From 28eb911a2a7e95bf5142813bbb2872c1a3f07a11 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Carlos=20Flor=C3=AAncio?= <carlosmflorencio@gmail.com>
Date: Sun, 6 Sep 2026 15:14:49 +0100
Subject: [PATCH] fix: suppress invalid task timestamps (#3434)

---
 .../chat/messages/message-actions.test.tsx    |  15 +++
 .../task/chat/messages/message-actions.tsx    |   5 +-
 .../lib/state/slices/session/turn-actions.ts  |  63 +---------
 apps/web/lib/utils.test.ts                    |   7 ++
 apps/web/lib/utils.ts                         |   2 +
 apps/web/lib/utils/strict-timestamp.ts        |  66 +++++++++++
 docs/plans/invalid-task-date-display/plan.md  | 109 ++++++++++++++++++
 ...-01-suppress-invalid-message-timestamps.md |  95 +++++++++++++++
 .../task-prompt-transcript-visibility.md      |   1 +
 .../task-prompt-transcript-visibility.md      |   9 +-
 10 files changed, 309 insertions(+), 63 deletions(-)
 create mode 100644 apps/web/lib/utils/strict-timestamp.ts
 create mode 100644 docs/plans/invalid-task-date-display/plan.md
 create mode 100644 docs/plans/invalid-task-date-display/task-01-suppress-invalid-message-timestamps.md

diff --git a/apps/web/components/task/chat/messages/message-actions.test.tsx b/apps/web/components/task/chat/messages/message-actions.test.tsx
index e96867583f..377252c3b1 100644
--- a/apps/web/components/task/chat/messages/message-actions.test.tsx
+++ b/apps/web/components/task/chat/messages/message-actions.test.tsx
@@ -98,6 +98,21 @@ describe("MessageActions timestamp tooltip", () => {
     expect(timeEl?.getAttribute("title")).toBe(new Date(MESSAGE_TIMESTAMP).toLocaleString());
   });
 
+  it.each(["", "not-a-date", "0", "2026-02-30T10:00:00Z"])(
+    "omits the timestamp affordance for invalid created_at value %j",
+    (createdAt) => {
+      const { container } = render(
+        <StateProvider>
+          <MessageActions message={assistantMessage({ created_at: createdAt })} />
+        </StateProvider>,
+      );
+
+      expect(container.querySelector("time")).toBeNull();
+      expect(container.textContent).not.toContain("Invalid Date");
+      expect(screen.getByRole("button", { name: /copy message to clipboard/i })).toBeTruthy();
+    },
+  );
+
   it("omits the timestamp element entirely when showTimestamp is false", () => {
     const { container } = render(
       <StateProvider>
diff --git a/apps/web/components/task/chat/messages/message-actions.tsx b/apps/web/components/task/chat/messages/message-actions.tsx
index b240bba6bc..8700cdfbcc 100644
--- a/apps/web/components/task/chat/messages/message-actions.tsx
+++ b/apps/web/components/task/chat/messages/message-actions.tsx
@@ -15,6 +15,7 @@ import {
 } from "@tabler/icons-react";
 import { cn } from "@/lib/utils";
 import { formatRelativeTime } from "@/lib/utils";
+import { parseStrictRfc3339Timestamp } from "@/lib/utils/strict-timestamp";
 import { formatPromptDuration, messageTurnDurationSeconds } from "@/lib/prompt-history";
 import { useCopyToClipboard } from "@/hooks/use-copy-to-clipboard";
 import { useAppStore } from "@/components/state-provider";
@@ -247,7 +248,9 @@ function MessageTimestamp({ createdAt }: { createdAt: string }) {
   const { t } = useTranslation();
   const usesTouchDrawer = useTouchDrawer();
   const [open, setOpen] = useState(false);
-  const absoluteTime = new Date(createdAt).toLocaleString();
+  if (parseStrictRfc3339Timestamp(createdAt) === null) return null;
+  const date = new Date(createdAt);
+  const absoluteTime = date.toLocaleString();
   const timeEl = (
     <time
       dateTime={createdAt}
diff --git a/apps/web/lib/state/slices/session/turn-actions.ts b/apps/web/lib/state/slices/session/turn-actions.ts
index a8dcedefac..83f6cae0ae 100644
--- a/apps/web/lib/state/slices/session/turn-actions.ts
+++ b/apps/web/lib/state/slices/session/turn-actions.ts
@@ -2,6 +2,7 @@ import type { StateCreator } from "zustand";
 import type { Draft } from "immer";
 import type { SessionSlice } from "./types";
 import type { Turn, TaskSession } from "@/lib/types/http";
+import { parseStrictRfc3339Timestamp } from "@/lib/utils/strict-timestamp";
 
 type ImmerSet = Parameters<
   StateCreator<SessionSlice, [["zustand/immer", never]], [], SessionSlice>
@@ -9,43 +10,6 @@ type ImmerSet = Parameters<
 
 type TurnReconciliationDraft = Pick<Draft<SessionSlice>, "turns" | "taskSessions">;
 
-// Strict wire format: full RFC3339 with explicit offset/UTC marker. JS
-// Date.parse is permissive (e.g. "0" parses as 2000-01-01, partial dates as
-// midnight, timezone-less values in the local zone), so shape validation must
-// come first or malformed rows would win freshness comparisons.
-const RFC3339_PATTERN =
-  /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.(\d+))?(Z|[+-](\d{2}):(\d{2}))$/;
-
-const DAYS_IN_MONTH = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
-
-/** Whether the year is a leap year (proleptic Gregorian calendar). */
-function isLeapYear(year: number): boolean {
-  return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
-}
-
-/** Whether the day exists in the month (leap-year aware). */
-function validDayForMonth(year: number, month: number, day: number): boolean {
-  if (day < 1) return false;
-  const maxDay = month === 2 && isLeapYear(year) ? 29 : DAYS_IN_MONTH[month - 1];
-  return day <= maxDay;
-}
-
-/**
- * Returns the UTC-offset delta in SECONDS for a matched zone, or null when
- * the offset components are out of range.
- */
-function parseOffsetSeconds(
-  zone: string,
-  offsetHourText: string | undefined,
-  offsetMinuteText: string | undefined,
-): number | null {
-  if (zone === "Z") return 0;
-  const hour = Number(offsetHourText);
-  const minute = Number(offsetMinuteText);
-  if (hour > 23 || minute > 59) return null;
-  return (zone.startsWith("-") ? -1 : 1) * (hour * 60 + minute) * 60;
-}
-
 /**
  * Whether `incoming` is not newer than `existing`. A malformed incoming
  * timestamp counts as stale (never newer).
@@ -224,30 +188,7 @@ export function reconcileActiveTurnAfterHydrationDraft(
  * would collapse legitimate sub-100ns differences and misorder rows.
  */
 export function parseTurnTimestamp(value: string | undefined): bigint | null {
-  if (!value) return null;
-  const match = RFC3339_PATTERN.exec(value);
-  if (!match) return null;
-  const year = Number(match[1]);
-  const month = Number(match[2]);
-  if (month < 1 || month > 12) return null;
-  if (!validDayForMonth(year, month, Number(match[3]))) return null;
-  if (Number(match[4]) > 23 || Number(match[5]) > 59 || Number(match[6]) > 59) {
-    return null;
-  }
-  const offsetSeconds = parseOffsetSeconds(match[8], match[9], match[10]);
-  if (offsetSeconds === null) return null;
-  // RFC3339Nano caps the fraction at 9 digits; longer fractions would
-  // overflow into the seconds component (padEnd does not cap).
-  if (match[7] !== undefined && match[7].length > 9) return null;
-  // setUTCFullYear handles years 0-99 correctly (Date.UTC maps them to
-  // 1900+year); the components are validated above, so no normalization can
-  // occur here.
-  const utc = new Date(0);
-  utc.setUTCFullYear(year, month - 1, Number(match[3]));
-  utc.setUTCHours(Number(match[4]), Number(match[5]), Number(match[6]), 0);
-  const wholeSeconds = BigInt(Math.floor(utc.getTime() / 1000)) - BigInt(offsetSeconds);
-  const fractionNs = BigInt((match[7] ?? "").padEnd(9, "0"));
-  return wholeSeconds * BigInt(1_000_000_000) + fractionNs;
+  return parseStrictRfc3339Timestamp(value);
 }
 
 /**
diff --git a/apps/web/lib/utils.test.ts b/apps/web/lib/utils.test.ts
index 9e1f07d5b0..0621b55b98 100644
--- a/apps/web/lib/utils.test.ts
+++ b/apps/web/lib/utils.test.ts
@@ -184,6 +184,13 @@ describe("formatRelativeTime", () => {
     expect(formatRelativeTime(ago(9 * SECOND))).toBe("just now");
   });
 
+  it.each(["", "not-a-date", "0", "2026-02-30T10:00:00Z"])(
+    "returns an empty label for invalid input %j",
+    (value) => {
+      expect(formatRelativeTime(value)).toBe("");
+    },
+  );
+
   it("localizes EVERY branch, so a timestamp does not change language as it ages", async () => {
     // The regression: "just now" was routed through the catalog while the rungs
     // below it stayed hardcoded, so a non-English locale showed a translated
diff --git a/apps/web/lib/utils.ts b/apps/web/lib/utils.ts
index 4473572a75..a2498142df 100644
--- a/apps/web/lib/utils.ts
+++ b/apps/web/lib/utils.ts
@@ -1,6 +1,7 @@
 import { clsx, type ClassValue } from "clsx";
 import { twMerge } from "tailwind-merge";
 import { t } from "@/lib/i18n";
+import { parseStrictRfc3339Timestamp } from "@/lib/utils/strict-timestamp";
 
 export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs));
@@ -205,6 +206,7 @@ export const DEFAULT_LOCAL_EXECUTOR_TYPE = "worktree";
  * @returns Formatted relative time string
  */
 export function formatRelativeTime(dateString: string): string {
+  if (parseStrictRfc3339Timestamp(dateString) === null) return "";
   const date = new Date(dateString);
   const now = new Date();
   const diffMs = now.getTime() - date.getTime();
diff --git a/apps/web/lib/utils/strict-timestamp.ts b/apps/web/lib/utils/strict-timestamp.ts
new file mode 100644
index 0000000000..62280ffce1
--- /dev/null
+++ b/apps/web/lib/utils/strict-timestamp.ts
@@ -0,0 +1,66 @@
+// Strict wire format: full RFC3339 with an explicit offset or UTC marker.
+// Date.parse is permissive, so shape validation must happen before any date
+// formatting or malformed values can be normalized into a different instant.
+const RFC3339_PATTERN =
+  /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.(\d+))?(Z|[+-](\d{2}):(\d{2}))$/;
+
+const DAYS_IN_MONTH = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
+
+/** Whether the year is a leap year in the proleptic Gregorian calendar. */
+function isLeapYear(year: number): boolean {
+  return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
+}
+
+/** Whether the day exists in the month, including leap-day validation. */
+function validDayForMonth(year: number, month: number, day: number): boolean {
+  if (day < 1) return false;
+  const maxDay = month === 2 && isLeapYear(year) ? 29 : DAYS_IN_MONTH[month - 1];
+  return day <= maxDay;
+}
+
+/** Returns the UTC offset in seconds for a matched RFC3339 zone. */
+function parseOffsetSeconds(
+  zone: string,
+  offsetHourText: string | undefined,
+  offsetMinuteText: string | undefined,
+): number | null {
+  if (zone === "Z") return 0;
+  const hour = Number(offsetHourText);
+  const minute = Number(offsetMinuteText);
+  if (hour > 23 || minute > 59) return null;
+  return (zone.startsWith("-") ? -1 : 1) * (hour * 60 + minute) * 60;
+}
+
+/**
+ * Parses an RFC3339/RFC3339Nano timestamp into epoch nanoseconds.
+ *
+ * Missing, empty, malformed, and semantically invalid values return `null`.
+ * Calendar and time components are validated explicitly because Date.parse
+ * normalizes values such as February 30th instead of rejecting them.
+ */
+export function parseStrictRfc3339Timestamp(value: string | undefined): bigint | null {
+  if (!value) return null;
+  const match = RFC3339_PATTERN.exec(value);
+  if (!match) return null;
+  const year = Number(match[1]);
+  const month = Number(match[2]);
+  if (month < 1 || month > 12) return null;
+  if (!validDayForMonth(year, month, Number(match[3]))) return null;
+  if (Number(match[4]) > 23 || Number(match[5]) > 59 || Number(match[6]) > 59) {
+    return null;
+  }
+  const offsetSeconds = parseOffsetSeconds(match[8], match[9], match[10]);
+  if (offsetSeconds === null) return null;
+  // RFC3339Nano caps the fraction at 9 digits; longer fractions would
+  // overflow into the seconds component when padded.
+  if (match[7] !== undefined && match[7].length > 9) return null;
+
+  // setUTCFullYear handles years 0-99 correctly; Date.UTC maps them to
+  // 1900-1999 and would therefore normalize an otherwise valid timestamp.
+  const utc = new Date(0);
+  utc.setUTCFullYear(year, month - 1, Number(match[3]));
+  utc.setUTCHours(Number(match[4]), Number(match[5]), Number(match[6]), 0);
+  const wholeSeconds = BigInt(Math.floor(utc.getTime() / 1000)) - BigInt(offsetSeconds);
+  const fractionNs = BigInt((match[7] ?? "").padEnd(9, "0"));
+  return wholeSeconds * BigInt(1_000_000_000) + fractionNs;
+}
diff --git a/docs/plans/invalid-task-date-display/plan.md b/docs/plans/invalid-task-date-display/plan.md
new file mode 100644
index 0000000000..96c4fc7321
--- /dev/null
+++ b/docs/plans/invalid-task-date-display/plan.md
@@ -0,0 +1,109 @@
+---
+created: 2026-09-05
+status: done
+requirements:
+  - REQ-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001
+system_design:
+  - ../../specs/ui/system-design/task-prompt-transcript-visibility.md
+legacy_specs: []
+---
+
+# Implementation Plan: Suppress invalid task timestamps
+
+## Overview
+
+Prevent the task transcript from displaying the browser's `Invalid Date` text
+when it renders the frontend-only task-description fallback. The fallback has
+content but no persisted message timestamp, so the timestamp action is omitted;
+the shared legacy relative formatter also returns an empty label for malformed
+input. The implementation is one sequential frontend slice with focused unit
+regressions.
+
+## Root cause
+
+`useProcessedMessages` creates `TASK_DESCRIPTION_SYNTHETIC_ID` with
+`created_at: ""` when history is exhausted without a stored user prompt.
+`MessageActions` always renders `MessageTimestamp`, and the legacy
+`formatRelativeTime` helper treats the invalid `Date` as though its numeric
+comparisons were valid before falling through to `date.toLocaleDateString()`.
+The browser therefore supplies the visible string `Invalid Date`.
+
+## Scope
+
+### In scope
+
+- Validate a message timestamp before rendering the timestamp action and its
+  absolute-time disclosure.
+- Make the shared legacy relative formatter return an empty string for empty or
+  malformed input.
+- Add unit coverage for the observed synthetic fallback path and the shared
+  formatter contract.
+
+### Out of scope
+
+- Changing task-description fallback eligibility, message persistence, or API
+  timestamp fields.
+- Adding a new mobile composition or changing action-row sizing, touch targets,
+  navigation, or scrolling.
+- Reworking the separate locale-aware formatter in `lib/i18n/formats.ts`.
+
+## Technical approach
+
+- `apps/web/lib/utils.ts`: return `""` immediately when the parsed date is
+  invalid in `formatRelativeTime`, using the shared strict parser.
+- `apps/web/lib/utils/strict-timestamp.ts`: centralize strict RFC3339 calendar,
+  clock, offset, and fractional-second validation for timestamp consumers.
+- `apps/web/components/task/chat/messages/message-actions.tsx`: validate
+-  `createdAt` in `MessageTimestamp` with the shared parser before deriving the
+  absolute label or mounting the `<time>`/Drawer surface. Invalid timestamps
+  keep content actions visible but omit only the timestamp affordance.
+- `apps/web/lib/state/slices/session/turn-actions.ts`: delegate the existing
+  nanosecond parser API to the shared implementation so session reconciliation
+  and UI formatting enforce the same wire contract.
+- Keep the existing `formatRelativeTime` import and action-row presentation so
+  valid message timestamps and desktop/mobile interaction behavior are
+  unchanged.
+
+## Tests
+
+- `apps/web/lib/utils.test.ts`: add an invalid and empty input regression for
+  `formatRelativeTime`, including values that `Date.parse` would normalize.
+- `apps/web/components/task/chat/messages/message-actions.test.tsx`: add a
+  synthetic-fallback timestamp case asserting no `<time>` element or
+  `Invalid Date` text/disclosure is rendered for empty, malformed, and
+  normalized-malformed values. This covers
+  `AC-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001.15`.
+
+## E2E tests
+
+No new Playwright scenario is required. This repair changes only timestamp
+normalization and conditionally removes an affordance from an existing inline
+message row; it does not change mobile composition, touch behavior, navigation,
+scrolling, or viewport geometry. The focused message-action test covers the
+shared desktop/mobile rendering path, and existing task-chat mobile flows remain
+the rendered transcript smoke coverage.
+
+## Work orders
+
+- [x] [Task 01: Suppress invalid message timestamps](task-01-suppress-invalid-message-timestamps.md)
+
+## Verification results
+
+Passed:
+
+- `pnpm --filter @kandev/web test -- --run lib/utils.test.ts components/task/chat/messages/message-actions.test.tsx`: 51 tests passed.
+- `pnpm --filter @kandev/web test -- --run lib/state/slices/session/turn-actions.test.ts`: 54 tests passed.
+- ESLint passed for the four changed frontend source and test files.
+- `pnpm run typecheck` passed from `apps/web`.
+- `python3 scripts/lint-spec-files.py --all` passed.
+- `git diff --check` passed.
+- Managed Chromium and Pixel 5 capture specs passed with synthetic task data; both screenshots contain no `Invalid Date` text. Temporary capture specs were removed after capture.
+
+## Risks
+
+- A malformed timestamp supplied by a persisted message will lose its timestamp
+  affordance instead of showing a misleading value; message content and other
+  actions remain available.
+- The focused test command may regenerate frontend release-note/changelog
+  artifacts as part of the package pretest hook; unrelated generated changes
+  must not be included.
diff --git a/docs/plans/invalid-task-date-display/task-01-suppress-invalid-message-timestamps.md b/docs/plans/invalid-task-date-display/task-01-suppress-invalid-message-timestamps.md
new file mode 100644
index 0000000000..584ba588ea
--- /dev/null
+++ b/docs/plans/invalid-task-date-display/task-01-suppress-invalid-message-timestamps.md
@@ -0,0 +1,95 @@
+---
+id: "01-suppress-invalid-message-timestamps"
+title: "Suppress invalid message timestamps"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+requirements:
+  - REQ-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001
+acceptance_criteria:
+  - AC-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001.4
+  - AC-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001.15
+system_design:
+  - ../../specs/ui/system-design/task-prompt-transcript-visibility.md
+---
+
+# Task 01: Suppress invalid message timestamps
+
+## Summary
+
+Make empty and malformed message timestamps render no timestamp affordance and
+never expose the browser's `Invalid Date` string. Preserve the existing message
+action row and valid timestamp behavior.
+
+## In scope
+
+- Add a failing formatter regression for empty and invalid values.
+- Add a failing message-action regression for the synthetic task-description
+  message shape (`created_at: ""`).
+- Add the minimum guards in the shared formatter and `MessageTimestamp`.
+
+## Out of scope
+
+- Task/session persistence or backend message contracts.
+- Changes to transcript pagination, fallback eligibility, responsive layout,
+  touch targets, or action semantics for valid timestamps.
+
+## Acceptance
+
+- `formatRelativeTime` returns an empty string for empty, malformed, and
+  normalized-malformed values such as `"0"` and February 30th.
+- `MessageActions` renders no timestamp element or invalid-date disclosure for
+  an empty or malformed `created_at` while copy/raw/favorite actions remain
+  available.
+- Valid timestamp tests continue to pass unchanged.
+
+## Verification
+
+```bash
+(cd apps && pnpm --filter @kandev/web test -- --run lib/utils.test.ts components/task/chat/messages/message-actions.test.tsx)
+(cd apps/web && pnpm exec eslint lib/utils.ts lib/utils.test.ts lib/utils/strict-timestamp.ts lib/state/slices/session/turn-actions.ts components/task/chat/messages/message-actions.tsx components/task/chat/messages/message-actions.test.tsx)
+python3 scripts/lint-spec-files.py --all
+git diff --check
+```
+
+## Files likely touched
+
+- `apps/web/lib/utils.ts`
+- `apps/web/lib/utils/strict-timestamp.ts`
+- `apps/web/lib/utils.test.ts`
+- `apps/web/lib/state/slices/session/turn-actions.ts`
+- `apps/web/components/task/chat/messages/message-actions.tsx`
+- `apps/web/components/task/chat/messages/message-actions.test.tsx`
+- `docs/specs/ui/requirements/task-prompt-transcript-visibility.md`
+- `docs/specs/ui/system-design/task-prompt-transcript-visibility.md`
+
+## Dependencies
+
+None.
+
+## Risks
+
+- Invalid persisted timestamps will be intentionally hidden rather than
+  displayed as raw values, matching the fallback's lack of a trustworthy date.
+
+## Parallelism
+
+`sequential`
+
+## Inputs
+
+- `REQ-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001.4` and `.15`.
+- The fallback timestamp boundary in the transcript system design.
+- Existing `MessageActions` and `formatRelativeTime` tests.
+
+## Results
+
+Implemented and verified:
+
+- `parseStrictRfc3339Timestamp` is shared by session reconciliation and UI date formatting.
+- `formatRelativeTime` returns an empty label for empty, malformed, and normalized-malformed dates.
+- `MessageTimestamp` omits the timestamp element and coarse-pointer disclosure for invalid dates while preserving other message actions.
+- Focused frontend tests pass with 51 tests; turn timestamp tests pass with 54 tests.
+- ESLint, TypeScript typecheck, specification lint, and `git diff --check` pass.
+- Managed Chromium and Pixel 5 capture specs pass against the production build; the captured fallback shows no `Invalid Date` text. Temporary capture specs were removed.
diff --git a/docs/specs/ui/requirements/task-prompt-transcript-visibility.md b/docs/specs/ui/requirements/task-prompt-transcript-visibility.md
index 0fad0f4b03..cb33968b8c 100644
--- a/docs/specs/ui/requirements/task-prompt-transcript-visibility.md
+++ b/docs/specs/ui/requirements/task-prompt-transcript-visibility.md
@@ -51,6 +51,7 @@ start.
 - **AC-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001.12:** When a previously opened session is revisited after more than one message page was persisted while it was inactive, the transcript shall reconcile to a contiguous newest window and upward pagination shall reach every persisted user prompt without gaps or duplicate rows.
 - **AC-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001.13:** When an upward lazy-load operation sizes its batch, it shall target 20 newly loaded text parts. Tool calls and other activity rows shall not advance that target; the operation can stop earlier when prompt `#1` is loaded, persisted history is exhausted, a request makes no progress, or its bounded safety limit is reached.
 - **AC-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001.14:** When a previously hidden transcript becomes visible with its restored viewport at the oldest loaded edge and older history remains, the transcript shall resume older-history loading without requiring the user to move away from and return to that edge.
+- **AC-UI-TASK-PROMPT-TRANSCRIPT-VISIBILITY-001.15:** When the task-description fallback has no valid persisted message timestamp, the transcript shall omit that row's timestamp affordance and shall not render an invalid-date placeholder.
 
 ## Exclusions
 
diff --git a/docs/specs/ui/system-design/task-prompt-transcript-visibility.md b/docs/specs/ui/system-design/task-prompt-transcript-visibility.md
index 101ceff138..f1a48dd777 100644
--- a/docs/specs/ui/system-design/task-prompt-transcript-visibility.md
+++ b/docs/specs/ui/system-design/task-prompt-transcript-visibility.md
@@ -85,6 +85,12 @@ are true:
 A tool-only newest window with `has_more: true` does not satisfy this contract.
 It renders only the stored rows in that window until the user navigates upward.
 
+The fallback row is frontend-synthesized content, not a persisted message, so
+its timestamp is empty. The message action renderer must validate timestamps
+before creating the timestamp element. An empty or malformed timestamp omits
+the timestamp affordance, including its absolute-time disclosure, instead of
+passing an invalid `Date` to browser formatting APIs.
+
 ## Opening boundary
 
 The session message fetch requests only the newest bounded window when a task
@@ -229,7 +235,8 @@ anchor behavior as desktop.
 
 - Processed-message tests cover an uninitialized window, a tool-only window
   with older history, exhausted legacy history, an empty description, and a
-  loaded stored prompt.
+  loaded stored prompt. Message-action tests cover the no-timestamp fallback
+  path without an invalid-date placeholder.
 - Session-state tests cover initialization through boot hydration, newest
   fetch, purge, and refetch without inferring exhaustion from defaults.
 - Sentinel tests cover committed in-region continuation, out-of-region stop,