fix: correct message ordering after task cancellation — v0.50.163 (#883)

fix: correct message ordering after task cancellation — v0.50.163 (#883)

Fixes the message-ordering glitch from #882: clicking Cancel while the
agent is responding could cause a subsequent response to render above
the "*Task cancelled.*" marker.

Root cause: the cancel handler pushed the marker only to local S.messages
without persisting to the server. When the done event fired shortly after
and replaced S.messages from server state, the marker disappeared from
client state while the next response anchored to the server-authoritative
position.

Fix has three parts:
- Server (cancel_stream): append *Task cancelled.* to session.messages
  with _error:True + timestamp, then save. _error ensures
  _sanitize_messages_for_api() strips it from conversation_history on
  the next agent turn, so the LLM never sees it as a prior assistant
  turn. Precedent: same flag used for the apperror marker at line 1343.
- Client (SSE cancel handler): fetch /api/session instead of pushing
  locally (same pattern as the done handler). Falls back to local push
  if the fetch fails.
- Tests: fix test window width for cancel handler (1200→dynamic); add
  two regression tests pinning _error flag and _sanitize invariant.

1941 tests passing.

Co-authored-by: piliang <piliang1@jd.com>

tests/test_sprint36.py

@@ -158,11 +158,13 @@ def test_sse_cancel_handler_still_present():
 def test_sse_cancel_handler_calls_set_busy():
     """The SSE cancel handler must still call setBusy(false)."""
     src = read("static/messages.js")
-    idx = src.find("addEventListener('cancel'") 
+    idx = src.find("addEventListener('cancel'")
     if idx == -1:
         idx = src.find('addEventListener("cancel"')
     assert idx != -1
-    block = src[idx:idx + 1200]
+    # Find the closing of this handler block (next top-level addEventListener)
+    next_handler = src.find("source.addEventListener(", idx + 50)
+    block = src[idx:next_handler] if next_handler != -1 else src[idx:idx + 3000]
     assert "setBusy(false)" in block, (
         "SSE cancel handler no longer calls setBusy(false)"
     )
@@ -180,3 +182,53 @@ def test_cancel_failed_i18n_key_exists_in_all_locales():
         f"cancel_failed key only found {count} times in i18n.js — "
         f"expected at least {locale_count} (one per locale)"
     )
+
+
+# ── 8. Server-persisted cancel marker doesn't leak into agent history ────────
+
+def test_cancel_marker_flagged_as_error_to_skip_in_api_history():
+    """The server-side cancel marker appended in cancel_stream() must carry
+    _error: True so _sanitize_messages_for_api() strips it from the
+    conversation_history sent to the agent on the next user message.
+
+    Without this flag, the LLM sees "*Task cancelled.*" as a prior assistant
+    turn and may reference it in subsequent responses ("As I mentioned, I was
+    cancelled...") — a behavioral regression introduced when this PR started
+    persisting the marker to the session.
+    """
+    src = read("api/streaming.py")
+    idx = src.find("'content': '*Task cancelled.*'")
+    if idx == -1:
+        idx = src.find('"content": "*Task cancelled.*"')
+    assert idx != -1, "cancel marker content string not found in cancel_stream()"
+
+    # Walk back to the start of the dict literal (opening brace)
+    brace_open = src.rfind("{", 0, idx)
+    brace_close = src.find("}", idx)
+    assert brace_open != -1 and brace_close != -1, "couldn't locate cancel marker dict"
+
+    marker_dict = src[brace_open:brace_close + 1]
+    assert "_error" in marker_dict and "True" in marker_dict, (
+        "cancel marker is missing _error: True — it will leak into the agent's "
+        "conversation_history via _sanitize_messages_for_api() on the next turn. "
+        "See line 591-593 of api/streaming.py for the error-marker filter."
+    )
+
+
+def test_sanitize_strips_error_flagged_assistant_messages():
+    """_sanitize_messages_for_api() must drop messages with _error: True —
+    this is the invariant the cancel marker's _error flag relies on."""
+    from api.streaming import _sanitize_messages_for_api
+    messages = [
+        {"role": "user", "content": "hello"},
+        {"role": "assistant", "content": "hi"},
+        {"role": "assistant", "content": "*Task cancelled.*", "_error": True},
+        {"role": "user", "content": "next"},
+    ]
+    sanitized = _sanitize_messages_for_api(messages)
+    assert len(sanitized) == 3, (
+        f"expected 3 messages (cancel marker stripped), got {len(sanitized)}: {sanitized}"
+    )
+    assert all("Task cancelled" not in (m.get("content") or "") for m in sanitized), (
+        "_sanitize_messages_for_api must filter cancel markers from API history"
+    )