant messages
+    msgs := []Message{sys}
+    for i := 0; i < 5; i++ {
+        msgs = append(msgs, m(RoleUser, repeat("U", 2000)))
+        msgs = append(msgs, m(RoleAssistant, repeat("A", 2000)))
+    }
+    // Force heavy trim
+    limit := EstimateTokens(msgs) / 2
+    out := TrimMessagesToFit(msgs, limit)
+    if EstimateTokens(out) > limit {
+        t.Fatalf("expected tokens <= limit; got=%d limit=%d", EstimateTokens(out), limit)
+    }
+    // Ensure the newest non-pinned message remains (the last assistant)
+    if out[len(out)-1].Role != RoleAssistant {
+        t.Fatalf("expected newest assistant at tail; got %q", out[len(out)-1].Role)
+    }
+}
+
+func TestTrimMessagesToFit_OnlySystemDeveloperTooLarge_TruncatesContent(t *testing.T) {
+    sys := m(RoleSystem, repeat("S", 20000))    // ~5000 tokens
+    dev := m(RoleDeveloper, repeat("D", 20000)) // ~5000 tokens
+    in := []Message{sys, dev}
+    limit := 3000 // far below combined estimate
+    out := TrimMessagesToFit(in, limit)
+    if EstimateTokens(out) > limit {
+        t.Fatalf("expected tokens <= limit after truncation; got=%d limit=%d", EstimateTokens(out), limit)
+    }
+    if len(out) != 2 {
+        t.Fatalf("should keep both system and developer; got %d", len(out))
+    }
+    if len(out[0].Content) >= len(sys.Content) {
+        t.Fatalf("system content was not truncated")
+    }
+    if len(out[1].Content) >= len(dev.Content) {
+        t.Fatalf("developer content was not truncated")
+    }
+}
+
+// repeat returns a string consisting of count repetitions of s.
+func repeat(s string, count int) string {
+    if count <= 0 {
+        return ""
+    }
+    b := make([]byte, 0, len(s)*count)
+    for i := 0; i < count; i++ {
+        b = append(b, s...)
+    }
+    return string(b)
+}