refactor(chat): update datetime prefixing logic for user messages and improve formatting

packages/stage-ui/src/stores/chat.ts

@@ -350,28 +350,28 @@ export const useChatOrchestratorStore = defineStore('chat-orchestrator', () => {
         ],
       })
 
-      // Per-message datetime injection (replaces the old `<context>` XML block):
-      // every user/assistant message gets a `[YYYY-MM-DD HH:MM]` prefix
-      // derived from its persisted `createdAt`. The full date appears on every
-      // turn so the model can read "today" from the most recent message; the
-      // system prompt itself stays 100% static for permanent KV-cache reuse.
-      // Legacy entries without a persisted `createdAt` fall back to "now"
-      // rather than a fabricated older timestamp.
+      // Inject `[YYYY-MM-DD HH:MM]` prefix only into user messages, derived
+      // from their persisted `createdAt`. Assistant messages stay clean —
+      // otherwise the model learns the format and mirrors it back into its
+      // own replies (e.g. `[2026-05-09 00:23] > ...`). The model still reads
+      // "today" from the latest user message, and the system prompt stays
+      // 100% static for permanent KV-cache reuse. Legacy entries without a
+      // persisted `createdAt` fall back to "now" rather than a fabricated
+      // older timestamp.
       // See `./chat/datetime-prefix.ts` for the rationale.
       const nowTs = Date.now()
 
       const newMessages = sessionMessagesForSend.map((msg) => {
         const { context: _context, id: _id, createdAt, ...withoutContext } = msg
         const rawMessage = toRaw(withoutContext)
-        const ts = createdAt ?? nowTs
 
         if (rawMessage.role === 'user') {
-          return prependTextToContent(rawMessage, formatTimePrefix(ts))
+          return prependTextToContent(rawMessage, formatTimePrefix(createdAt ?? nowTs))
         }
 
         if (rawMessage.role === 'assistant') {
           const { slices: _slices, tool_results: _toolResults, categorization: _categorization, ...rest } = rawMessage as ChatAssistantMessage
-          return prependTextToContent(toRaw(rest), formatTimePrefix(ts))
+          return toRaw(rest)
         }
 
         return rawMessage

packages/stage-ui/src/stores/chat/datetime-prefix.ts

@@ -6,14 +6,16 @@
  * invalidated KV-cache prefixes on every send).
  *
  * Strategy:
- * - Each user/assistant message is prefixed with `[YYYY-MM-DD HH:MM]` derived
- *   from its persisted `createdAt`. Stored timestamps never change, so the
- *   prefixed history stays byte-stable across turns and accumulates KV-cache
- *   prefix matches.
- * - The full date is included on every message so the model can infer "today"
- *   from the most recent message — there is no separate system-prompt date
- *   anchor, which keeps the system prompt 100% static and permanently
- *   cacheable across turns and across day boundaries.
+ * - Only user messages are prefixed with `[YYYY-MM-DD HH:MM]` derived from
+ *   their persisted `createdAt`. Assistant messages stay clean — prefixing
+ *   them caused models to learn the format and emit `[date] > ...` in their
+ *   own replies.
+ * - Stored timestamps never change, so the prefixed user history stays
+ *   byte-stable across turns and accumulates KV-cache prefix matches.
+ * - The full date is included on every user message so the model can infer
+ *   "today" from the most recent user turn — there is no separate
+ *   system-prompt date anchor, which keeps the system prompt 100% static and
+ *   permanently cacheable across turns and across day boundaries.
  *
  * Format choice:
  * - `[YYYY-MM-DD HH:MM]` is ISO-like, structurally compact (~17 chars), and
@@ -26,23 +28,18 @@
  *   correlates with verbatim copy-back.
  */
 
-const DATE_TIME = new Intl.DateTimeFormat('en-CA', {
-  year: 'numeric',
-  month: '2-digit',
-  day: '2-digit',
-  hour: '2-digit',
-  minute: '2-digit',
-  hour12: false,
-})
+import { format } from 'date-fns'
 
 /**
  * Formats a timestamp as `[YYYY-MM-DD HH:MM] ` in the user's local timezone.
  *
  * Use when:
- * - Annotating user/assistant messages so the model has a concrete time
- *   anchor on every turn — historic and current alike use the same shape so
- *   that prefix-cache stays valid when a "current" turn becomes "historic" on
+ * - Annotating user messages so the model has a concrete time anchor on
+ *   every turn — historic and current user turns use the same shape so that
+ *   prefix-cache stays valid when a "current" turn becomes "historic" on
  *   the next send.
+ * - Not used for assistant messages — that caused the model to mirror the
+ *   prefix into its own output.
  *
  * Returns:
  * - String including a trailing space, e.g. `"[2026-04-25 18:47] "`.
@@ -54,8 +51,5 @@ const DATE_TIME = new Intl.DateTimeFormat('en-CA', {
  * - "[2026-04-25 18:47] "
  */
 export function formatTimePrefix(createdAt: number): string {
-  // Intl en-CA locale uses ISO-style `YYYY-MM-DD, HH:MM`. Strip the comma to
-  // produce the bracketed `YYYY-MM-DD HH:MM` form.
-  const formatted = DATE_TIME.format(new Date(createdAt)).replace(', ', ' ')
-  return `[${formatted}] `
+  return `[${format(createdAt, 'yyyy-MM-dd HH:mm')}] `
 }