fix(calendar-tool): anchor today's weekday in prompts + verify expected_weekday on create/update

A user asked Fable to schedule "this Friday at 8am" on Wednesday 4/29
2026. The model picked 4/30 (Thursday) and confidently labeled it
"Friday." The TZ pipeline did everything correctly given the model's
date — the bug was upstream: the model was guessing weekdays from ISO
dates without an anchor, and the calendar tools had no way to verify.

Three layered fixes:

1. **System prompts now name the weekday alongside the ISO date.**
   Both the journal-conversation prompt and the general chat prompt
   used to say "Today is 2026-04-29 (America/New_York)." They now say
   "Today is Wednesday, 2026-04-29 (...)." LLMs are unreliable at
   deriving weekday names from ISO dates; supplying the name removes
   the guess.

2. **`expected_weekday` parameter on create_event / update_event.**
   When the model passes `expected_weekday="friday"`, the backend
   computes the resolved start_date's weekday in the user's local
   timezone and rejects mismatches with a self-correcting error
   ("Date 2026-04-30 falls on Thursday, not Friday. Recompute..."),
   without creating the event. The check is local-aware: a Friday
   23:00 event in Tokyo crosses midnight UTC but the local view
   stays Friday, and the validator respects that.

3. **Tool descriptions instruct echo-and-confirm.** create_event and
   update_event descriptions now tell the model: when the user names
   a weekday, state the resolved date in the reply BEFORE calling
   the tool, and pass `expected_weekday`. Costs nothing in code,
   reinforces the validator.

6 new tests — match success, mismatch rejection (with create/update
not invoked), omitted-param backcompat, invalid weekday name, local-
not-UTC weekday computation, and the update_event variant. All 18
calendar-tool tests + 33 event-related tests pass; ruff clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_calendar_tool_tz.py

@@ -395,3 +395,219 @@ async def test_update_event_split_fields_reschedule_no_drift():
     assert result["success"] is True
     utc = captured["start_dt"].astimezone(timezone.utc)
     assert (utc.year, utc.month, utc.day, utc.hour) == (2026, 5, 1, 12)
+
+
+# ── expected_weekday verification (catches "this Friday" → Thursday bugs) ────
+
+
+@pytest.mark.asyncio
+async def test_create_event_expected_weekday_match_succeeds():
+    """When expected_weekday agrees with the resolved local date's
+    weekday, the event is created normally."""
+    from fabledassistant.services.tools.calendar import create_event_tool
+
+    captured = {}
+
+    async def fake_create_event(**kwargs):
+        captured.update(kwargs)
+        event = AsyncMock()
+        event.to_dict.return_value = {"id": 1}
+        return event
+
+    with patch(
+        "fabledassistant.services.tools.calendar.get_user_tz",
+        AsyncMock(return_value=__import__("zoneinfo").ZoneInfo("America/New_York")),
+    ), patch(
+        "fabledassistant.services.tools.calendar.events_create_event",
+        side_effect=fake_create_event,
+    ):
+        result = await create_event_tool(
+            user_id=1,
+            arguments={
+                "title": "Meeting",
+                "start_date": "2026-05-01",  # is a Friday
+                "start_time": "08:00",
+                "expected_weekday": "friday",
+            },
+        )
+
+    assert result["success"] is True
+    assert captured["start_dt"] is not None
+
+
+@pytest.mark.asyncio
+async def test_create_event_expected_weekday_mismatch_rejects_and_names_actual():
+    """The reported failure mode: model picks Thursday and calls it
+    Friday. With expected_weekday set, the create is rejected and the
+    error names the actual weekday so the model can self-correct."""
+    from fabledassistant.services.tools.calendar import create_event_tool
+
+    create_called = False
+
+    async def fake_create_event(**kwargs):
+        nonlocal create_called
+        create_called = True
+        event = AsyncMock()
+        event.to_dict.return_value = {"id": 1}
+        return event
+
+    with patch(
+        "fabledassistant.services.tools.calendar.get_user_tz",
+        AsyncMock(return_value=__import__("zoneinfo").ZoneInfo("America/New_York")),
+    ), patch(
+        "fabledassistant.services.tools.calendar.events_create_event",
+        side_effect=fake_create_event,
+    ):
+        result = await create_event_tool(
+            user_id=1,
+            arguments={
+                "title": "Dentist",
+                "start_date": "2026-04-30",  # Thursday
+                "start_time": "08:00",
+                "expected_weekday": "friday",
+            },
+        )
+
+    assert result["success"] is False
+    assert "Thursday" in result["error"]
+    assert "Friday" in result["error"]
+    # Critical: the event must NOT have been created when the check failed.
+    assert create_called is False
+
+
+@pytest.mark.asyncio
+async def test_create_event_expected_weekday_omitted_skips_check():
+    """Backcompat: when expected_weekday isn't passed, no validation
+    runs — the existing create flow is unchanged."""
+    from fabledassistant.services.tools.calendar import create_event_tool
+
+    captured = {}
+
+    async def fake_create_event(**kwargs):
+        captured.update(kwargs)
+        event = AsyncMock()
+        event.to_dict.return_value = {"id": 1}
+        return event
+
+    with patch(
+        "fabledassistant.services.tools.calendar.get_user_tz",
+        AsyncMock(return_value=__import__("zoneinfo").ZoneInfo("America/New_York")),
+    ), patch(
+        "fabledassistant.services.tools.calendar.events_create_event",
+        side_effect=fake_create_event,
+    ):
+        result = await create_event_tool(
+            user_id=1,
+            arguments={
+                "title": "x",
+                "start_date": "2026-04-30",
+                "start_time": "08:00",
+            },
+        )
+
+    assert result["success"] is True
+
+
+@pytest.mark.asyncio
+async def test_create_event_expected_weekday_invalid_value_rejects():
+    """Garbage in expected_weekday produces a clear validation error,
+    not a silent pass."""
+    from fabledassistant.services.tools.calendar import create_event_tool
+
+    with patch(
+        "fabledassistant.services.tools.calendar.get_user_tz",
+        AsyncMock(return_value=__import__("zoneinfo").ZoneInfo("America/New_York")),
+    ):
+        result = await create_event_tool(
+            user_id=1,
+            arguments={
+                "title": "x",
+                "start_date": "2026-05-01",
+                "start_time": "08:00",
+                "expected_weekday": "fri",  # abbreviation not accepted
+            },
+        )
+
+    assert result["success"] is False
+    assert "weekday" in result["error"].lower()
+
+
+@pytest.mark.asyncio
+async def test_update_event_expected_weekday_mismatch_rejects():
+    """update_event must enforce the same weekday check on reschedules."""
+    from fabledassistant.services.tools.calendar import update_event_tool
+
+    update_called = False
+
+    async def fake_find(*, user_id, query):
+        ev = AsyncMock()
+        ev.id = 99
+        ev.title = "Coffee"
+        return [ev]
+
+    async def fake_update(*, user_id, event_id, **fields):
+        nonlocal update_called
+        update_called = True
+        ev = AsyncMock()
+        ev.to_dict.return_value = {"id": event_id}
+        return ev
+
+    with patch(
+        "fabledassistant.services.tools.calendar.get_user_tz",
+        AsyncMock(return_value=__import__("zoneinfo").ZoneInfo("America/New_York")),
+    ), patch(
+        "fabledassistant.services.tools.calendar.find_events_by_query",
+        side_effect=fake_find,
+    ), patch(
+        "fabledassistant.services.tools.calendar.events_update_event",
+        side_effect=fake_update,
+    ):
+        result = await update_event_tool(
+            user_id=1,
+            arguments={
+                "query": "Coffee",
+                "start_date": "2026-04-30",  # Thursday
+                "start_time": "08:00",
+                "expected_weekday": "friday",
+            },
+        )
+
+    assert result["success"] is False
+    assert "Thursday" in result["error"]
+    assert update_called is False
+
+
+@pytest.mark.asyncio
+async def test_create_event_weekday_check_uses_local_not_utc():
+    """The weekday check must use the LOCAL date, not the UTC date.
+    A late-evening Friday event in Tokyo (UTC+9) crosses midnight UTC,
+    so a UTC-day check would call it Saturday — but the user's calendar
+    says Friday. The check must respect the user's local view."""
+    from fabledassistant.services.tools.calendar import create_event_tool
+
+    captured = {}
+
+    async def fake_create_event(**kwargs):
+        captured.update(kwargs)
+        event = AsyncMock()
+        event.to_dict.return_value = {"id": 1}
+        return event
+
+    with patch(
+        "fabledassistant.services.tools.calendar.get_user_tz",
+        AsyncMock(return_value=__import__("zoneinfo").ZoneInfo("Asia/Tokyo")),
+    ), patch(
+        "fabledassistant.services.tools.calendar.events_create_event",
+        side_effect=fake_create_event,
+    ):
+        result = await create_event_tool(
+            user_id=1,
+            arguments={
+                "title": "Friday night",
+                "start_date": "2026-05-01",  # Friday in Tokyo
+                "start_time": "23:00",       # 14:00 UTC same day; safe
+                "expected_weekday": "friday",
+            },
+        )
+
+    assert result["success"] is True