feat(sidebar): use lastEventTime from agent callback for stable sorting

- Add after_agent_callback on ai_writer that writes lastEventTime to
  session state; only fires once per agent turn, not on every LLM call
- Sort sidebar by lastEventTime (falls back to lastUpdateTime for old
  sessions); opening a session no longer causes reordering
- Restore lastOpenedAt in session.state via markSessionOpened for
  cross-device unread tracking; stored in seconds to match time.time()
- Invalidate ['sessions'] query after SSE stream ends so lastEventTime
  is reflected in the sidebar immediately after agent responds

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: MrOrz

adk/cofacts_ai/agent.py

@@ -11,6 +11,7 @@
 from typing import Dict, Optional
 import re
 import json
+import time
 
 from dotenv import load_dotenv
 from google.adk.apps import App
@@ -34,6 +35,17 @@
 # Initialize Langfuse instrumentation for observability
 setup_instrumentation()
 
+# lastEventTime: records when the agent turn last completed, used by the sidebar
+# for sorting and unread-dot logic. We cannot rely on ADK's built-in lastUpdateTime
+# because any session state PATCH (including the client writing lastOpenedAt)
+# bumps it, which would cause sidebar reordering on every session open.
+SESSION_LAST_EVENT_TIME_KEY = 'lastEventTime'
+
+
+async def update_last_event_time(callback_context: CallbackContext) -> None:
+    """Records the current time in session state after each ai_writer agent turn."""
+    callback_context.state[SESSION_LAST_EVENT_TIME_KEY] = time.time()
+
 
 async def append_grounding_sources(
     callback_context: CallbackContext,
@@ -380,6 +392,7 @@ async def append_grounding_sources(
     name="writer",
     model="gemini-2.5-pro",
     description="AI agent that orchestrates fact-checking process and composes final fact-check replies for Cofacts.",
+    after_agent_callback=update_last_event_time,
     instruction=f"""
     You are an AI Writer and orchestrator for the Cofacts fact-checking system. Today is {datetime.now().strftime("%Y-%m-%d")}.
 

src/components/Sidebar.tsx

@@ -4,7 +4,6 @@ import { useQueryClient } from '@tanstack/react-query'
 import type { SessionListItem } from '@/lib/sessions.functions'
 import { useSessions } from '@/hooks/useSessions'
 import { updateSession } from '@/lib/sessions.functions'
-import { getLastOpenedTime } from '@/lib/sessionOpenedTime'
 
 interface SidebarProps {
   isOpen: boolean
@@ -23,13 +22,14 @@ function SessionItem({ session, isActive, onClose }: SessionItemProps) {
   const [editTitle, setEditTitle] = useState('')
 
   const title = session.name
+  const lastActivityTime = session.lastEventTime ?? session.lastUpdateTime
   const hasNew =
     !isActive &&
-    session.lastUpdateTime > 0 &&
-    session.lastUpdateTime > getLastOpenedTime(session.id)
+    lastActivityTime > 0 &&
+    lastActivityTime > (session.lastOpenedAt ?? 0)
 
-  const lastActiveLabel = session.lastUpdateTime
-    ? new Date(session.lastUpdateTime * 1000).toLocaleDateString('zh-TW', {
+  const lastActiveLabel = lastActivityTime
+    ? new Date(lastActivityTime * 1000).toLocaleDateString('zh-TW', {
         month: 'short',
         day: 'numeric',
       })
@@ -141,7 +141,11 @@ export function Sidebar({ isOpen, onClose }: SidebarProps) {
   const { data: rawSessions, isLoading } = useSessions()
   const sessions = rawSessions
     ?.slice()
-    .sort((a, b) => b.lastUpdateTime - a.lastUpdateTime)
+    .sort(
+      (a, b) =>
+        (b.lastEventTime ?? b.lastUpdateTime) -
+        (a.lastEventTime ?? a.lastUpdateTime),
+    )
 
   return (
     <>

src/lib/chatCache.ts

@@ -145,6 +145,8 @@ export async function startChatStream({
     })
     if (abortControllers.get(sessionId) === controller) {
       abortControllers.delete(sessionId)
+      // Refresh session list so lastEventTime (written by agent callback) is picked up immediately.
+      queryClient.invalidateQueries({ queryKey: ['sessions'] })
     }
   }
 }

src/lib/sessionOpenedTime.ts

@@ -4,10 +4,20 @@ import { handleAdkError, handleAdkResponseError } from './adk-errors'
 
 const SESSION_TITLE_KEY = 'title'
 
+// lastEventTime: set by Python after_agent_callback when an agent turn completes.
+// We avoid using ADK's built-in lastUpdateTime because any PATCH to session state
+// (including writing lastOpenedAt) bumps it, causing sidebar sort to jump on session open.
+const SESSION_LAST_EVENT_TIME_KEY = 'lastEventTime'
+
+// lastOpenedAt: set by the client when the user opens a session, for cross-device unread tracking.
+const SESSION_LAST_OPENED_KEY = 'lastOpenedAt'
+
 export interface SessionListItem {
   id: string
   name: string
   lastUpdateTime: number
+  lastEventTime?: number
+  lastOpenedAt?: number
 }
 
 export const listSessions = createServerFn({ method: 'GET' }).handler(
@@ -31,7 +41,15 @@ export const listSessions = createServerFn({ method: 'GET' }).handler(
                 (e) => e.content?.role === 'user' && e.content.parts?.[0]?.text,
               )
               ?.content?.parts?.[0]?.text?.slice(0, 40) ?? session.id)
-      return { id: session.id, name, lastUpdateTime: session.lastUpdateTime }
+      const lastEventTime = session.state?.[SESSION_LAST_EVENT_TIME_KEY]
+      const lastOpenedAt = session.state?.[SESSION_LAST_OPENED_KEY]
+      return {
+        id: session.id,
+        name,
+        lastUpdateTime: session.lastUpdateTime,
+        lastEventTime: typeof lastEventTime === 'number' ? lastEventTime : undefined,
+        lastOpenedAt: typeof lastOpenedAt === 'number' ? lastOpenedAt : undefined,
+      }
     })
   },
 )
@@ -113,3 +131,26 @@ export const updateSession = createServerFn({ method: 'POST' })
     if (error) handleAdkError(error)
     return data
   })
+
+export const markSessionOpened = createServerFn({ method: 'POST' })
+  .inputValidator((sessionId: string) => sessionId)
+  .handler(async ({ data: sessionId }) => {
+    const { error } = await adkClient.PATCH(
+      '/apps/{app_name}/users/{user_id}/sessions/{session_id}',
+      {
+        params: {
+          path: {
+            app_name: ADK_APP_NAME,
+            user_id: ADK_USER_ID,
+            session_id: sessionId,
+          },
+        },
+        // Store as seconds (Date.now()/1000) to match Python's time.time() for comparison.
+        body: {
+          stateDelta: { [SESSION_LAST_OPENED_KEY]: Date.now() / 1000 },
+        },
+      },
+    )
+    if (error) handleAdkError(error)
+    return { ok: true }
+  })

src/lib/sessions.functions.ts

@@ -1,22 +1,26 @@
 import { createFileRoute, useParams } from '@tanstack/react-router'
 import { useEffect } from 'react'
+import { useQueryClient } from '@tanstack/react-query'
 import { ChatArea } from '@/components/ChatArea'
 import { useChat } from '@/hooks/useChat'
-import { setLastOpenedTime } from '@/lib/sessionOpenedTime'
+import { markSessionOpened } from '@/lib/sessions.functions'
 
 export const Route = createFileRoute('/_app/session/$sessionId')({
   component: SessionPage,
 })
 
 function SessionPage() {
   const { sessionId } = useParams({ from: '/_app/session/$sessionId' })
+  const queryClient = useQueryClient()
   const { messages, isStreaming, error, sendMessage, stopGeneration } = useChat(
     { sessionId },
   )
 
   useEffect(() => {
-    setLastOpenedTime(sessionId)
-  }, [sessionId])
+    markSessionOpened({ data: sessionId }).then(() => {
+      queryClient.invalidateQueries({ queryKey: ['sessions'] })
+    })
+  }, [sessionId, queryClient])
 
   return (
     <>