diff --git a/src/features/background-agent/manager.polling.test.ts b/src/features/background-agent/manager.polling.test.ts
index 3da723934..6b3a38f9c 100644
--- a/src/features/background-agent/manager.polling.test.ts
+++ b/src/features/background-agent/manager.polling.test.ts
@@ -238,6 +238,43 @@ describe("BackgroundManager pollRunningTasks", () => {
       expect(task.status).toBe("completed")
       expect(messagesCallCount).toBe(0)
     })
+
+    test("#when todo state was already observed from events #then it completes without fetching todos", async () => {
+      //#given
+      let todoCallCount = 0
+      const manager = createManagerWithClient({
+        status: async () => ({ data: { "ses-idle-todo-cached": { type: "idle" } } }),
+        todo: async () => {
+          todoCallCount += 1
+          return { data: [] }
+        },
+      })
+      const task = createRunningTask("ses-idle-todo-cached")
+      injectTask(manager, task)
+
+      manager.handleEvent({
+        type: "message.part.updated",
+        properties: { sessionID: "ses-idle-todo-cached", type: "text" },
+      })
+      manager.handleEvent({
+        type: "todo.updated",
+        properties: {
+          sessionID: "ses-idle-todo-cached",
+          todos: [
+            { id: "todo-1", content: "done", status: "completed", priority: "high" },
+          ],
+        },
+      })
+
+      //#when
+      const poll = (manager as unknown as { pollRunningTasks: () => Promise<void> }).pollRunningTasks
+      await poll.call(manager)
+      manager.shutdown()
+
+      //#then
+      expect(task.status).toBe("completed")
+      expect(todoCallCount).toBe(0)
+    })
   })
 
   describe("#given a running task whose session status is busy", () => {
diff --git a/src/features/background-agent/manager.ts b/src/features/background-agent/manager.ts
index f9ec17156..b66659abf 100644
--- a/src/features/background-agent/manager.ts
+++ b/src/features/background-agent/manager.ts
@@ -160,6 +160,7 @@ export class BackgroundManager {
   private idleDeferralTimers: Map<string, ReturnType<typeof setTimeout>> = new Map()
   private notificationQueueByParent: Map<string, Promise<void>> = new Map()
   private observedOutputSessions: Set<string> = new Set()
+  private observedIncompleteTodosBySession: Map<string, boolean> = new Map()
   private rootDescendantCounts: Map<string, number>
   private preStartDescendantReservations: Set<string>
   private enableParentSessionNotifications: boolean
@@ -882,17 +883,27 @@ export class BackgroundManager {
   }
 
   private async checkSessionTodos(sessionID: string): Promise<boolean> {
+    const observedIncompleteTodos = this.observedIncompleteTodosBySession.get(sessionID)
+    if (observedIncompleteTodos !== undefined) {
+      return observedIncompleteTodos
+    }
+
     try {
       const response = await this.client.session.todo({
         path: { id: sessionID },
       })
       const todos = normalizeSDKResponse(response, [] as Todo[], { preferResponseOnMissingData: true })
-      if (!todos || todos.length === 0) return false
+      if (!todos || todos.length === 0) {
+        this.observedIncompleteTodosBySession.set(sessionID, false)
+        return false
+      }
 
       const incomplete = todos.filter(
         (t) => t.status !== "completed" && t.status !== "cancelled"
       )
-      return incomplete.length > 0
+      const hasIncompleteTodos = incomplete.length > 0
+      this.observedIncompleteTodosBySession.set(sessionID, hasIncompleteTodos)
+      return hasIncompleteTodos
     } catch (error) {
       log("[background-agent] Failed to check session todos:", {
         sessionID,
@@ -910,6 +921,10 @@ export class BackgroundManager {
     this.observedOutputSessions.delete(sessionID)
   }
 
+  private clearSessionTodoObservation(sessionID: string): void {
+    this.observedIncompleteTodosBySession.delete(sessionID)
+  }
+
   private hasOutputSignalFromPart(partInfo: MessagePartInfo | undefined): boolean {
     if (!partInfo?.sessionID) return false
     if (partInfo.tool) return true
@@ -1047,6 +1062,20 @@ export class BackgroundManager {
       }
     }
 
+    if (event.type === "todo.updated") {
+      const sessionID = typeof props?.sessionID === "string" ? props.sessionID : undefined
+      const todos = Array.isArray(props?.todos) ? props.todos : undefined
+      if (!sessionID || !todos) return
+
+      const hasIncompleteTodos = todos.some((todo) => {
+        if (!todo || typeof todo !== "object") return false
+        const status = (todo as { status?: unknown }).status
+        return status !== "completed" && status !== "cancelled"
+      })
+      this.observedIncompleteTodosBySession.set(sessionID, hasIncompleteTodos)
+      return
+    }
+
     if (event.type === "session.idle") {
       if (!props || typeof props !== "object") return
       handleSessionIdleBackgroundEvent({
@@ -1091,6 +1120,7 @@ export class BackgroundManager {
       if (!info || typeof info.id !== "string") return
       const sessionID = info.id
       this.clearSessionOutputObserved(sessionID)
+      this.clearSessionTodoObservation(sessionID)
 
       const tasksToCancel = new Map<string, BackgroundTask>()
       const directTask = this.findBySession(sessionID)
@@ -1250,6 +1280,7 @@ export class BackgroundManager {
     return result.then((retried) => {
       if (retried && previousSessionID) {
         this.clearSessionOutputObserved(previousSessionID)
+        this.clearSessionTodoObservation(previousSessionID)
         subagentSessions.delete(previousSessionID)
       }
       return retried
diff --git a/src/features/tmux-subagent/manager.ts b/src/features/tmux-subagent/manager.ts
index 2b688d998..7bab2d215 100644
--- a/src/features/tmux-subagent/manager.ts
+++ b/src/features/tmux-subagent/manager.ts
@@ -923,6 +923,10 @@ export class TmuxSessionManager {
     }
   }
 
+  onEvent(event: { type: string; properties?: Record<string, unknown> }): void {
+    this.pollingManager.handleEvent(event)
+  }
+
   createEventHandler(): (input: { event: { type: string; properties?: unknown } }) => Promise<void> {
     return async (input) => {
       await this.onSessionCreated(input.event as SessionCreatedEvent)
diff --git a/src/features/tmux-subagent/polling-manager.test.ts b/src/features/tmux-subagent/polling-manager.test.ts
index 11781d238..060ee23f5 100644
--- a/src/features/tmux-subagent/polling-manager.test.ts
+++ b/src/features/tmux-subagent/polling-manager.test.ts
@@ -55,4 +55,55 @@ describe("TmuxPollingManager overlap", () => {
     expect(maxActiveCalls).toBe(1)
     expect(statusCallCount).toBe(1)
   })
+
+  test("closes stable idle sessions without fetching full messages when activity was already observed from events", async () => {
+    //#given
+    const sessions = new Map<string, TrackedSession>()
+    sessions.set("ses-1", {
+      sessionId: "ses-1",
+      paneId: "%1",
+      description: "test",
+      createdAt: new Date(Date.now() - 15_000),
+      lastSeenAt: new Date(),
+      closePending: false,
+      closeRetryCount: 0,
+      activityVersion: 0,
+    })
+
+    let messagesCallCount = 0
+    const closedSessionIds: string[] = []
+    const client = {
+      session: {
+        status: async () => ({ data: { "ses-1": { type: "idle" } } }),
+        messages: async () => {
+          messagesCallCount += 1
+          return { data: [] }
+        },
+      },
+    }
+
+    const manager = new TmuxPollingManager(
+      client as unknown as import("../../tools/delegate-task/types").OpencodeClient,
+      sessions,
+      async (sessionId) => {
+        closedSessionIds.push(sessionId)
+      },
+    )
+
+    manager.handleEvent({
+      type: "message.part.delta",
+      properties: { sessionID: "ses-1", field: "text", delta: "done" },
+    })
+
+    //#when
+    const pollSessions = (manager as unknown as { pollSessions: () => Promise<void> }).pollSessions
+    await pollSessions.call(manager)
+    await pollSessions.call(manager)
+    await pollSessions.call(manager)
+    await pollSessions.call(manager)
+
+    //#then
+    expect(messagesCallCount).toBe(0)
+    expect(closedSessionIds).toEqual(["ses-1"])
+  })
 })
diff --git a/src/features/tmux-subagent/polling-manager.ts b/src/features/tmux-subagent/polling-manager.ts
index 5cbb45b8f..d7a972d40 100644
--- a/src/features/tmux-subagent/polling-manager.ts
+++ b/src/features/tmux-subagent/polling-manager.ts
@@ -19,6 +19,16 @@ export class TmuxPollingManager {
     private closeSessionById: (sessionId: string) => Promise<void>
   ) {}
 
+  handleEvent(event: { type: string; properties?: Record<string, unknown> }): void {
+    const sessionId = this.getEventSessionId(event)
+    if (!sessionId) return
+
+    const tracked = this.sessions.get(sessionId)
+    if (!tracked) return
+
+    tracked.activityVersion = (tracked.activityVersion ?? 0) + 1
+  }
+
   startPolling(): void {
     if (this.pollInterval) return
 
@@ -73,42 +83,29 @@ export class TmuxPollingManager {
         let shouldCloseViaStability = false
 
         if (isIdle && elapsedMs >= MIN_STABILITY_TIME_MS) {
-          try {
-            const messagesResult = await this.client.session.messages({ 
-              path: { id: sessionId } 
-            })
-            const currentMsgCount = Array.isArray(messagesResult.data) 
-              ? messagesResult.data.length 
-              : 0
+          const activityVersion = tracked.activityVersion ?? 0
 
-            if (tracked.lastMessageCount === currentMsgCount) {
-              tracked.stableIdlePolls = (tracked.stableIdlePolls ?? 0) + 1
-              
-              if (tracked.stableIdlePolls >= STABLE_POLLS_REQUIRED) {
-                const recheckResult = await this.client.session.status({ path: undefined })
-                const recheckStatuses = normalizeSDKResponse(recheckResult, {} as Record<string, { type: string }>)
-                const recheckStatus = recheckStatuses[sessionId]
-                
-                if (recheckStatus?.type === "idle") {
-                  shouldCloseViaStability = true
-                } else {
-                  tracked.stableIdlePolls = 0
-                  log("[tmux-session-manager] stability reached but session not idle on recheck, resetting", {
-                    sessionId,
-                    recheckStatus: recheckStatus?.type,
-                  })
-                }
+          if (tracked.observedIdleActivityVersion === activityVersion) {
+            tracked.stableIdlePolls = (tracked.stableIdlePolls ?? 0) + 1
+
+            if (tracked.stableIdlePolls >= STABLE_POLLS_REQUIRED) {
+              const recheckResult = await this.client.session.status({ path: undefined })
+              const recheckStatuses = normalizeSDKResponse(recheckResult, {} as Record<string, { type: string }>)
+              const recheckStatus = recheckStatuses[sessionId]
+
+              if (recheckStatus?.type === "idle") {
+                shouldCloseViaStability = true
+              } else {
+                tracked.stableIdlePolls = 0
+                log("[tmux-session-manager] stability reached but session not idle on recheck, resetting", {
+                  sessionId,
+                  recheckStatus: recheckStatus?.type,
+                })
               }
-            } else {
-              tracked.stableIdlePolls = 0
             }
-            
-            tracked.lastMessageCount = currentMsgCount
-          } catch (msgErr) {
-            log("[tmux-session-manager] failed to fetch messages for stability check", {
-              sessionId,
-              error: String(msgErr),
-            })
+          } else {
+            tracked.stableIdlePolls = 0
+            tracked.observedIdleActivityVersion = activityVersion
           }
         } else if (!isIdle) {
           tracked.stableIdlePolls = 0
@@ -120,7 +117,8 @@ export class TmuxPollingManager {
           isIdle,
           elapsedMs,
           stableIdlePolls: tracked.stableIdlePolls,
-          lastMessageCount: tracked.lastMessageCount,
+          activityVersion: tracked.activityVersion,
+          observedIdleActivityVersion: tracked.observedIdleActivityVersion,
           missingSince,
           missingTooLong,
           isTimedOut,
@@ -142,4 +140,28 @@ export class TmuxPollingManager {
       this.pollingInFlight = false
     }
   }
+
+  private getEventSessionId(event: { type: string; properties?: Record<string, unknown> }): string | undefined {
+    const properties = event.properties
+    if (!properties) return undefined
+
+    if (event.type === "message.updated") {
+      const info = properties.info
+      if (!info || typeof info !== "object") return undefined
+      const sessionId = (info as { sessionID?: unknown }).sessionID
+      return typeof sessionId === "string" ? sessionId : undefined
+    }
+
+    if (
+      event.type === "message.part.updated"
+      || event.type === "message.part.delta"
+      || event.type === "message.part.removed"
+      || event.type === "message.removed"
+    ) {
+      const sessionId = properties.sessionID
+      return typeof sessionId === "string" ? sessionId : undefined
+    }
+
+    return undefined
+  }
 }
diff --git a/src/features/tmux-subagent/tracked-session-state.ts b/src/features/tmux-subagent/tracked-session-state.ts
index 87ba19f51..9bcf94674 100644
--- a/src/features/tmux-subagent/tracked-session-state.ts
+++ b/src/features/tmux-subagent/tracked-session-state.ts
@@ -16,6 +16,7 @@ export function createTrackedSession(params: {
     lastSeenAt: now,
     closePending: false,
     closeRetryCount: 0,
+    activityVersion: 0,
   }
 }
 
diff --git a/src/features/tmux-subagent/types.ts b/src/features/tmux-subagent/types.ts
index 15d47ab83..db8f88d69 100644
--- a/src/features/tmux-subagent/types.ts
+++ b/src/features/tmux-subagent/types.ts
@@ -9,6 +9,8 @@ export interface TrackedSession {
   // Stability detection fields (prevents premature closure)
   lastMessageCount?: number
   stableIdlePolls?: number
+  activityVersion?: number
+  observedIdleActivityVersion?: number
 }
 
 export const MIN_PANE_WIDTH = 52
diff --git a/src/hooks/background-notification/hook.test.ts b/src/hooks/background-notification/hook.test.ts
index c8566a39d..f32ce14c1 100644
--- a/src/hooks/background-notification/hook.test.ts
+++ b/src/hooks/background-notification/hook.test.ts
@@ -34,4 +34,27 @@ describe("createBackgroundNotificationHook", () => {
     //#then
     expect(handleEvent).toHaveBeenCalledWith(event)
   })
+
+  test("#given todo.updated event #when event handler runs #then it forwards to manager", async () => {
+    //#given
+    const handleEvent = mock(() => {})
+    const hook = createBackgroundNotificationHook({
+      handleEvent,
+      injectPendingNotificationsIntoChatMessage: () => {},
+    } as never)
+
+    const event = {
+      type: "todo.updated",
+      properties: {
+        sessionID: "ses-1",
+        todos: [{ id: "todo-1", content: "done", status: "completed", priority: "high" }],
+      },
+    }
+
+    //#when
+    await hook.event({ event })
+
+    //#then
+    expect(handleEvent).toHaveBeenCalledWith(event)
+  })
 })
diff --git a/src/hooks/background-notification/hook.ts b/src/hooks/background-notification/hook.ts
index e52b7beb4..0e31ba36f 100644
--- a/src/hooks/background-notification/hook.ts
+++ b/src/hooks/background-notification/hook.ts
@@ -21,6 +21,7 @@ const FORWARDED_EVENT_TYPES = new Set([
   "message.updated",
   "message.part.updated",
   "message.part.delta",
+  "todo.updated",
   "session.idle",
   "session.error",
   "session.deleted",
diff --git a/src/plugin/event.test.ts b/src/plugin/event.test.ts
index 3f8e909ad..81addf3bc 100644
--- a/src/plugin/event.test.ts
+++ b/src/plugin/event.test.ts
@@ -447,6 +447,44 @@ afterEach(() => {
 })
 
 describe("createEventHandler - event forwarding", () => {
+	it("forwards message activity events to tmux session manager", async () => {
+		//#given
+		const forwardedEvents: EventInput[] = []
+		const eventHandler = createEventHandler({
+			ctx: asEventHandlerContext({}),
+			pluginConfig: asPluginConfig({}),
+			firstMessageVariantGate: {
+				markSessionCreated: () => {},
+				clear: () => {},
+			},
+			managers: createEventHandlerManagers({
+				skillMcpManager: {
+					disconnectSession: async () => {},
+				},
+				tmuxSessionManager: {
+					onEvent: (event: EventInput["event"]) => {
+						forwardedEvents.push({ event })
+					},
+					onSessionCreated: async () => {},
+					onSessionDeleted: async () => {},
+				},
+			}),
+			hooks: createEventHandlerHooks({}),
+		})
+
+		//#when
+		await eventHandler(asEventHandlerInput({
+			event: {
+				type: "message.part.delta",
+				properties: { sessionID: "ses_tmux_activity", field: "text", delta: "x" },
+			},
+		}))
+
+		//#then
+		expect(forwardedEvents.length).toBe(1)
+		expect(forwardedEvents[0]?.event.type).toBe("message.part.delta")
+	})
+
 	it("forwards session.deleted to write-existing-file-guard hook", async () => {
 		//#given
 		const forwardedEvents: EventInput[] = []
diff --git a/src/plugin/event.ts b/src/plugin/event.ts
index 594f82919..f57947931 100644
--- a/src/plugin/event.ts
+++ b/src/plugin/event.ts
@@ -265,6 +265,13 @@ export function createEventHandler(args: {
   const recentSyntheticIdles = new Map<string, number>();
   const recentRealIdles = new Map<string, number>();
   const DEDUP_WINDOW_MS = 500;
+  const TMUX_ACTIVITY_EVENT_TYPES = new Set([
+    "message.updated",
+    "message.part.updated",
+    "message.part.delta",
+    "message.part.removed",
+    "message.removed",
+  ]);
 
   const shouldAutoRetrySession = (sessionID: string): boolean => {
     if (syncSubagentSessions.has(sessionID)) return true;
@@ -337,6 +344,10 @@ export function createEventHandler(args: {
     const { event } = input;
     const props = event.properties as Record<string, unknown> | undefined;
 
+    if (TMUX_ACTIVITY_EVENT_TYPES.has(event.type)) {
+      managers.tmuxSessionManager.onEvent?.(event as { type: string; properties?: Record<string, unknown> });
+    }
+
     if (event.type === "session.created") {
       const sessionInfo = props?.info as { id?: string; title?: string; parentID?: string } | undefined;