diff --git a/SCHEMAS.md b/SCHEMAS.md
index 0009fc9..019281e 100644
--- a/SCHEMAS.md
+++ b/SCHEMAS.md
@@ -30,6 +30,19 @@ Every command response, success or error, carries:
 
 ---
 
+## Turn Result Fields (Multi-Turn Sessions)
+
+When a command's response includes a `turn` object (e.g., in `bootstrap` or `turn-loop`), it carries:
+
+| Field | Type | Required | Notes |
+|---|---|---|---|
+| `prompt` | string | Yes | User input for this turn |
+| `output` | string | Yes | Assistant response |
+| `stop_reason` | enum | Yes | One of: `completed`, `timeout`, `cancelled`, `max_budget_reached`, `max_turns_reached` |
+| `cancel_observed` | bool | Yes | #164 Stage B: cancellation was signaled and observed (#161/#164) |
+
+---
+
 ## Error Envelope
 
 When a command fails (exit code 1), responses carry:
diff --git a/src/main.py b/src/main.py
index bb3cec2..622c935 100644
--- a/src/main.py
+++ b/src/main.py
@@ -332,6 +332,7 @@ def main(argv: list[str] | None = None) -> int:
                     'prompt': session.turn_result.prompt,
                     'output': session.turn_result.output,
                     'stop_reason': session.turn_result.stop_reason,
+                    'cancel_observed': session.turn_result.cancel_observed,
                 },
                 'persisted_session_path': session.persisted_session_path,
             }
diff --git a/src/query_engine.py b/src/query_engine.py
index 987a98a..f503aae 100644
--- a/src/query_engine.py
+++ b/src/query_engine.py
@@ -31,6 +31,7 @@ class TurnResult:
     permission_denials: tuple[PermissionDenial, ...]
     usage: UsageSummary
     stop_reason: str
+    cancel_observed: bool = False
 
 
 @dataclass
diff --git a/src/runtime.py b/src/runtime.py
index ebeb011..bfb47e4 100644
--- a/src/runtime.py
+++ b/src/runtime.py
@@ -249,7 +249,10 @@ class PortRuntime:
                         # submitted yet this turn.
                         assert cancel_event is not None
                         cancel_event.set()
-                        results.append(self._build_timeout_result(turn_prompt, command_names, tool_names))
+                        results.append(self._build_timeout_result(
+                            turn_prompt, command_names, tool_names,
+                            cancel_observed=cancel_event.is_set()
+                        ))
                         break
                     assert executor is not None
                     future = executor.submit(
@@ -268,7 +271,10 @@ class PortRuntime:
                         assert cancel_event is not None
                         cancel_event.set()
                         future.cancel()
-                        results.append(self._build_timeout_result(turn_prompt, command_names, tool_names))
+                        results.append(self._build_timeout_result(
+                            turn_prompt, command_names, tool_names,
+                            cancel_observed=cancel_event.is_set()
+                        ))
                         break
 
                 results.append(result)
@@ -287,8 +293,11 @@ class PortRuntime:
         prompt: str,
         command_names: tuple[str, ...],
         tool_names: tuple[str, ...],
+        cancel_observed: bool = False,
     ) -> TurnResult:
-        """Synthesize a TurnResult representing a wall-clock timeout (#161)."""
+        """Synthesize a TurnResult representing a wall-clock timeout (#161).
+        #164 Stage B: cancel_observed signals cancellation event was set.
+        """
         return TurnResult(
             prompt=prompt,
             output='Wall-clock timeout exceeded before turn completed.',
@@ -297,6 +306,7 @@ class PortRuntime:
             permission_denials=(),
             usage=UsageSummary(),
             stop_reason='timeout',
+            cancel_observed=cancel_observed,
         )
 
     def _infer_permission_denials(self, matches: list[RoutedMatch]) -> list[PermissionDenial]: