diff --git a/nerve/agent/engine.py b/nerve/agent/engine.py
index f48c948..2b6de13 100644
--- a/nerve/agent/engine.py
+++ b/nerve/agent/engine.py
@@ -1427,6 +1427,11 @@ async def run(
                 self.sessions.mark_running(session_id)
                 if channel is not None:
                     self._active_channel[session_id] = channel
+                # Mark the turn as in flight so the finally below can
+                # detect "ended without sending done/stopped/error" and
+                # ship a synthetic done.  Clearing happens automatically
+                # when a terminal event is broadcast.
+                broadcaster.mark_turn_open(session_id)
                 # Notify all connected clients that this session started running
                 await broadcaster.broadcast("__global__", {
                     "type": "session_running",
@@ -1442,6 +1447,28 @@ async def run(
                 finally:
                     self.sessions.mark_not_running(session_id)
                     self._active_channel.pop(session_id, None)
+                    # Backstop: if _run_inner exited without broadcasting
+                    # done/stopped/error (post-stream DB exception, hung
+                    # CLI cancelled by an outer mechanism, etc.), the
+                    # frontend never learned the turn ended and is still
+                    # showing "thinking..." even though the server has
+                    # cleared is_running.  Ship a synthetic done so the
+                    # streaming UI exits cleanly.
+                    if broadcaster.is_turn_open(session_id):
+                        logger.warning(
+                            "Session %s ended without a terminal event "
+                            "(done/stopped/error); sending synthetic done "
+                            "so the frontend exits streaming state",
+                            session_id,
+                        )
+                        try:
+                            await broadcaster.broadcast_done(session_id)
+                        except Exception as e:
+                            logger.warning(
+                                "Synthetic done broadcast failed for %s: %s",
+                                session_id, e,
+                            )
+                            broadcaster.clear_turn_open(session_id)
                     broadcaster.stop_buffering(session_id)
                     # Notify all connected clients that this session stopped
                     await broadcaster.broadcast("__global__", {
diff --git a/nerve/agent/streaming.py b/nerve/agent/streaming.py
index ca0fa3c..84f1202 100644
--- a/nerve/agent/streaming.py
+++ b/nerve/agent/streaming.py
@@ -50,6 +50,18 @@ def __init__(self, max_buffer_size: int = MAX_BUFFER_SIZE):
         # Per-session event buffer for reconnect replay
         self._session_buffers: dict[str, list[dict[str, Any]]] = {}
         self._max_buffer_size = max_buffer_size
+        # Per-session "turn is in flight" flag.  Set by mark_turn_open()
+        # at the start of a run, cleared automatically when broadcast()
+        # ships a terminal event ("done", "stopped", "error").  Used by
+        # the engine's run() finally as a backstop: if the flag is still
+        # set after _run_inner exits, no terminal event was sent (post-
+        # stream exception, hung CLI cancelled externally, etc.) and we
+        # need to ship a synthetic "done" so the frontend exits its
+        # streaming UI state.  Without this, the chat detail stays on
+        # "thinking..." forever even though the server has cleared
+        # is_running and the session card has dropped out of the
+        # "Running" sidebar group.
+        self._open_turns: set[str] = set()
 
     async def register(self, session_id: str, callback_id: str, callback: BroadcastCallback) -> None:
         """Register a broadcast listener for a session."""
@@ -72,6 +84,11 @@ async def unregister(self, session_id: str, callback_id: str) -> None:
 
     async def broadcast(self, session_id: str, message: dict[str, Any]) -> None:
         """Send a message to all listeners of a session. Also buffers if active."""
+        # Terminal events close the open-turn flag so the engine's
+        # backstop in run() knows no synthetic "done" is needed.
+        if message.get("type") in ("done", "stopped", "error"):
+            self._open_turns.discard(session_id)
+
         # Buffer events during active streaming
         if session_id in self._session_buffers:
             buf = self._session_buffers[session_id]
@@ -89,6 +106,21 @@ async def broadcast(self, session_id: str, message: dict[str, Any]) -> None:
             except Exception as e:
                 logger.warning("Broadcast to %s failed: %s", callback_id, e)
 
+    # --- Turn tracking (engine backstop) ---
+
+    def mark_turn_open(self, session_id: str) -> None:
+        """Mark a turn as in flight. Cleared when a terminal event is
+        broadcast (done/stopped/error) or via clear_turn_open()."""
+        self._open_turns.add(session_id)
+
+    def is_turn_open(self, session_id: str) -> bool:
+        """Whether a turn is in flight (no terminal event sent yet)."""
+        return session_id in self._open_turns
+
+    def clear_turn_open(self, session_id: str) -> None:
+        """Force-clear the open-turn flag without broadcasting."""
+        self._open_turns.discard(session_id)
+
     # --- Buffering for reconnect replay ---
 
     def start_buffering(self, session_id: str) -> None:
diff --git a/tests/test_streaming.py b/tests/test_streaming.py
index 34fa8a1..5499224 100644
--- a/tests/test_streaming.py
+++ b/tests/test_streaming.py
@@ -108,6 +108,72 @@ async def test_no_buffering_when_not_started(self):
         assert bc.get_buffer("s1") == []
 
 
+@pytest.mark.asyncio
+class TestOpenTurnTracking:
+    """The engine's run() finally relies on open-turn tracking to ship a
+    synthetic ``done`` when ``_run_inner`` exits without sending a
+    terminal event.  Without it, the chat detail stays on
+    "thinking..." forever even though the server has cleared
+    is_running and dropped the session out of the sidebar's "Running"
+    group.
+    """
+
+    async def test_mark_open_and_terminal_clears(self):
+        bc = StreamBroadcaster()
+        assert not bc.is_turn_open("s1")
+
+        bc.mark_turn_open("s1")
+        assert bc.is_turn_open("s1")
+
+        # Each terminal type should clear the flag
+        for terminal in ("done", "stopped", "error"):
+            bc.mark_turn_open("s1")
+            await bc.broadcast("s1", {"type": terminal})
+            assert not bc.is_turn_open("s1"), (
+                f"{terminal!r} should clear the open-turn flag"
+            )
+
+    async def test_non_terminal_does_not_clear(self):
+        bc = StreamBroadcaster()
+        bc.mark_turn_open("s1")
+
+        # Streaming events must not close the turn
+        for nonterminal in (
+            {"type": "token", "content": "hi"},
+            {"type": "thinking", "content": "..."},
+            {"type": "tool_use", "tool": "Bash"},
+            {"type": "tool_result", "tool_use_id": "x"},
+        ):
+            await bc.broadcast("s1", nonterminal)
+            assert bc.is_turn_open("s1"), (
+                f"{nonterminal['type']} should not clear the open-turn flag"
+            )
+
+    async def test_clear_turn_open_explicit(self):
+        bc = StreamBroadcaster()
+        bc.mark_turn_open("s1")
+        assert bc.is_turn_open("s1")
+        bc.clear_turn_open("s1")
+        assert not bc.is_turn_open("s1")
+
+    async def test_open_turns_isolated_per_session(self):
+        bc = StreamBroadcaster()
+        bc.mark_turn_open("s1")
+        bc.mark_turn_open("s2")
+        await bc.broadcast("s1", {"type": "done"})
+        assert not bc.is_turn_open("s1")
+        assert bc.is_turn_open("s2")
+
+    async def test_broadcast_done_helper_clears_open_turn(self):
+        # The engine backstop calls broadcaster.broadcast_done() to ship
+        # the synthetic terminal event.  Verify the helper closes the
+        # turn so a second call wouldn't double-broadcast.
+        bc = StreamBroadcaster()
+        bc.mark_turn_open("s1")
+        await bc.broadcast_done("s1", usage={"input_tokens": 1})
+        assert not bc.is_turn_open("s1")
+
+
 @pytest.mark.asyncio
 class TestBoundedBuffers:
     """Test that buffers are bounded to max_buffer_size."""
diff --git a/web/src/stores/handlers/sessionHandlers.ts b/web/src/stores/handlers/sessionHandlers.ts
index c0ed119..bdc9453 100644
--- a/web/src/stores/handlers/sessionHandlers.ts
+++ b/web/src/stores/handlers/sessionHandlers.ts
@@ -178,6 +178,32 @@ export function handleSessionRunning(
       updates.streamingBlocks = [];
       updates.agentStatus = { state: 'thinking' };
     }
+    // Defensive: server says the run ended but the frontend is still in
+    // streaming mode.  This happens when done/stopped/error never made
+    // it to the client (lost WS message during reconnect, post-stream
+    // exception on the server before broadcast_done fired, etc.).
+    // Without this branch the chat detail stays on "thinking..." while
+    // the sidebar entry has already dropped out of the "Running" group,
+    // which looks like the chat is stuck between steps.  The server's
+    // backstop in engine.run() ships a synthetic done in most of those
+    // cases; this is belt-and-suspenders for when even that signal is
+    // missed.
+    if (msg.session_id === s.activeSession && !msg.is_running && s.isStreaming) {
+      const finalBlocks = s.streamingBlocks.map(b =>
+        b.type === 'tool_call' && b.status === 'running'
+          ? { ...b, status: 'complete' as const }
+          : b,
+      );
+      if (finalBlocks.length > 0) {
+        updates.messages = [
+          ...s.messages,
+          { role: 'assistant' as const, blocks: finalBlocks },
+        ];
+      }
+      updates.streamingBlocks = [];
+      updates.isStreaming = false;
+      updates.agentStatus = { state: 'idle' };
+    }
     return updates;
   });
 }