diff --git a/packages/core/src/agent/agent-session.test.ts b/packages/core/src/agent/agent-session.test.ts
index 235b4eb013..e3ff1c5dc0 100644
--- a/packages/core/src/agent/agent-session.test.ts
+++ b/packages/core/src/agent/agent-session.test.ts
@@ -117,6 +117,7 @@ describe('AgentSession', () => {
     expect(events).toHaveLength(0);
     expect(protocol.events).toHaveLength(1);
     expect(protocol.events[0].type).toBe('session_update');
+    expect(protocol.events[0].streamId).toEqual(expect.any(String));
   });
 
   it('should skip events that occur before agent_start', async () => {
@@ -171,6 +172,181 @@ describe('AgentSession', () => {
       expect(streamedEvents).toEqual(allEvents.slice(2));
     });
 
+    it('should complete immediately when resuming from agent_end', async () => {
+      const protocol = new MockAgentProtocol();
+      const session = new AgentSession(protocol);
+
+      protocol.pushResponse([{ type: 'message' }]);
+      const { streamId } = await session.send({
+        message: [{ type: 'text', text: 'request' }],
+      });
+      await new Promise((resolve) => setTimeout(resolve, 10));
+
+      const endEvent = session.events.findLast(
+        (event): event is AgentEvent<'agent_end'> =>
+          event.type === 'agent_end' && event.streamId === streamId,
+      );
+      expect(endEvent).toBeDefined();
+
+      const iterator = session
+        .stream({ eventId: endEvent!.id })
+        [Symbol.asyncIterator]();
+      await expect(iterator.next()).resolves.toEqual({
+        value: undefined,
+        done: true,
+      });
+    });
+
+    it('should throw for an unknown eventId', async () => {
+      const protocol = new MockAgentProtocol();
+      const session = new AgentSession(protocol);
+
+      const iterator = session
+        .stream({ eventId: 'missing-event' })
+        [Symbol.asyncIterator]();
+      await expect(iterator.next()).rejects.toThrow(
+        'Unknown eventId: missing-event',
+      );
+    });
+
+    it('should throw when resuming from an event before agent_start on a stream with no agent activity', async () => {
+      const protocol = new MockAgentProtocol();
+      const session = new AgentSession(protocol);
+
+      const { streamId } = await session.send({ update: { title: 'draft' } });
+      expect(streamId).toBeNull();
+
+      const updateEvent = session.events.find(
+        (event): event is AgentEvent<'session_update'> =>
+          event.type === 'session_update',
+      );
+      expect(updateEvent).toBeDefined();
+
+      const iterator = session
+        .stream({ eventId: updateEvent!.id })
+        [Symbol.asyncIterator]();
+      await expect(iterator.next()).rejects.toThrow(
+        `Cannot resume from eventId ${updateEvent!.id} before agent_start for stream ${updateEvent!.streamId}`,
+      );
+    });
+
+    it('should replay from agent_start when resuming from a pre-agent_start event after activity is in history', async () => {
+      const protocol = new MockAgentProtocol();
+      const session = new AgentSession(protocol);
+
+      protocol.pushResponse([
+        {
+          type: 'message',
+          role: 'agent',
+          content: [{ type: 'text', text: 'hello' }],
+        },
+      ]);
+      await session.send({
+        message: [{ type: 'text', text: 'request' }],
+      });
+      await new Promise((resolve) => setTimeout(resolve, 10));
+
+      const userMessage = session.events.find(
+        (event): event is AgentEvent<'message'> =>
+          event.type === 'message' && event.role === 'user',
+      );
+      expect(userMessage).toBeDefined();
+
+      const streamedEvents: AgentEvent[] = [];
+      for await (const event of session.stream({ eventId: userMessage!.id })) {
+        streamedEvents.push(event);
+      }
+
+      expect(streamedEvents.map((event) => event.type)).toEqual([
+        'agent_start',
+        'message',
+        'agent_end',
+      ]);
+      expect(streamedEvents[0]?.streamId).toBe(userMessage!.streamId);
+    });
+
+    it('should throw when resuming from a pre-agent_start event before activity is in history', async () => {
+      const protocol = new MockAgentProtocol([
+        {
+          id: 'e-1',
+          timestamp: '2026-01-01T00:00:00.000Z',
+          streamId: 'stream-1',
+          type: 'message',
+          role: 'user',
+          content: [{ type: 'text', text: 'request' }],
+        },
+      ]);
+      const session = new AgentSession(protocol);
+
+      const iterator = session
+        .stream({ eventId: 'e-1' })
+        [Symbol.asyncIterator]();
+      await expect(iterator.next()).rejects.toThrow(
+        'Cannot resume from eventId e-1 before agent_start for stream stream-1',
+      );
+    });
+
+    it('should resume from an in-stream event within the same stream only', async () => {
+      const protocol = new MockAgentProtocol();
+      const session = new AgentSession(protocol);
+
+      protocol.pushResponse([
+        {
+          type: 'message',
+          role: 'agent',
+          content: [{ type: 'text', text: 'first answer 1' }],
+        },
+        {
+          type: 'message',
+          role: 'agent',
+          content: [{ type: 'text', text: 'first answer 2' }],
+        },
+      ]);
+      const { streamId: streamId1 } = await session.send({
+        message: [{ type: 'text', text: 'first request' }],
+      });
+      await new Promise((resolve) => setTimeout(resolve, 10));
+
+      protocol.pushResponse([
+        {
+          type: 'message',
+          role: 'agent',
+          content: [{ type: 'text', text: 'second answer' }],
+        },
+      ]);
+      await session.send({
+        message: [{ type: 'text', text: 'second request' }],
+      });
+      await new Promise((resolve) => setTimeout(resolve, 10));
+
+      const resumeEvent = session.events.find(
+        (event): event is AgentEvent<'message'> =>
+          event.type === 'message' &&
+          event.streamId === streamId1 &&
+          event.role === 'agent' &&
+          event.content[0]?.type === 'text' &&
+          event.content[0].text === 'first answer 1',
+      );
+      expect(resumeEvent).toBeDefined();
+
+      const streamedEvents: AgentEvent[] = [];
+      for await (const event of session.stream({ eventId: resumeEvent!.id })) {
+        streamedEvents.push(event);
+      }
+
+      expect(
+        streamedEvents.every((event) => event.streamId === streamId1),
+      ).toBe(true);
+      expect(streamedEvents.map((event) => event.type)).toEqual([
+        'message',
+        'agent_end',
+      ]);
+      const resumedMessage = streamedEvents[0] as AgentEvent<'message'>;
+      expect(resumedMessage.content).toEqual([
+        { type: 'text', text: 'first answer 2' },
+      ]);
+    });
+
     it('should replay events for streamId starting with agent_start', async () => {
       const protocol = new MockAgentProtocol();
       const session = new AgentSession(protocol);
@@ -223,6 +399,33 @@ describe('AgentSession', () => {
       expect(streamedEvents.at(-1)?.type).toBe('agent_end');
     });
 
+    it('should not drop agent_end that arrives while replay events are being yielded', async () => {
+      const protocol = new MockAgentProtocol();
+      const session = new AgentSession(protocol);
+
+      protocol.pushResponse([{ type: 'message' }], { keepOpen: true });
+      const { streamId } = await session.send({ update: { title: 't1' } });
+      await new Promise((resolve) => setTimeout(resolve, 10));
+
+      const iterator = session
+        .stream({ streamId: streamId! })
+        [Symbol.asyncIterator]();
+
+      const first = await iterator.next();
+      expect(first.value?.type).toBe('agent_start');
+
+      protocol.pushToStream(streamId!, [], { close: true });
+
+      const second = await iterator.next();
+      expect(second.value?.type).toBe('message');
+
+      const third = await iterator.next();
+      expect(third.value?.type).toBe('agent_end');
+
+      const fourth = await iterator.next();
+      expect(fourth.done).toBe(true);
+    });
+
     it('should follow an active stream if no options provided', async () => {
       const protocol = new MockAgentProtocol();
       const session = new AgentSession(protocol);
diff --git a/packages/core/src/agent/agent-session.ts b/packages/core/src/agent/agent-session.ts
index 0d9fc86bb0..6a4c295fc8 100644
--- a/packages/core/src/agent/agent-session.ts
+++ b/packages/core/src/agent/agent-session.ts
@@ -34,7 +34,7 @@ export class AgentSession implements AgentProtocol {
     return this._protocol.abort();
   }
 
-  get events(): AgentEvent[] {
+  get events(): readonly AgentEvent[] {
     return this._protocol.events;
   }
 
@@ -77,6 +77,30 @@ export class AgentSession implements AgentProtocol {
     let done = false;
     let trackedStreamId = options.streamId;
     let started = false;
+    let agentActivityStarted = false;
+
+    const queueVisibleEvent = (event: AgentEvent): void => {
+      if (trackedStreamId && event.streamId !== trackedStreamId) {
+        return;
+      }
+
+      if (!agentActivityStarted) {
+        if (event.type !== 'agent_start') {
+          return;
+        }
+        trackedStreamId = event.streamId;
+        agentActivityStarted = true;
+      }
+
+      if (!trackedStreamId) {
+        return;
+      }
+
+      eventQueue.push(event);
+      if (event.type === 'agent_end' && event.streamId === trackedStreamId) {
+        done = true;
+      }
+    };
 
     // 1. Subscribe early to avoid missing any events that occur during replay setup
     const unsubscribe = this._protocol.subscribe((event) => {
@@ -87,23 +111,7 @@ export class AgentSession implements AgentProtocol {
         return;
       }
 
-      if (trackedStreamId && event.streamId !== trackedStreamId) return;
-
-      // If we don't have a tracked stream yet, the first agent_start we see becomes it.
-      if (!trackedStreamId && event.type === 'agent_start') {
-        trackedStreamId = event.streamId ?? undefined;
-      }
-
-      // If we still don't have a tracked stream and we aren't replaying everything (eventId), ignore.
-      if (!trackedStreamId && !options.eventId) return;
-
-      eventQueue.push(event);
-      if (
-        event.type === 'agent_end' &&
-        event.streamId === (trackedStreamId ?? null)
-      ) {
-        done = true;
-      }
+      queueVisibleEvent(event);
 
       const currentResolve = resolve;
       next = new Promise<void>((r) => {
@@ -118,8 +126,42 @@ export class AgentSession implements AgentProtocol {
 
       if (options.eventId) {
         const index = currentEvents.findIndex((e) => e.id === options.eventId);
-        if (index !== -1) {
+        if (index === -1) {
+          throw new Error(`Unknown eventId: ${options.eventId}`);
+        }
+
+        const resumeEvent = currentEvents[index];
+        trackedStreamId = resumeEvent.streamId;
+        const firstAgentStartIndex = currentEvents.findIndex(
+          (event) =>
+            event.type === 'agent_start' && event.streamId === trackedStreamId,
+        );
+
+        if (resumeEvent.type === 'agent_end') {
           replayStartIndex = index + 1;
+          agentActivityStarted = true;
+          done = true;
+        } else if (
+          firstAgentStartIndex !== -1 &&
+          firstAgentStartIndex <= index
+        ) {
+          replayStartIndex = index + 1;
+          agentActivityStarted = true;
+        } else if (firstAgentStartIndex !== -1) {
+          // A pre-agent_start cursor can be resumed once the corresponding
+          // agent activity is already present in history. Because stream()
+          // yields only agent_start -> agent_end, replay begins at agent_start
+          // rather than at the original pre-start event.
+          replayStartIndex = firstAgentStartIndex;
+        } else {
+          // Consumers can only resume by eventId once the corresponding stream
+          // has entered the agent_start -> agent_end lifecycle in history.
+          // Without a recorded agent_start, this wrapper cannot distinguish
+          // "agent activity may start later" from "this send was acknowledged
+          // without agent activity" without risking an infinite wait.
+          throw new Error(
+            `Cannot resume from eventId ${options.eventId} before agent_start for stream ${trackedStreamId}`,
+          );
         }
       } else if (options.streamId) {
         const index = currentEvents.findIndex(
@@ -128,29 +170,7 @@ export class AgentSession implements AgentProtocol {
         if (index !== -1) {
           replayStartIndex = index;
         }
-      }
-
-      if (replayStartIndex !== -1) {
-        for (let i = replayStartIndex; i < currentEvents.length; i++) {
-          const event = currentEvents[i];
-          if (options.streamId && event.streamId !== options.streamId) continue;
-
-          eventQueue.push(event);
-          if (event.type === 'agent_start' && !trackedStreamId) {
-            trackedStreamId = event.streamId ?? undefined;
-          }
-          if (
-            event.type === 'agent_end' &&
-            event.streamId === (trackedStreamId ?? null)
-          ) {
-            done = true;
-            break;
-          }
-        }
-      }
-
-      if (!done && !trackedStreamId) {
-        // Find active stream in history
+      } else {
         const activeStarts = currentEvents.filter(
           (e) => e.type === 'agent_start',
         );
@@ -161,36 +181,28 @@ export class AgentSession implements AgentProtocol {
               (e) => e.type === 'agent_end' && e.streamId === start.streamId,
             )
           ) {
-            trackedStreamId = start.streamId ?? undefined;
+            trackedStreamId = start.streamId;
+            replayStartIndex = currentEvents.findIndex(
+              (e) => e.id === start.id,
+            );
             break;
           }
         }
       }
 
-      // If we replayed to the end and no stream is active, and we were specifically
-      // replaying from an eventId (or we've already finished the stream we were looking for), we are done.
-      if (!done && !trackedStreamId && options.eventId) {
-        done = true;
+      if (replayStartIndex !== -1) {
+        for (let i = replayStartIndex; i < currentEvents.length; i++) {
+          const event = currentEvents[i];
+          queueVisibleEvent(event);
+          if (done) break;
+        }
       }
-
       started = true;
 
       // Process events that arrived while we were replaying
       for (const event of earlyEvents) {
         if (done) break;
-        if (trackedStreamId && event.streamId !== trackedStreamId) continue;
-        if (!trackedStreamId && event.type === 'agent_start') {
-          trackedStreamId = event.streamId ?? undefined;
-        }
-        if (!trackedStreamId && !options.eventId) continue;
-
-        eventQueue.push(event);
-        if (
-          event.type === 'agent_end' &&
-          event.streamId === (trackedStreamId ?? null)
-        ) {
-          done = true;
-        }
+        queueVisibleEvent(event);
       }
 
       while (true) {
@@ -200,6 +212,7 @@ export class AgentSession implements AgentProtocol {
           for (const event of eventsToYield) {
             yield event;
           }
+          continue;
         }
 
         if (done) break;
diff --git a/packages/core/src/agent/mock.test.ts b/packages/core/src/agent/mock.test.ts
index 4f102d5dbd..f5138e388a 100644
--- a/packages/core/src/agent/mock.test.ts
+++ b/packages/core/src/agent/mock.test.ts
@@ -235,7 +235,7 @@ describe('MockAgentProtocol', () => {
     expect(streamId).toBeNull();
     expect(session.events).toHaveLength(1);
     expect(session.events[0].type).toBe('session_update');
-    expect(session.events[0].streamId).toBeNull();
+    expect(session.events[0].streamId).toEqual(expect.any(String));
   });
 
   it('should throw on action', async () => {
diff --git a/packages/core/src/agent/mock.ts b/packages/core/src/agent/mock.ts
index 683e3e0b2a..80d8ebae2f 100644
--- a/packages/core/src/agent/mock.ts
+++ b/packages/core/src/agent/mock.ts
@@ -8,8 +8,8 @@ import type {
   AgentEvent,
   AgentEventCommon,
   AgentEventData,
-  AgentSend,
   AgentProtocol,
+  AgentSend,
   Unsubscribe,
 } from './types.js';
 
@@ -86,13 +86,7 @@ export class MockAgentProtocol implements AgentProtocol {
   ) {
     const now = new Date().toISOString();
     for (const eventData of events) {
-      // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-      const event: AgentEvent = {
-        ...eventData,
-        id: eventData.id ?? `e-${this._nextEventId++}`,
-        timestamp: eventData.timestamp ?? now,
-        streamId: eventData.streamId ?? streamId,
-      } as AgentEvent;
+      const event = this._normalizeEvent(eventData, now, streamId);
       this._emit(event);
     }
 
@@ -100,13 +94,13 @@ export class MockAgentProtocol implements AgentProtocol {
       options?.close &&
       !events.some((eventData) => eventData.type === 'agent_end')
     ) {
-      this._emit({
-        id: `e-${this._nextEventId++}`,
-        timestamp: now,
-        streamId,
-        type: 'agent_end',
-        reason: 'completed',
-      } as AgentEvent);
+      this._emit(
+        this._normalizeEvent(
+          { type: 'agent_end', reason: 'completed' },
+          now,
+          streamId,
+        ),
+      );
     }
   }
 
@@ -124,16 +118,18 @@ export class MockAgentProtocol implements AgentProtocol {
 
     const now = new Date().toISOString();
     const eventsToEmit: AgentEvent[] = [];
+    let fallbackStreamId: string | undefined;
 
-    // Helper to normalize and prepare for emission
+    // All emitted events stay correlated to a stream even if this send does not
+    // start agent activity and therefore returns `streamId: null`.
     const normalize = (eventData: MockAgentEvent): AgentEvent =>
-      // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-      ({
-        ...eventData,
-        id: eventData.id ?? `e-${this._nextEventId++}`,
-        timestamp: eventData.timestamp ?? now,
-        streamId: eventData.streamId ?? streamId,
-      }) as AgentEvent;
+      this._normalizeEvent(
+        eventData,
+        now,
+        eventData.streamId ??
+          streamId ??
+          (fallbackStreamId ??= `mock-stream-${this._nextStreamId++}`),
+      );
 
     // 1. User/Update event (BEFORE agent_start)
     if ('message' in payload && payload.message) {
@@ -225,16 +221,32 @@ export class MockAgentProtocol implements AgentProtocol {
     return { streamId };
   }
 
+  private _normalizeEvent(
+    eventData: MockAgentEvent,
+    timestamp: string,
+    streamId: string,
+  ): AgentEvent {
+    // TypeScript loses the specific union member when we add common event
+    // fields here, so keep the narrowing local to this mock-only helper.
+    // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+    return {
+      ...eventData,
+      id: eventData.id ?? `e-${this._nextEventId++}`,
+      timestamp: eventData.timestamp ?? timestamp,
+      streamId: eventData.streamId ?? streamId,
+    } as AgentEvent;
+  }
+
   async abort(): Promise<void> {
     if (this._lastStreamId && this._activeStreamIds.has(this._lastStreamId)) {
       const streamId = this._lastStreamId;
-      this._emit({
-        id: `e-${this._nextEventId++}`,
-        timestamp: new Date().toISOString(),
-        streamId,
-        type: 'agent_end',
-        reason: 'aborted',
-      } as AgentEvent);
+      this._emit(
+        this._normalizeEvent(
+          { type: 'agent_end', reason: 'aborted' },
+          new Date().toISOString(),
+          streamId,
+        ),
+      );
     }
   }
 }
diff --git a/packages/core/src/agent/types.ts b/packages/core/src/agent/types.ts
index 014998d68b..4ec369d066 100644
--- a/packages/core/src/agent/types.ts
+++ b/packages/core/src/agent/types.ts
@@ -11,9 +11,10 @@ export type Unsubscribe = () => void;
 export interface AgentProtocol extends Trajectory {
   /**
    * Send data to the agent. Promise resolves when action is acknowledged.
-   * Returns the `streamId` of the stream the message was correlated to --
-   * this may be a new stream if idle, an existing stream, or null if no
-   * stream was triggered.
+   * Returns the agent-activity `streamId` affected by the send. This may be a
+   * new stream if idle, an existing stream, or null if the send was
+   * acknowledged without starting agent activity. Emitted events should still
+   * remain correlated to a stream via their `streamId`.
    *
    * When a new stream is created by a send, the streamId MUST be returned
    * before the `agent_start` event is emitted for the stream.
@@ -36,7 +37,7 @@ export interface AgentProtocol extends Trajectory {
   /**
    * AgentProtocol implements the Trajectory interface and can retrieve existing events.
    */
-  readonly events: AgentEvent[];
+  readonly events: readonly AgentEvent[];
 }
 
 type RequireExactlyOne<T> = {
@@ -54,7 +55,7 @@ interface AgentSendPayloads {
 export type AgentSend = RequireExactlyOne<AgentSendPayloads> & WithMeta;
 
 export interface Trajectory {
-  readonly events: AgentEvent[];
+  readonly events: readonly AgentEvent[];
 }
 
 export interface AgentEventCommon {
@@ -62,8 +63,8 @@ export interface AgentEventCommon {
   id: string;
   /** Identifies the subagent thread, omitted for "main thread" events. */
   threadId?: string;
-  /** Identifies a particular stream of a particular thread. */
-  streamId?: string | null;
+  /** Identifies the stream this event belongs to. */
+  streamId: string;
   /** ISO Timestamp for the time at which the event occurred. */
   timestamp: string;
   /** The concrete type of the event. */