diff --git a/tests/test_meanwhile_turn_flow.py b/tests/test_meanwhile_turn_flow.py
index 0290b2b..9549813 100644
--- a/tests/test_meanwhile_turn_flow.py
+++ b/tests/test_meanwhile_turn_flow.py
@@ -570,3 +570,173 @@ def test_meanwhile_turn_registered_in_in_flight_tasks(
     # Post-flight: the entry has been cleaned up so the next turn (or
     # the cancel route) doesn't see a stale task.
     assert "chat_bot_a" not in _in_flight_tasks
+
+
+def test_meanwhile_turn_cancellation_via_route(app_state_setup, tmp_path):
+    """T85.2: a cancellation that fires while a meanwhile beat is
+    streaming truncates the assistant_turn and skips the post-turn
+    memory + state-update writes — the same end-to-end shape the
+    /turns/cancel route produces.
+
+    Drives the cancel by hijacking ``client.stream`` to raise
+    CancelledError on its first iteration — the exact pattern proven
+    by ``test_cancelled_turn_still_closes_scene_when_user_prose_signals_close``
+    in ``tests/test_turn_flow.py``. This mirrors what
+    ``cancel_turn`` does in production (``task.cancel()`` schedules a
+    CancelledError on the next await); doing the raise inline avoids
+    the TestClient-loop-reentry problem that prevents driving a second
+    POST mid-stream from the same synchronous test thread, while
+    exercising the same code path: the meanwhile streamer's
+    ``except asyncio.CancelledError`` block at meanwhile.py:276 sets
+    ``cancelled=True`` + ``truncated=True``, the assistant_turn lands
+    with the partial, and the memory/state-update branch is skipped.
+
+    The ``_in_flight_tasks`` registration that wires the cancel route
+    to the meanwhile streamer is independently pinned by
+    ``test_meanwhile_turn_registered_in_in_flight_tasks`` above; this
+    test pins the downstream behavioural shape the registration
+    enables — together they cover the full Stop-button lifecycle for
+    meanwhile beats.
+
+    Behavioural pins:
+
+    * ``assistant_turn`` lands with ``truncated=True``,
+      ``meanwhile_scene_id=2``, ``speaker_id="bot_a"``.
+    * No ``memory_written`` events fire (cancel skips per-bot writes).
+    * No post-turn ``edge_update`` events fire (cancel skips state updates).
+    * ``_in_flight_tasks`` is empty post-flight.
+    """
+    from typing import AsyncIterator, Sequence
+
+    from chat.llm.client import Message
+    from chat.web.turns import _in_flight_tasks
+
+    _seed_meanwhile_chat(tmp_path / "test.db")
+
+    class _CancelOnStreamMock(MockLLMClient):
+        """Yields CancelledError on first iteration of ``stream`` —
+        simulates ``cancel_turn`` having fired ``task.cancel()`` on the
+        in-flight streaming task. ``generate`` is delegated to the
+        canned-queue base so parse_turn still resolves cleanly.
+        """
+
+        async def stream(
+            self, messages: Sequence[Message], *, model: str, **params
+        ) -> AsyncIterator[str]:
+            raise asyncio.CancelledError
+            yield  # pragma: no cover — keeps this an async generator.
+
+    canned_parse = json.dumps(
+        {"segments": [{"kind": "narration", "text": "they exchange a glance"}]}
+    )
+    # Canned queue: only parse_turn — the narrative slot is never pulled
+    # because stream raises before consuming it, and post-turn
+    # state-update is skipped by the cancel branch.
+    mock = _CancelOnStreamMock(canned=[canned_parse])
+    from chat.web.kickoff import get_llm_client
+
+    app.dependency_overrides[get_llm_client] = lambda: mock
+    try:
+        # The meanwhile controller re-raises CancelledError after the
+        # partial assistant_turn is recorded (meanwhile.py:387). The
+        # outer post_turn route has no catch for CancelledError on the
+        # meanwhile path (turns.py:244-254 only catches ValueError), so
+        # the exception propagates up through Starlette. TestClient
+        # surfaces that as a 500 or a propagated exception depending on
+        # Starlette/asyncio versions; we don't pin the response.
+        try:
+            app_state_setup.post(
+                "/chats/chat_bot_a/turns",
+                data={"prose": "they exchange a glance"},
+            )
+        except BaseException:
+            pass
+    finally:
+        app.dependency_overrides.clear()
+
+    with open_db(tmp_path / "test.db") as conn:
+        assistant_rows = conn.execute(
+            "SELECT payload_json FROM event_log "
+            "WHERE kind = 'assistant_turn' ORDER BY id"
+        ).fetchall()
+        memory_count = conn.execute(
+            "SELECT COUNT(*) FROM event_log WHERE kind = 'memory_written'"
+        ).fetchone()[0]
+        # Edge updates AFTER the assistant_turn (i.e. excluding seeded ones).
+        max_at_row = conn.execute(
+            "SELECT MAX(id) FROM event_log WHERE kind = 'assistant_turn'"
+        ).fetchone()
+        max_at = max_at_row[0] if max_at_row[0] is not None else 0
+        post_turn_edge_updates = conn.execute(
+            "SELECT COUNT(*) FROM event_log "
+            "WHERE kind = 'edge_update' AND id > ?",
+            (max_at,),
+        ).fetchone()[0]
+
+    # The cancelled assistant_turn was still recorded with truncated=True,
+    # carrying whatever partial text accumulated before cancel propagated
+    # (zero text here since the cancel hits on the first iteration).
+    assert len(assistant_rows) == 1
+    payload = json.loads(assistant_rows[0][0])
+    assert payload["truncated"] is True, payload
+    assert payload["meanwhile_scene_id"] == 2
+    assert payload["speaker_id"] == "bot_a"
+
+    # No per-bot memory writes — cancellation short-circuits the memory
+    # + state-update branch (see chat/web/meanwhile.py:308).
+    assert memory_count == 0
+
+    # No post-turn edge_updates — same short-circuit.
+    assert post_turn_edge_updates == 0
+
+    # Post-flight: registry cleared so the cancel route won't try to
+    # re-cancel a defunct task on a follow-up POST.
+    assert "chat_bot_a" not in _in_flight_tasks
+
+
+def test_meanwhile_cancel_route_no_op_after_turn_completes(
+    app_state_setup, tmp_path
+):
+    """T85.2: POST ``/chats/<id>/turns/cancel`` AFTER a meanwhile turn
+    has fully completed is a silent 204 no-op — there is no in-flight
+    task to cancel, the registry is empty, and the route must not error.
+
+    Pins the cancel endpoint's robustness against the common-but-racy
+    sequence where the user clicks Stop just after the stream finished
+    (the SSE channel hasn't yet flipped the client-side ``isStreaming``
+    flag). This is a complement to the snapshot test: the snapshot test
+    pins that the registry IS populated mid-flight, this test pins that
+    it isn't AFTER and that the route copes gracefully.
+    """
+    from chat.web.turns import _in_flight_tasks
+
+    _seed_meanwhile_chat(tmp_path / "test.db")
+    canned_parse = json.dumps(
+        {"segments": [{"kind": "narration", "text": "they exchange a glance"}]}
+    )
+    canned = [
+        canned_parse,
+        "BotA leans in. *quietly*",
+        _zero_state(),
+        _zero_state(),
+    ]
+    mock = _override_llm(canned)
+    try:
+        response = app_state_setup.post(
+            "/chats/chat_bot_a/turns",
+            data={"prose": "they exchange a glance"},
+        )
+        assert response.status_code == 204
+    finally:
+        app.dependency_overrides.clear()
+    assert mock._canned == []
+
+    # Registry was cleaned up after the stream completed.
+    assert "chat_bot_a" not in _in_flight_tasks
+
+    # Cancel after-the-fact: 204, no error, registry stays empty.
+    cancel_response = app_state_setup.post(
+        "/chats/chat_bot_a/turns/cancel"
+    )
+    assert cancel_response.status_code == 204
+    assert "chat_bot_a" not in _in_flight_tasks