diff --git a/packages/server/src/server/agent/providers/acp-agent.test.ts b/packages/server/src/server/agent/providers/acp-agent.test.ts
index 546ff8c80..828b1b4de 100644
--- a/packages/server/src/server/agent/providers/acp-agent.test.ts
+++ b/packages/server/src/server/agent/providers/acp-agent.test.ts
@@ -2208,6 +2208,93 @@ describe("ACPAgentSession", () => {
     expect(assistantMessages[2].messageId).not.toBe(assistantMessages[0].messageId);
   });
 
+  test("starts an autonomous turn for spontaneous session updates outside a foreground turn", async () => {
+    vi.useFakeTimers({ shouldAdvanceTime: true });
+    const session = createSession();
+    asInternals<ACPSessionInternals>(session).sessionId = "session-1";
+
+    const events: AgentStreamEvent[] = [];
+    session.subscribe((event) => {
+      events.push(event);
+    });
+
+    await session.sessionUpdate({
+      sessionId: "session-1",
+      update: {
+        sessionUpdate: "agent_message_chunk",
+        messageId: "autonomous-msg",
+        content: { type: "text", text: "Autonomous update" },
+      } as SessionUpdate,
+    });
+
+    // Should emit turn_started before the timeline item.
+    const turnStartedIndex = events.findIndex((e) => e.type === "turn_started");
+    expect(turnStartedIndex).toBeGreaterThanOrEqual(0);
+    const timelineIndex = events.findIndex(
+      (e) =>
+        e.type === "timeline" &&
+        e.item.type === "assistant_message" &&
+        e.item.text === "Autonomous update",
+    );
+    expect(timelineIndex).toBeGreaterThan(turnStartedIndex);
+
+    const turnStarted = events[turnStartedIndex];
+    expect(turnStarted.type).toBe("turn_started");
+    const autonomousTurnId = (turnStarted as { turnId?: string }).turnId;
+    expect(autonomousTurnId).toEqual(expect.any(String));
+
+    // Timeline item should be tagged with the autonomous turn id.
+    const timelineEvent = events[timelineIndex];
+    expect(timelineEvent.type).toBe("timeline");
+    expect((timelineEvent as { turnId?: string }).turnId).toBe(autonomousTurnId);
+
+    // Advance timers to complete the autonomous turn.
+    await vi.advanceTimersByTimeAsync(ACPAgentSession["AUTONOMOUS_TURN_TIMEOUT_MS"] + 10);
+    const turnCompleted = events.find((e) => e.type === "turn_completed");
+    expect(turnCompleted).toBeDefined();
+    expect((turnCompleted as { turnId?: string }).turnId).toBe(autonomousTurnId);
+
+    vi.useRealTimers();
+  });
+
+  test("completes an existing autonomous turn before starting a foreground turn", async () => {
+    vi.useFakeTimers({ shouldAdvanceTime: true });
+    const session = createSession();
+    asInternals<ACPSessionInternals>(session).sessionId = "session-1";
+    asInternals<ACPSessionInternals>(session).connection = {
+      prompt: vi.fn(() => new Promise(() => {})),
+    };
+
+    const events: AgentStreamEvent[] = [];
+    session.subscribe((event) => {
+      events.push(event);
+    });
+
+    await session.sessionUpdate({
+      sessionId: "session-1",
+      update: {
+        sessionUpdate: "agent_message_chunk",
+        messageId: "autonomous-msg",
+        content: { type: "text", text: "Autonomous update" },
+      } as SessionUpdate,
+    });
+
+    const turnStarted = events.find((e) => e.type === "turn_started");
+    expect(turnStarted).toBeDefined();
+    const autonomousTurnId = (turnStarted as { turnId?: string }).turnId;
+
+    // Starting a foreground turn should complete the autonomous turn first.
+    void session.startTurn("user prompt");
+    await vi.runOnlyPendingTimersAsync();
+
+    const turnCompleted = events.find(
+      (e) => e.type === "turn_completed" && (e as { turnId?: string }).turnId === autonomousTurnId,
+    );
+    expect(turnCompleted).toBeDefined();
+
+    vi.useRealTimers();
+  });
+
   test("startTurn returns before the ACP prompt settles and completes later via subscribers", async () => {
     const session = createSession();
     const events: Array<{ type: string; turnId?: string }> = [];
diff --git a/packages/server/src/server/agent/providers/acp-agent.ts b/packages/server/src/server/agent/providers/acp-agent.ts
index 6da37dad6..51e24c669 100644
--- a/packages/server/src/server/agent/providers/acp-agent.ts
+++ b/packages/server/src/server/agent/providers/acp-agent.ts
@@ -1331,6 +1331,9 @@ export class ACPAgentSession implements AgentSession, ACPClient {
   private readonly extensionCommandsParser?: ACPExtensionCommandsParser;
   private currentTurnUsage: AgentUsage | undefined;
   private activeForegroundTurnId: string | null = null;
+  private autonomousTurnId: string | null = null;
+  private autonomousTurnTimer: ReturnType<typeof setTimeout> | null = null;
+  private static readonly AUTONOMOUS_TURN_TIMEOUT_MS = 30_000;
   private fallbackAssistantMessageId: string | null = null;
   private closed = false;
   private historyPending = false;
@@ -1472,6 +1475,7 @@ export class ACPAgentSession implements AgentSession, ACPClient {
     if (this.activeForegroundTurnId) {
       throw new Error("A foreground turn is already active");
     }
+    this.completeAutonomousTurn();
 
     const turnId = randomUUID();
     const messageId = options?.messageId ?? randomUUID();
@@ -2115,7 +2119,7 @@ export class ACPAgentSession implements AgentSession, ACPClient {
         agentId: this.agentId,
         provider: this.provider,
         sessionId: this.sessionId,
-        turnId: this.activeForegroundTurnId ?? undefined,
+        turnId: this.activeForegroundTurnId ?? this.autonomousTurnId ?? undefined,
         rawEvent: params,
         events,
       },
@@ -2130,6 +2134,11 @@ export class ACPAgentSession implements AgentSession, ACPClient {
       return;
     }
 
+    if (events.length > 0 && !this.activeForegroundTurnId) {
+      this.startAutonomousTurn();
+      this.resetAutonomousTurnTimer();
+    }
+
     for (const event of events) {
       this.pushEvent(event);
     }
@@ -2663,23 +2672,25 @@ export class ACPAgentSession implements AgentSession, ACPClient {
       type: "timeline",
       provider: this.provider,
       item,
-      turnId: this.activeForegroundTurnId ?? undefined,
+      turnId: this.activeForegroundTurnId ?? this.autonomousTurnId ?? undefined,
     };
   }
 
   private pushEvent(event: AgentStreamEvent): void {
+    const turnId = this.activeForegroundTurnId ?? this.autonomousTurnId;
+    const tagged = event.type === "timeline" && turnId ? { ...event, turnId } : event;
     this.logger.trace(
       {
         agentId: this.agentId,
         provider: this.provider,
         sessionId: this.sessionId,
-        turnId: getAgentStreamEventTurnId(event) ?? this.activeForegroundTurnId ?? undefined,
-        event,
+        turnId: getAgentStreamEventTurnId(tagged) ?? turnId ?? undefined,
+        event: tagged,
       },
       "provider.acp.event_emit",
     );
     for (const subscriber of this.subscribers) {
-      subscriber(event);
+      subscriber(tagged);
     }
   }
 
@@ -2727,6 +2738,41 @@ export class ACPAgentSession implements AgentSession, ACPClient {
     this.pushEvent(event);
   }
 
+  private startAutonomousTurn(): void {
+    if (this.autonomousTurnId) {
+      return;
+    }
+    this.autonomousTurnId = randomUUID();
+    this.pushEvent({
+      type: "turn_started",
+      provider: this.provider,
+      turnId: this.autonomousTurnId,
+    });
+  }
+
+  private completeAutonomousTurn(): void {
+    if (!this.autonomousTurnId) {
+      return;
+    }
+    if (this.autonomousTurnTimer) {
+      clearTimeout(this.autonomousTurnTimer);
+      this.autonomousTurnTimer = null;
+    }
+    const turnId = this.autonomousTurnId;
+    this.autonomousTurnId = null;
+    this.pushEvent({ type: "turn_completed", provider: this.provider, turnId });
+  }
+
+  private resetAutonomousTurnTimer(): void {
+    if (this.autonomousTurnTimer) {
+      clearTimeout(this.autonomousTurnTimer);
+    }
+    this.autonomousTurnTimer = setTimeout(() => {
+      this.completeAutonomousTurn();
+    }, ACPAgentSession.AUTONOMOUS_TURN_TIMEOUT_MS);
+    this.autonomousTurnTimer.unref?.();
+  }
+
   private isSubmittedUserMessageEcho(
     item: Extract<AgentTimelineItem, { type: "user_message" }>,
   ): boolean {