fix(agent): drop the hour from the system-prompt date line (day-stable prompt-cache prefix)

backend/src/agents/main_agent/core/system_prompt_builder.py

@@ -190,6 +190,8 @@ def build(self, include_date: bool = True) -> str:
             str: Complete system prompt
         """
         if include_date:
+            # This line sits in the Bedrock prompt-cache prefix, so it must be
+            # byte-stable within a day: date + weekday + timezone only, no hour.
             current_date = get_current_date_pacific()
             prompt = f"{self.base_prompt}\n\nCurrent date: {current_date}"
             logger.info(f"Built system prompt with current date: {current_date}")

backend/src/agents/main_agent/utils/timezone.py

@@ -22,10 +22,21 @@
 
 def get_current_date_pacific() -> str:
     """
-    Get current date and hour in US Pacific timezone (America/Los_Angeles)
+    Get the current calendar date in US Pacific timezone (America/Los_Angeles).
+
+    The result is rendered into the system prompt, which is the head of the
+    Bedrock prompt-cache prefix (see the prompt-cache contract in CLAUDE.md).
+    It therefore deliberately contains **no hour**: the string is byte-stable
+    for a whole Pacific day, so the cached prefix is re-written once per day
+    instead of at every hour boundary. Earlier versions appended ``HH:00`` and
+    a 2026-09 prod cost audit attributed ~2.6% of cache-write spend to the
+    resulting hourly ``systemPromptHash`` flips. If a flow ever needs the
+    time of day, put it in the user turn (or another turn-scoped message),
+    never back in this prefix.
 
     Returns:
-        str: Formatted date string with timezone (e.g., "2024-01-15 (Monday) 14:00 PST")
+        str: Date, weekday and timezone abbreviation
+             (e.g., "2024-01-15 (Monday) PST")
     """
     try:
         if TIMEZONE_AVAILABLE:
@@ -44,12 +55,12 @@ def get_current_date_pacific() -> str:
                 # Get timezone abbreviation (PST/PDT)
                 tz_abbr = now.strftime("%Z")
 
-            return now.strftime(f"%Y-%m-%d (%A) %H:00 {tz_abbr}")
+            return now.strftime(f"%Y-%m-%d (%A) {tz_abbr}")
         else:
             # Fallback to UTC if no timezone library available
             now = datetime.now(timezone.utc)
-            return now.strftime("%Y-%m-%d (%A) %H:00 UTC")
+            return now.strftime("%Y-%m-%d (%A) UTC")
     except Exception as e:
         logger.warning(f"Failed to get Pacific time: {e}, using UTC")
         now = datetime.now(timezone.utc)
-        return now.strftime("%Y-%m-%d (%A) %H:00 UTC")
+        return now.strftime("%Y-%m-%d (%A) UTC")

backend/tests/agents/main_agent/core/test_system_prompt_builder.py

@@ -32,17 +32,17 @@ class TestBuildWithDateTrue:
 
     @patch(
         "agents.main_agent.core.system_prompt_builder.get_current_date_pacific",
-        return_value="2024-06-15 (Saturday) 10:00 PDT",
+        return_value="2024-06-15 (Saturday) PDT",
     )
     def test_appends_current_date_line(self, mock_date):
         builder = SystemPromptBuilder()
         result = builder.build(include_date=True)
 
-        assert result.endswith("Current date: 2024-06-15 (Saturday) 10:00 PDT")
+        assert result.endswith("Current date: 2024-06-15 (Saturday) PDT")
 
     @patch(
         "agents.main_agent.core.system_prompt_builder.get_current_date_pacific",
-        return_value="2024-06-15 (Saturday) 10:00 PDT",
+        return_value="2024-06-15 (Saturday) PDT",
     )
     def test_includes_base_prompt(self, mock_date):
         builder = SystemPromptBuilder()
@@ -52,13 +52,13 @@ def test_includes_base_prompt(self, mock_date):
 
     @patch(
         "agents.main_agent.core.system_prompt_builder.get_current_date_pacific",
-        return_value="2024-01-01 (Monday) 08:00 PST",
+        return_value="2024-01-01 (Monday) PST",
     )
     def test_date_separated_by_blank_line(self, mock_date):
         builder = SystemPromptBuilder()
         result = builder.build(include_date=True)
 
-        expected = f"{DEFAULT_SYSTEM_PROMPT}\n\nCurrent date: 2024-01-01 (Monday) 08:00 PST"
+        expected = f"{DEFAULT_SYSTEM_PROMPT}\n\nCurrent date: 2024-01-01 (Monday) PST"
         assert result == expected
 
 
@@ -102,14 +102,14 @@ def test_uses_custom_prompt(self):
 
     @patch(
         "agents.main_agent.core.system_prompt_builder.get_current_date_pacific",
-        return_value="2024-03-20 (Wednesday) 15:00 PDT",
+        return_value="2024-03-20 (Wednesday) PDT",
     )
     def test_build_with_date_uses_custom_prompt(self, mock_date):
         custom = "Custom prompt."
         builder = SystemPromptBuilder(base_prompt=custom)
         result = builder.build(include_date=True)
 
-        assert result == "Custom prompt.\n\nCurrent date: 2024-03-20 (Wednesday) 15:00 PDT"
+        assert result == "Custom prompt.\n\nCurrent date: 2024-03-20 (Wednesday) PDT"
 
     def test_none_base_prompt_falls_back_to_default(self):
         builder = SystemPromptBuilder(base_prompt=None)

backend/tests/agents/main_agent/utils/test_timezone.py

@@ -1,29 +1,50 @@
 """
 Tests for timezone utilities.
 Requirements: 21.1–21.3
+
+The rendered string is part of the Bedrock prompt-cache prefix, so it must be
+byte-stable within a Pacific day: date, weekday and timezone only — no hour.
 """
 
 import re
+from datetime import datetime
 from unittest.mock import patch
-
-import pytest
+from zoneinfo import ZoneInfo
 
 from agents.main_agent.utils.timezone import get_current_date_pacific
 
 
-# Regex for "YYYY-MM-DD (DayName) HH:00 TZ"
+# Regex for "YYYY-MM-DD (DayName) TZ"
 DATE_FORMAT_RE = re.compile(
     r"^\d{4}-\d{2}-\d{2} "           # YYYY-MM-DD
     r"\([A-Z][a-z]+\) "              # (DayName)
-    r"\d{2}:00 "                     # HH:00
     r"[A-Z]{3,4}$"                   # TZ abbreviation
 )
 
 VALID_DAYS = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"}
 
+PACIFIC = ZoneInfo("America/Los_Angeles")
+
+
+class _FrozenDatetime(datetime):
+    """datetime subclass whose now() returns a fixed instant (in the requested tz)."""
+
+    frozen: datetime
+
+    @classmethod
+    def now(cls, tz=None):
+        return cls.frozen.astimezone(tz) if tz else cls.frozen
+
+
+def _at(instant: datetime) -> str:
+    """Call get_current_date_pacific() as if it were `instant` right now."""
+    _FrozenDatetime.frozen = instant
+    with patch("agents.main_agent.utils.timezone.datetime", _FrozenDatetime):
+        return get_current_date_pacific()
+
 
 class TestGetCurrentDatePacific:
-    """Req 21.1: Verify format matches 'YYYY-MM-DD (DayName) HH:00 TZ'."""
+    """Req 21.1: Verify format matches 'YYYY-MM-DD (DayName) TZ'."""
 
     def test_format_matches_expected_pattern(self):
         result = get_current_date_pacific()
@@ -34,12 +55,32 @@ def test_contains_valid_day_name(self):
         day = re.search(r"\((\w+)\)", result).group(1)
         assert day in VALID_DAYS, f"Day '{day}' is not a valid day name"
 
-    def test_hour_is_zero_padded_with_00_minutes(self):
+    def test_has_no_time_of_day_component(self):
+        # Prompt-cache contract: the line must not change at hour boundaries.
         result = get_current_date_pacific()
-        hour_match = re.search(r"(\d{2}):00", result)
-        assert hour_match is not None
-        hour = int(hour_match.group(1))
-        assert 0 <= hour <= 23
+        assert not re.search(r"\d{1,2}:\d{2}", result), f"Unexpected time-of-day in '{result}'"
+        assert len(result.split()) == 3
+
+
+class TestDayStability:
+    """The rendered string must be identical for every hour of the same Pacific day
+    and change only when the Pacific calendar date changes."""
+
+    def test_identical_across_every_hour_of_a_day(self):
+        base = datetime(2026, 9, 15, 0, 0, tzinfo=PACIFIC)
+        rendered = {_at(base.replace(hour=h, minute=m)) for h in range(24) for m in (0, 59)}
+        assert rendered == {"2026-09-15 (Tuesday) PDT"}, rendered
+
+    def test_changes_at_pacific_midnight(self):
+        before = _at(datetime(2026, 9, 15, 23, 59, tzinfo=PACIFIC))
+        after = _at(datetime(2026, 9, 16, 0, 0, tzinfo=PACIFIC))
+        assert before == "2026-09-15 (Tuesday) PDT"
+        assert after == "2026-09-16 (Wednesday) PDT"
+
+    def test_uses_pacific_date_not_utc_date(self):
+        # 03:00 UTC on the 16th is still 20:00 PDT on the 15th.
+        utc_instant = datetime(2026, 9, 16, 3, 0, tzinfo=ZoneInfo("UTC"))
+        assert _at(utc_instant) == "2026-09-15 (Tuesday) PDT"
 
 
 class TestTimezoneAbbreviation:
@@ -50,6 +91,9 @@ def test_timezone_is_pst_or_pdt(self):
         tz = result.split()[-1]
         assert tz in ("PST", "PDT"), f"Timezone '{tz}' is not PST or PDT"
 
+    def test_standard_time_renders_pst(self):
+        assert _at(datetime(2026, 1, 15, 12, 0, tzinfo=PACIFIC)) == "2026-01-15 (Thursday) PST"
+
 
 class TestUTCFallback:
     """Req 21.3: Verify UTC fallback when timezone libraries unavailable."""
@@ -63,7 +107,7 @@ def test_falls_back_to_utc_when_timezone_unavailable(self):
     def test_utc_fallback_format_matches(self):
         with patch("agents.main_agent.utils.timezone.TIMEZONE_AVAILABLE", False):
             result = get_current_date_pacific()
-        # Should still have YYYY-MM-DD (DayName) HH:00 UTC
+        # Should still have YYYY-MM-DD (DayName) UTC — and no hour
         parts = result.split()
-        assert len(parts) == 4
-        assert parts[3] == "UTC"
+        assert len(parts) == 3
+        assert parts[2] == "UTC"

frontend/ai.client/src/app/session/components/message-list/components/file-preview/docx-viewer.component.spec.ts

@@ -16,6 +16,10 @@ describe('DocxViewerComponent', () => {
 
   beforeEach(async () => {
     renderAsync.mockReset();
+    // The component memoizes its dynamic import('docx-preview') on a static
+    // field, and the builder runs vitest with isolate: false, so a sibling spec
+    // that rendered first would pin *its* mocked module for this file too.
+    (DocxViewerComponent as unknown as { libraryPromise: unknown }).libraryPromise = null;
     renderAsync.mockImplementation((_data, host: HTMLElement) => {
       const wrapper = document.createElement('div');
       wrapper.className = 'docx-wrapper';

frontend/ai.client/src/app/session/components/message-list/components/file-preview/file-preview-panel.component.spec.ts

@@ -1,6 +1,7 @@
 import { ComponentFixture, TestBed } from '@angular/core/testing';
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { FilePreviewPanelComponent } from './file-preview-panel.component';
+import { DocxViewerComponent } from './docx-viewer.component';
 import { FilePreviewStateService } from '../../../../services/file-preview/file-preview-state.service';
 import {
   FilePreviewError,
@@ -22,6 +23,10 @@ describe('FilePreviewPanelComponent', () => {
 
   beforeEach(async () => {
     renderAsync.mockReset();
+    // The component memoizes its dynamic import('docx-preview') on a static
+    // field, and the builder runs vitest with isolate: false, so a sibling spec
+    // that rendered first would pin *its* mocked module for this file too.
+    (DocxViewerComponent as unknown as { libraryPromise: unknown }).libraryPromise = null;
     renderAsync.mockImplementation((_data, host: HTMLElement) => {
       host.appendChild(document.createElement('section'));
       return Promise.resolve();