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>

src/fabledassistant/services/journal_pipeline.py

@@ -134,7 +134,11 @@ async def build_journal_system_prompt(
     static_block = f"{JOURNAL_PERSONA}\n\n{JOURNAL_CALIBRATION}"
 
     today_iso = day_date.isoformat()
-    tz_block = f"Today is {today_iso} ({user_timezone})."
+    # Include the day-of-week explicitly. LLMs are unreliable at deriving
+    # weekday names from ISO dates, which causes "this Friday" / "next
+    # Monday" to land on the wrong calendar day.
+    weekday = day_date.strftime("%A")
+    tz_block = f"Today is {weekday}, {today_iso} ({user_timezone})."
 
     profile_context = await build_profile_context(user_id)
     profile_section = f"\n\n{profile_context}" if profile_context else ""

src/fabledassistant/services/llm.py

@@ -602,7 +602,12 @@ async def build_context(
             }
 
     assistant_name = await get_setting(user_id, "assistant_name", "Fable")
-    today = date_type.today().isoformat()
+    _today_obj = date_type.today()
+    today = _today_obj.isoformat()
+    # Day-of-week paired with the ISO date so the model doesn't have to
+    # derive the weekday — that derivation is a documented failure mode
+    # for "this Friday" / "next Monday"-style requests.
+    today_weekday = _today_obj.strftime("%A")
     has_caldav = await is_caldav_configured(user_id)
 
     # Build tool usage guidance based on available integrations
@@ -678,7 +683,7 @@ async def build_context(
     entities_context = await get_people_and_places_context(user_id)
     entities_section = f"\n\n{entities_context}" if entities_context else ""
 
-    dynamic_tail = f"\n\nToday's date is {today}.{tz_line}{profile_section}{entities_section}"
+    dynamic_tail = f"\n\nToday is {today_weekday}, {today}.{tz_line}{profile_section}{entities_section}"
 
     # --- System message: stable content only ---
     # Workspace context and history summary stay here because they carry

src/fabledassistant/services/tools/calendar.py

@@ -108,6 +108,34 @@ async def _resolve_event_end(
     return None
 
 
+def _validate_weekday(start_dt_utc: datetime, user_tz, expected: str | None) -> str | None:
+    """Verify the resolved local date falls on the expected day of the week.
+
+    Models routinely miscompute "this Friday" / "next Monday" when the
+    system prompt only carries an ISO date without a weekday. When the
+    model passes `expected_weekday`, the backend rejects mismatches with
+    a self-correcting error message naming the actual weekday.
+
+    Returns an error string on mismatch, or ``None`` when the check
+    passes (or no expected weekday was supplied).
+    """
+    if not expected:
+        return None
+    expected_norm = expected.strip().lower()
+    valid = {"monday", "tuesday", "wednesday", "thursday", "friday", "saturday", "sunday"}
+    if expected_norm not in valid:
+        return f"expected_weekday must be a full English weekday name; got {expected!r}."
+    local = start_dt_utc.astimezone(user_tz)
+    actual = local.strftime("%A").lower()
+    if actual == expected_norm:
+        return None
+    return (
+        f"Date {local.date().isoformat()} falls on {actual.title()}, "
+        f"not {expected_norm.title()}. Recompute the date for "
+        f"{expected_norm.title()} or confirm with the user before retrying."
+    )
+
+
 @tool(
     name="create_event",
     description=(
@@ -117,7 +145,11 @@ async def _resolve_event_end(
         "separate fields in the user's local time — never combine them and "
         "never include a timezone suffix. The server attaches the user's "
         "configured timezone. Omit `start_time` (or set `all_day=true`) "
-        "for all-day events like birthdays or holidays."
+        "for all-day events like birthdays or holidays. "
+        "When the user names a weekday ('this Friday', 'next Monday'), "
+        "state the resolved calendar date in your reply BEFORE calling "
+        "this tool, and pass `expected_weekday` so the server can verify "
+        "the date falls on the day you intended."
     ),
     parameters={
         "title": {"type": "string", "description": "A descriptive event title"},
@@ -135,6 +167,7 @@ async def _resolve_event_end(
         "attendees": {"type": "array", "items": {"type": "string"}, "description": "Optional list of attendee email addresses"},
         "calendar_name": {"type": "string", "description": "Optional calendar name to create the event in. Falls back to default calendar."},
         "project": {"type": "string", "description": "Optional project name to associate this event with"},
+        "expected_weekday": {"type": "string", "description": "Optional weekday name (e.g. 'friday') the start_date should fall on. Pass this whenever the user names a weekday so the server can verify the date is correct. Rejects with a corrective error if the date falls on a different day."},
         # Legacy combined fields kept for backward compatibility with saved
         # tool-call payloads in conversation history. New calls should use
         # start_date + start_time. Hidden from typical model output via the
@@ -154,6 +187,10 @@ async def create_event_tool(*, user_id, arguments, **_ctx):
         return {"success": False, "error": f"Invalid start: {exc}"}
     if start_was_date_only:
         all_day = True
+    user_tz = await get_user_tz(user_id)
+    weekday_err = _validate_weekday(start_dt, user_tz, arguments.get("expected_weekday"))
+    if weekday_err:
+        return {"success": False, "error": weekday_err}
     try:
         end_dt = await _resolve_event_end(user_id, arguments)
     except ValueError as exc:
@@ -254,7 +291,11 @@ async def search_events_tool(*, user_id, arguments, **_ctx):
         "change, move, reschedule, or modify an event. Pass `start_date` "
         "(YYYY-MM-DD) and `start_time` (HH:MM) as separate fields in the "
         "user's local time when rescheduling — never combine them, never "
-        "include a timezone suffix."
+        "include a timezone suffix. "
+        "When the user names a weekday ('move to Friday'), state the "
+        "resolved calendar date in your reply BEFORE calling this tool, "
+        "and pass `expected_weekday` so the server can verify the date "
+        "falls on the day you intended."
     ),
     parameters={
         "query": {"type": "string", "description": "Search term to find the event to update (matches against title)"},
@@ -269,6 +310,7 @@ async def search_events_tool(*, user_id, arguments, **_ctx):
         "color": {"type": "string", "description": "New hex color for the event (e.g. '#6366f1')"},
         "recurrence": {"type": "string", "description": "New iCalendar RRULE"},
         "reminder_minutes": {"type": "integer", "description": "Reminder N minutes before the event. Pass 0 to remove an existing reminder."},
+        "expected_weekday": {"type": "string", "description": "Optional weekday name (e.g. 'friday') the new start_date should fall on. Pass whenever the user names a weekday."},
         # Legacy combined fields kept for backcompat — see create_event.
         "start": {"type": "string", "description": "[Deprecated] Combined start datetime — prefer start_date + start_time."},
         "end": {"type": "string", "description": "[Deprecated] Combined end datetime — prefer end_date + end_time."},
@@ -296,6 +338,10 @@ async def update_event_tool(*, user_id, arguments, **_ctx):
             start_dt, _ = await _resolve_event_start(user_id, arguments)
         except (ValueError, TypeError) as exc:
             return {"success": False, "error": f"Invalid start: {exc}"}
+        user_tz = await get_user_tz(user_id)
+        weekday_err = _validate_weekday(start_dt, user_tz, arguments.get("expected_weekday"))
+        if weekday_err:
+            return {"success": False, "error": weekday_err}
         fields["start_dt"] = start_dt
     if arguments.get("end_date") or arguments.get("end"):
         try: