diff --git a/packages/core/src/agents/remote-subagent-protocol.test.ts b/packages/core/src/agents/remote-subagent-protocol.test.ts
new file mode 100644
index 0000000000..ccf1390e73
--- /dev/null
+++ b/packages/core/src/agents/remote-subagent-protocol.test.ts
@@ -0,0 +1,947 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  describe,
+  it,
+  expect,
+  vi,
+  beforeEach,
+  afterEach,
+  type Mock,
+} from 'vitest';
+import { RemoteSubagentSession } from './remote-subagent-protocol.js';
+
+import { A2AAuthProviderFactory } from './auth-provider/factory.js';
+import type { RemoteAgentDefinition, SubagentProgress } from './types.js';
+import { createMockMessageBus } from '../test-utils/mock-message-bus.js';
+import type { AgentLoopContext } from '../config/agent-loop-context.js';
+import type { AgentEvent } from '../agent/types.js';
+import type { Config } from '../config/config.js';
+import type { A2AAuthProvider } from './auth-provider/types.js';
+
+// Mock A2AClientManager at module level
+vi.mock('./a2a-client-manager.js', () => ({
+  A2AClientManager: vi.fn().mockImplementation(() => ({
+    getClient: vi.fn(),
+    loadAgent: vi.fn(),
+    sendMessageStream: vi.fn(),
+  })),
+}));
+
+// Mock A2AAuthProviderFactory
+vi.mock('./auth-provider/factory.js', () => ({
+  A2AAuthProviderFactory: {
+    create: vi.fn(),
+  },
+}));
+
+const mockDefinition: RemoteAgentDefinition = {
+  name: 'test-remote-agent',
+  kind: 'remote',
+  agentCardUrl: 'http://test-agent/card',
+  displayName: 'Test Remote Agent',
+  description: 'A test remote agent',
+  inputConfig: {
+    inputSchema: { type: 'object' },
+  },
+};
+
+function makeChunk(text: string) {
+  return {
+    kind: 'message' as const,
+    messageId: `msg-${Math.random()}`,
+    role: 'agent' as const,
+    parts: [{ kind: 'text' as const, text }],
+  };
+}
+
+describe('RemoteSubagentSession (protocol)', () => {
+  let mockClientManager: {
+    getClient: Mock;
+    loadAgent: Mock;
+    sendMessageStream: Mock;
+  };
+  let mockContext: AgentLoopContext;
+  let mockMessageBus: ReturnType<typeof createMockMessageBus>;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+
+    // Each test creates fresh session instances. contextId/taskId persist as
+    // instance fields within a session, not via static state.
+
+    mockClientManager = {
+      getClient: vi.fn().mockReturnValue(undefined), // client not yet loaded
+      loadAgent: vi.fn().mockResolvedValue(undefined),
+      sendMessageStream: vi.fn(),
+    };
+
+    const mockConfig = {
+      getA2AClientManager: vi.fn().mockReturnValue(mockClientManager),
+      injectionService: {
+        getLatestInjectionIndex: vi.fn().mockReturnValue(0),
+      },
+    } as unknown as Config;
+
+    mockContext = { config: mockConfig } as unknown as AgentLoopContext;
+    mockMessageBus = createMockMessageBus();
+
+    // Default: sendMessageStream yields one chunk with "Hello"
+    mockClientManager.sendMessageStream.mockImplementation(async function* () {
+      yield makeChunk('Hello');
+    });
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  // Helper: run a session with the default or custom stream and collect events
+  async function runSession(
+    definition: RemoteAgentDefinition = mockDefinition,
+    query = 'test query',
+  ) {
+    const session = new RemoteSubagentSession(
+      definition,
+      mockContext,
+      mockMessageBus,
+    );
+    const events: AgentEvent[] = [];
+    session.subscribe((e) => events.push(e));
+    await session.send({
+      message: { content: [{ type: 'text', text: query }] },
+    });
+    const result = await session.getResult();
+    return { session, events, result };
+  }
+
+  // ---------------------------------------------------------------------------
+  // Lifecycle events
+  // ---------------------------------------------------------------------------
+
+  describe('lifecycle events', () => {
+    it('emits agent_start then agent_end(completed) on success', async () => {
+      const { events } = await runSession();
+
+      const types = events.map((e) => e.type);
+      expect(types[0]).toBe('agent_start');
+      expect(types[types.length - 1]).toBe('agent_end');
+      const end = events[events.length - 1];
+      if (end.type === 'agent_end') {
+        expect(end.reason).toBe('completed');
+      }
+    });
+
+    it('emits agent_start exactly once', async () => {
+      const { events } = await runSession();
+      expect(events.filter((e) => e.type === 'agent_start')).toHaveLength(1);
+    });
+
+    it('emits agent_end exactly once on error path', async () => {
+      mockClientManager.sendMessageStream.mockReturnValue({
+        [Symbol.asyncIterator]() {
+          return {
+            async next(): Promise<IteratorResult<never>> {
+              throw new Error('stream error');
+            },
+          };
+        },
+      });
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      const events: AgentEvent[] = [];
+      session.subscribe((e) => events.push(e));
+      await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await expect(session.getResult()).rejects.toThrow('stream error');
+
+      expect(events.filter((e) => e.type === 'agent_end')).toHaveLength(1);
+    });
+
+    it('all events share the same streamId', async () => {
+      const { events } = await runSession();
+      const streamIds = new Set(events.map((e) => e.streamId));
+      expect(streamIds.size).toBe(1);
+    });
+
+    it('message returns a non-null streamId; unsupported payload returns null', async () => {
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      const updateResult = await session.send({
+        update: { config: { key: 'val' } },
+      });
+      expect(updateResult.streamId).toBeNull();
+
+      const messageResult = await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      expect(messageResult.streamId).not.toBeNull();
+      // complete the session to avoid dangling execution
+      await session.getResult();
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // Chunk → AgentEvent translation
+  // ---------------------------------------------------------------------------
+
+  describe('chunk → AgentEvent translation', () => {
+    it('each A2A chunk produces a message event with incremental delta text', async () => {
+      mockClientManager.sendMessageStream.mockImplementation(
+        async function* () {
+          yield makeChunk('Hello');
+          yield makeChunk(' world');
+        },
+      );
+
+      const { events } = await runSession();
+
+      const msgEvents = events.filter((e) => e.type === 'message');
+      expect(msgEvents).toHaveLength(2);
+      // Each message event contains only the delta, not accumulated text
+      if (msgEvents[0]?.type === 'message') {
+        const text = msgEvents[0].content.find((c) => c.type === 'text');
+        expect(text?.type === 'text' && text.text).toBe('Hello');
+      }
+      if (msgEvents[1]?.type === 'message') {
+        const text = msgEvents[1].content.find((c) => c.type === 'text');
+        expect(text?.type === 'text' && text.text).toBe(' world');
+      }
+    });
+
+    it('getLatestProgress() is updated per chunk with state running', async () => {
+      let capturedProgress: SubagentProgress | undefined;
+
+      mockClientManager.sendMessageStream.mockImplementation(
+        async function* () {
+          yield makeChunk('Partial');
+        },
+      );
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      session.subscribe((e) => {
+        if (e.type === 'message') {
+          capturedProgress = session.getLatestProgress();
+        }
+      });
+
+      await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await session.getResult();
+
+      // During streaming, progress should be 'running'
+      expect(capturedProgress).toBeDefined();
+      // Note: by the time we check, progress may be 'completed'.
+      // During the message event, it was 'running'.
+      expect(capturedProgress?.isSubagentProgress).toBe(true);
+      expect(capturedProgress?.agentName).toBe('Test Remote Agent');
+    });
+
+    it('getLatestProgress() state is completed after getResult() resolves', async () => {
+      const { session } = await runSession();
+      const progress = session.getLatestProgress();
+      expect(progress?.state).toBe('completed');
+      expect(progress?.result).toBe('Hello');
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // getResult() promise
+  // ---------------------------------------------------------------------------
+
+  describe('getResult()', () => {
+    it('resolves with ToolResult containing llmContent and SubagentProgress returnDisplay', async () => {
+      mockClientManager.sendMessageStream.mockImplementation(
+        async function* () {
+          yield makeChunk('Result text');
+        },
+      );
+
+      const { result } = await runSession();
+
+      expect(result.llmContent).toEqual([{ text: 'Result text' }]);
+      const display = result.returnDisplay as SubagentProgress;
+      expect(display.isSubagentProgress).toBe(true);
+      expect(display.state).toBe('completed');
+      expect(display.result).toBe('Result text');
+      expect(display.agentName).toBe('Test Remote Agent');
+    });
+
+    it('rejects when stream throws a non-A2A error', async () => {
+      mockClientManager.sendMessageStream.mockReturnValue({
+        [Symbol.asyncIterator]() {
+          return {
+            async next(): Promise<IteratorResult<never>> {
+              throw new Error('network failure');
+            },
+          };
+        },
+      });
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await expect(session.getResult()).rejects.toThrow();
+    });
+
+    it('resolves even with empty stream (empty final output)', async () => {
+      mockClientManager.sendMessageStream.mockImplementation(
+        async function* () {
+          // yield nothing
+        },
+      );
+
+      const { result } = await runSession();
+      expect(result.llmContent).toEqual([{ text: '' }]);
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // Session state persistence
+  // ---------------------------------------------------------------------------
+
+  describe('session state persistence', () => {
+    it('second send reuses contextId captured from first send', async () => {
+      let callCount = 0;
+      mockClientManager.sendMessageStream.mockImplementation(async function* (
+        _name: string,
+        _query: string,
+        opts: { contextId?: string },
+      ) {
+        callCount++;
+        if (callCount === 1) {
+          yield {
+            kind: 'message' as const,
+            messageId: 'msg-1',
+            role: 'agent' as const,
+            contextId: 'ctx-from-server',
+            parts: [{ kind: 'text' as const, text: 'First response' }],
+          };
+        } else {
+          // Second send on same session should pass the contextId
+          expect(opts.contextId).toBe('ctx-from-server');
+          yield makeChunk('Second response');
+        }
+      });
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+
+      // First send — establishes contextId
+      await session.send({
+        message: { content: [{ type: 'text', text: 'first' }] },
+      });
+      await session.getResult();
+
+      // Second send on same session — should reuse contextId
+      await session.send({
+        message: { content: [{ type: 'text', text: 'second' }] },
+      });
+      await session.getResult();
+
+      expect(callCount).toBe(2);
+    });
+
+    it('separate session instances have independent state', async () => {
+      const capturedContextIds: Array<string | undefined> = [];
+      mockClientManager.sendMessageStream.mockImplementation(async function* (
+        _name: string,
+        _query: string,
+        opts: { contextId?: string },
+      ) {
+        capturedContextIds.push(opts.contextId);
+        yield {
+          kind: 'message' as const,
+          messageId: 'msg-1',
+          role: 'agent' as const,
+          contextId: 'ctx-from-server',
+          parts: [{ kind: 'text' as const, text: 'ok' }],
+        };
+      });
+
+      // Two separate sessions for the same agent — state is NOT shared
+      const session1 = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      await session1.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await session1.getResult();
+
+      const session2 = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      await session2.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await session2.getResult();
+
+      // Both start with no contextId — separate instances, no shared state
+      expect(capturedContextIds[0]).toBeUndefined();
+      expect(capturedContextIds[1]).toBeUndefined();
+    });
+
+    it('taskId is cleared when a terminal-state task chunk is received', async () => {
+      let callCount = 0;
+      const capturedTaskIds: Array<string | undefined> = [];
+
+      mockClientManager.sendMessageStream.mockImplementation(async function* (
+        _n: string,
+        _q: string,
+        opts: { taskId?: string },
+      ) {
+        callCount++;
+        capturedTaskIds.push(opts.taskId);
+        if (callCount === 1) {
+          yield {
+            kind: 'task' as const,
+            id: 'task-123',
+            contextId: 'ctx-1',
+            status: { state: 'completed' as const },
+          };
+        } else {
+          yield makeChunk('done');
+        }
+      });
+
+      // Use same session for multi-send
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+
+      await session.send({
+        message: { content: [{ type: 'text', text: 'first' }] },
+      });
+      await session.getResult();
+
+      await session.send({
+        message: { content: [{ type: 'text', text: 'second' }] },
+      });
+      await session.getResult();
+
+      expect(callCount).toBe(2);
+      // First call starts with no taskId
+      expect(capturedTaskIds[0]).toBeUndefined();
+      // Second call: taskId was cleared because terminal-state task chunk was received
+      expect(capturedTaskIds[1]).toBeUndefined();
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // Auth setup
+  // ---------------------------------------------------------------------------
+
+  describe('auth setup', () => {
+    it('no auth → loadAgent called without auth handler', async () => {
+      await runSession();
+
+      expect(mockClientManager.loadAgent).toHaveBeenCalledWith(
+        'test-remote-agent',
+        { type: 'url', url: 'http://test-agent/card' },
+        undefined,
+      );
+    });
+
+    it('definition.auth present → A2AAuthProviderFactory.create called', async () => {
+      const authDef: RemoteAgentDefinition = {
+        ...mockDefinition,
+        name: 'auth-agent',
+        auth: {
+          type: 'http' as const,
+          scheme: 'Bearer' as const,
+          token: 'secret',
+        },
+      };
+
+      const mockProvider = {
+        type: 'http' as const,
+        headers: vi.fn().mockResolvedValue({ Authorization: 'Bearer secret' }),
+        shouldRetryWithHeaders: vi.fn(),
+      } as unknown as A2AAuthProvider;
+      (A2AAuthProviderFactory.create as Mock).mockResolvedValue(mockProvider);
+
+      await runSession(authDef, 'q');
+
+      expect(A2AAuthProviderFactory.create).toHaveBeenCalledWith(
+        expect.objectContaining({
+          agentName: 'auth-agent',
+          agentCardUrl: 'http://test-agent/card',
+        }),
+      );
+      expect(mockClientManager.loadAgent).toHaveBeenCalledWith(
+        'auth-agent',
+        expect.any(Object),
+        mockProvider,
+      );
+    });
+
+    it('auth factory returns undefined → throws error that rejects getResult()', async () => {
+      const authDef: RemoteAgentDefinition = {
+        ...mockDefinition,
+        name: 'failing-auth-agent',
+        auth: {
+          type: 'http' as const,
+          scheme: 'Bearer' as const,
+          token: 'secret',
+        },
+      };
+
+      (A2AAuthProviderFactory.create as Mock).mockResolvedValue(undefined);
+
+      const session = new RemoteSubagentSession(
+        authDef,
+        mockContext,
+        mockMessageBus,
+      );
+      await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await expect(session.getResult()).rejects.toThrow(
+        "Failed to create auth provider for agent 'failing-auth-agent'",
+      );
+    });
+
+    it('agent already loaded → loadAgent not called again', async () => {
+      // Return a client object (truthy) so getClient returns defined
+      mockClientManager.getClient.mockReturnValue({});
+
+      await runSession();
+
+      expect(mockClientManager.loadAgent).not.toHaveBeenCalled();
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // Error handling
+  // ---------------------------------------------------------------------------
+
+  describe('error handling', () => {
+    it('stream error → error event + agent_end(failed)', async () => {
+      mockClientManager.sendMessageStream.mockReturnValue({
+        [Symbol.asyncIterator]() {
+          return {
+            async next(): Promise<IteratorResult<never>> {
+              throw new Error('network error');
+            },
+          };
+        },
+      });
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      const events: AgentEvent[] = [];
+      session.subscribe((e) => events.push(e));
+
+      await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await expect(session.getResult()).rejects.toThrow();
+
+      const errEvent = events.find((e) => e.type === 'error');
+      expect(errEvent).toBeDefined();
+
+      const endEvent = events.find((e) => e.type === 'agent_end');
+      expect(endEvent).toBeDefined();
+      if (endEvent?.type === 'agent_end') {
+        expect(endEvent.reason).toBe('failed');
+      }
+    });
+
+    it('missing A2AClientManager → rejects getResult()', async () => {
+      const mockConfig = {
+        getA2AClientManager: vi.fn().mockReturnValue(undefined),
+        injectionService: {
+          getLatestInjectionIndex: vi.fn().mockReturnValue(0),
+        },
+      } as unknown as Config;
+      const noClientContext = {
+        config: mockConfig,
+      } as unknown as AgentLoopContext;
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        noClientContext,
+        mockMessageBus,
+      );
+      await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await expect(session.getResult()).rejects.toThrow(
+        'A2AClientManager not available',
+      );
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // Subscription
+  // ---------------------------------------------------------------------------
+
+  describe('subscription', () => {
+    it('unsubscribe stops event delivery', async () => {
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      const received: AgentEvent[] = [];
+      const unsub = session.subscribe((e) => received.push(e));
+      unsub();
+
+      await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await session.getResult();
+
+      expect(received).toHaveLength(0);
+    });
+
+    it('multiple subscribers all receive events', async () => {
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      const events1: AgentEvent[] = [];
+      const events2: AgentEvent[] = [];
+      session.subscribe((e) => events1.push(e));
+      session.subscribe((e) => events2.push(e));
+
+      await session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+      await session.getResult();
+
+      expect(events1.length).toBeGreaterThan(0);
+      expect(events1).toEqual(events2);
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // Abort
+  // ---------------------------------------------------------------------------
+
+  describe('abort()', () => {
+    it('abort() causes agent_end(reason:aborted)', async () => {
+      let rejectWithAbort: ((err: Error) => void) | undefined;
+
+      // Stream that blocks until aborted, then throws AbortError
+      mockClientManager.sendMessageStream.mockImplementation(
+        // eslint-disable-next-line require-yield
+        async function* () {
+          await new Promise<void>((_resolve, reject) => {
+            rejectWithAbort = reject;
+          });
+        },
+      );
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      const events: AgentEvent[] = [];
+      session.subscribe((e) => events.push(e));
+
+      void session.send({
+        message: { content: [{ type: 'text', text: 'q' }] },
+      });
+
+      // Wait for agent_start to be emitted before aborting
+      await vi.waitFor(() => {
+        expect(events.some((e) => e.type === 'agent_start')).toBe(true);
+      });
+
+      await session.abort();
+
+      // Simulate the transport throwing AbortError when signal fires
+      const abortErr = new Error('AbortError');
+      abortErr.name = 'AbortError';
+      rejectWithAbort?.(abortErr);
+
+      const result = await session.getResult();
+      expect(result.llmContent).toEqual([{ text: '' }]);
+      expect(result.returnDisplay).toBe('');
+
+      const endEvent = events.find((e) => e.type === 'agent_end');
+      expect(endEvent).toBeDefined();
+      if (endEvent?.type === 'agent_end') {
+        expect(endEvent.reason).toBe('aborted');
+      }
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // sendMessageStream call args
+  // ---------------------------------------------------------------------------
+
+  describe('sendMessageStream call arguments', () => {
+    it('passes the query string from the message payload', async () => {
+      await runSession(mockDefinition, 'my specific query');
+
+      expect(mockClientManager.sendMessageStream).toHaveBeenCalledWith(
+        'test-remote-agent',
+        'my specific query',
+        expect.objectContaining({ signal: expect.any(Object) }),
+      );
+    });
+
+    it('uses DEFAULT_QUERY_STRING when message text is empty', async () => {
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      await session.send({
+        message: { content: [{ type: 'text', text: '' }] },
+      });
+      await session.getResult();
+
+      // DEFAULT_QUERY_STRING = 'Get Started!'
+      expect(mockClientManager.sendMessageStream).toHaveBeenCalledWith(
+        'test-remote-agent',
+        'Get Started!',
+        expect.objectContaining({ signal: expect.any(Object) }),
+      );
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // Concurrent send() guard
+  // ---------------------------------------------------------------------------
+
+  describe('concurrent send() guard', () => {
+    it('calling send() while a stream is active throws', async () => {
+      let resolveChunk!: () => void;
+
+      mockClientManager.sendMessageStream.mockImplementation(
+        async function* () {
+          // Block until test releases the chunk
+          await new Promise<void>((resolve) => {
+            resolveChunk = resolve;
+          });
+          yield makeChunk('late');
+        },
+      );
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+
+      void session.send({
+        message: { content: [{ type: 'text', text: 'first' }] },
+      });
+
+      // Wait for the stream to actually start (agent_start emitted)
+      const events: AgentEvent[] = [];
+      session.subscribe((e) => events.push(e));
+      await vi.waitFor(() => {
+        expect(events.some((e) => e.type === 'agent_start')).toBe(true);
+      });
+
+      // Second send() while first stream is active must throw
+      await expect(
+        session.send({
+          message: { content: [{ type: 'text', text: 'second' }] },
+        }),
+      ).rejects.toThrow('cannot be called while a stream is active');
+
+      // Clean up: release the blocked generator so getResult() can settle
+      resolveChunk();
+      await session.getResult().catch(() => {});
+    });
+  });
+
+  // ---------------------------------------------------------------------------
+  // Multi-send support
+  // ---------------------------------------------------------------------------
+
+  describe('multi-send', () => {
+    it('supports sequential sends after stream completion', async () => {
+      let callCount = 0;
+      mockClientManager.sendMessageStream.mockImplementation(
+        async function* () {
+          callCount++;
+          yield makeChunk(`Response ${callCount}`);
+        },
+      );
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+
+      // First send
+      const result1 = await session.send({
+        message: { content: [{ type: 'text', text: 'first' }] },
+      });
+      expect(result1.streamId).not.toBeNull();
+      const output1 = await session.getResult();
+      expect(output1.llmContent).toEqual([{ text: 'Response 1' }]);
+
+      // Second send — should work, not throw
+      const result2 = await session.send({
+        message: { content: [{ type: 'text', text: 'second' }] },
+      });
+      expect(result2.streamId).not.toBeNull();
+      expect(result2.streamId).not.toBe(result1.streamId);
+
+      const output2 = await session.getResult();
+      expect(output2.llmContent).toEqual([{ text: 'Response 2' }]);
+    });
+
+    it('getResult() returns the latest stream result', async () => {
+      let callCount = 0;
+      mockClientManager.sendMessageStream.mockImplementation(
+        async function* () {
+          callCount++;
+          yield makeChunk(`Result ${callCount}`);
+        },
+      );
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+
+      await session.send({
+        message: { content: [{ type: 'text', text: 'first' }] },
+      });
+      const result1 = await session.getResult();
+
+      await session.send({
+        message: { content: [{ type: 'text', text: 'second' }] },
+      });
+      const result2 = await session.getResult();
+
+      expect(result1.llmContent).toEqual([{ text: 'Result 1' }]);
+      expect(result2.llmContent).toEqual([{ text: 'Result 2' }]);
+    });
+
+    it('contextId/taskId persist across sends within the same session', async () => {
+      let sendCallCount = 0;
+      mockClientManager.sendMessageStream.mockImplementation(async function* (
+        _name: string,
+        _query: string,
+        _opts: Record<string, unknown>,
+      ) {
+        sendCallCount++;
+        // First call returns ids; second call should receive them
+        yield {
+          kind: 'message' as const,
+          messageId: `msg-${sendCallCount}`,
+          contextId: `ctx-${sendCallCount}`,
+          taskId: `task-${sendCallCount}`,
+          role: 'agent' as const,
+          parts: [{ kind: 'text' as const, text: `Response ${sendCallCount}` }],
+        };
+      });
+
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+
+      // First send — establishes contextId/taskId
+      await session.send({
+        message: { content: [{ type: 'text', text: 'first' }] },
+      });
+      await session.getResult();
+
+      // Second send — should pass the persisted contextId/taskId
+      await session.send({
+        message: { content: [{ type: 'text', text: 'second' }] },
+      });
+      await session.getResult();
+
+      // Verify the second call received the contextId/taskId from first call
+      expect(mockClientManager.sendMessageStream).toHaveBeenCalledTimes(2);
+      const secondCallOpts =
+        mockClientManager.sendMessageStream.mock.calls[1]?.[2];
+      expect(secondCallOpts).toHaveProperty('contextId', 'ctx-1');
+      expect(secondCallOpts).toHaveProperty('taskId', 'task-1');
+    });
+
+    it('getResult() rejects when called before any send', async () => {
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      await expect(session.getResult()).rejects.toThrow(
+        'No active or completed stream',
+      );
+    });
+
+    it('emits fresh agent_start/agent_end per stream', async () => {
+      const session = new RemoteSubagentSession(
+        mockDefinition,
+        mockContext,
+        mockMessageBus,
+      );
+      const events: AgentEvent[] = [];
+      session.subscribe((e) => events.push(e));
+
+      // First send
+      await session.send({
+        message: { content: [{ type: 'text', text: 'first' }] },
+      });
+      await session.getResult();
+
+      const firstStreamEvents = events.length;
+      expect(events[0]?.type).toBe('agent_start');
+      expect(events[firstStreamEvents - 1]?.type).toBe('agent_end');
+
+      // Second send
+      await session.send({
+        message: { content: [{ type: 'text', text: 'second' }] },
+      });
+      await session.getResult();
+
+      // Should have a second agent_start/agent_end pair
+      const secondStreamStart = events[firstStreamEvents];
+      const lastEvent = events[events.length - 1];
+      expect(secondStreamStart?.type).toBe('agent_start');
+      expect(lastEvent?.type).toBe('agent_end');
+      expect(secondStreamStart?.streamId).not.toBe(events[0]?.streamId);
+    });
+  });
+});
diff --git a/packages/core/src/agents/remote-subagent-protocol.ts b/packages/core/src/agents/remote-subagent-protocol.ts
new file mode 100644
index 0000000000..4179e5587b
--- /dev/null
+++ b/packages/core/src/agents/remote-subagent-protocol.ts
@@ -0,0 +1,431 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview RemoteSubagentProtocol — wraps A2A remote agent streaming
+ * behind the AgentProtocol interface.
+ *
+ * Pattern mirrors LocalSubagentProtocol and LegacyAgentProtocol, but the loop
+ * body drives A2AClientManager instead of LocalAgentExecutor.
+ */
+
+import { randomUUID } from 'node:crypto';
+import type { AgentLoopContext } from '../config/agent-loop-context.js';
+import { AgentSession } from '../agent/agent-session.js';
+import type {
+  AgentProtocol,
+  AgentSend,
+  AgentEvent,
+  StreamEndReason,
+  Unsubscribe,
+  ContentPart,
+} from '../agent/types.js';
+import type { ToolResult } from '../tools/tools.js';
+import {
+  DEFAULT_QUERY_STRING,
+  type RemoteAgentDefinition,
+  type SubagentProgress,
+  getRemoteAgentTargetUrl,
+  getAgentCardLoadOptions,
+} from './types.js';
+import { A2AResultReassembler, extractIdsFromResponse } from './a2aUtils.js';
+import type { AuthenticationHandler } from '@a2a-js/sdk/client';
+import { A2AAuthProviderFactory } from './auth-provider/factory.js';
+import { A2AAgentError } from './a2a-errors.js';
+import { debugLogger } from '../utils/debugLogger.js';
+import type { MessageBus } from '../confirmation-bus/message-bus.js';
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+function isAbortLikeError(err: unknown): boolean {
+  return err instanceof Error && err.name === 'AbortError';
+}
+
+// ---------------------------------------------------------------------------
+// RemoteSubagentProtocol
+// ---------------------------------------------------------------------------
+
+class RemoteSubagentProtocol implements AgentProtocol {
+  private _events: AgentEvent[] = [];
+  private _subscribers = new Set<(event: AgentEvent) => void>();
+  private _streamId: string = randomUUID();
+  private _eventCounter = 0;
+  private _agentStartEmitted = false;
+  private _agentEndEmitted = false;
+  private _activeStreamId: string | undefined;
+  private _abortController = new AbortController();
+
+  // A2A conversation state — persists across sends within this session instance
+  private contextId: string | undefined;
+  private taskId: string | undefined;
+  private authHandler: AuthenticationHandler | undefined;
+
+  // Agent display name (for SubagentProgress construction)
+  private readonly _agentName: string;
+
+  // Latest SubagentProgress — updated per chunk, used for error recovery
+  private _latestProgress: SubagentProgress | undefined;
+
+  // Result promise wiring — re-created per stream in _beginNewStream()
+  private _resultResolve!: (result: ToolResult) => void;
+  private _resultReject!: (err: unknown) => void;
+  private _resultPromise: Promise<ToolResult> | undefined;
+
+  constructor(
+    private readonly definition: RemoteAgentDefinition,
+    private readonly context: AgentLoopContext,
+    // Required for API parity across protocol constructors (local, remote, legacy)
+    _messageBus: MessageBus,
+  ) {
+    this._agentName = definition.displayName ?? definition.name;
+  }
+
+  // ---------------------------------------------------------------------------
+  // AgentProtocol interface
+  // ---------------------------------------------------------------------------
+
+  get events(): readonly AgentEvent[] {
+    return this._events;
+  }
+
+  subscribe(callback: (event: AgentEvent) => void): Unsubscribe {
+    this._subscribers.add(callback);
+    return () => {
+      this._subscribers.delete(callback);
+    };
+  }
+
+  async send(payload: AgentSend): Promise<{ streamId: string | null }> {
+    if ('message' in payload && payload.message) {
+      if (this._activeStreamId) {
+        throw new Error(
+          'RemoteSubagentProtocol.send() cannot be called while a stream is active.',
+        );
+      }
+
+      const query =
+        payload.message.content
+          .filter((p): p is ContentPart & { type: 'text' } => p.type === 'text')
+          .map((p) => p.text)
+          .join('') || DEFAULT_QUERY_STRING;
+
+      this._beginNewStream();
+      const streamId = this._streamId;
+
+      setTimeout(() => {
+        void this._runStreamInBackground(query);
+      }, 0);
+
+      return { streamId };
+    }
+
+    // update/action/elicitations not used for remote agents
+    return { streamId: null };
+  }
+
+  async abort(): Promise<void> {
+    this._abortController.abort();
+  }
+
+  // ---------------------------------------------------------------------------
+  // Protocol-specific: result access
+  // ---------------------------------------------------------------------------
+
+  getResult(): Promise<ToolResult> {
+    if (!this._resultPromise) {
+      return Promise.reject(new Error('No active or completed stream'));
+    }
+    return this._resultPromise;
+  }
+
+  getLatestProgress(): SubagentProgress | undefined {
+    return this._latestProgress;
+  }
+
+  // ---------------------------------------------------------------------------
+  // Core: A2A streaming
+  // ---------------------------------------------------------------------------
+
+  private _beginNewStream(): void {
+    this._streamId = randomUUID();
+    this._eventCounter = 0;
+    this._abortController = new AbortController();
+    this._agentStartEmitted = false;
+    this._agentEndEmitted = false;
+    this._activeStreamId = this._streamId;
+    this._resultPromise = new Promise<ToolResult>((resolve, reject) => {
+      this._resultResolve = resolve;
+      this._resultReject = reject;
+    });
+  }
+
+  private async _runStreamInBackground(query: string): Promise<void> {
+    this._ensureAgentStart();
+    try {
+      await this._runStream(query);
+    } catch (err: unknown) {
+      if (this._abortController.signal.aborted || isAbortLikeError(err)) {
+        this._ensureAgentEnd('aborted');
+        // Abort resolves with an empty result — partial output is intentionally
+        // dropped since the caller requested cancellation.
+        this._resultResolve({
+          llmContent: [{ text: '' }],
+          returnDisplay: '',
+        });
+      } else {
+        this._emitErrorAndAgentEnd(err);
+        this._resultReject(err);
+      }
+    } finally {
+      this._clearActiveStream();
+    }
+  }
+
+  private async _runStream(query: string): Promise<void> {
+    const clientManager = this.context.config.getA2AClientManager();
+    if (!clientManager) {
+      throw new Error(
+        `RemoteSubagentProtocol: A2AClientManager not available for '${this.definition.name}'.`,
+      );
+    }
+
+    const authHandler = await this._getAuthHandler();
+    if (!clientManager.getClient(this.definition.name)) {
+      await clientManager.loadAgent(
+        this.definition.name,
+        getAgentCardLoadOptions(this.definition),
+        authHandler,
+      );
+    }
+
+    const reassembler = new A2AResultReassembler();
+    let prevText = '';
+
+    const stream = clientManager.sendMessageStream(
+      this.definition.name,
+      query,
+      {
+        contextId: this.contextId,
+        taskId: this.taskId,
+        signal: this._abortController.signal,
+      },
+    );
+
+    for await (const chunk of stream) {
+      reassembler.update(chunk);
+
+      const {
+        contextId: newContextId,
+        taskId: newTaskId,
+        clearTaskId,
+      } = extractIdsFromResponse(chunk);
+      if (newContextId) this.contextId = newContextId;
+      this.taskId = clearTaskId ? undefined : (newTaskId ?? this.taskId);
+
+      const currentText = reassembler.toString();
+
+      // Update latest progress snapshot (for invocation's error recovery)
+      this._latestProgress = {
+        isSubagentProgress: true,
+        agentName: this._agentName,
+        state: 'running',
+        recentActivity: reassembler.toActivityItems(),
+        result: currentText,
+      };
+
+      // Emit delta as a message event
+      const delta = currentText.slice(prevText.length);
+      if (delta) {
+        this._emit([
+          this._makeEvent('message', {
+            role: 'agent',
+            content: [{ type: 'text', text: delta }],
+          }),
+        ]);
+        prevText = currentText;
+      }
+    }
+
+    const finalOutput = reassembler.toString();
+    debugLogger.debug(
+      `[RemoteSubagentProtocol] ${this.definition.name} finished, output length: ${finalOutput.length}`,
+    );
+
+    const finalProgress: SubagentProgress = {
+      isSubagentProgress: true,
+      agentName: this._agentName,
+      state: 'completed',
+      result: finalOutput,
+      recentActivity: reassembler.toActivityItems(),
+    };
+    this._latestProgress = finalProgress;
+
+    this._finishStream('completed');
+
+    this._resultResolve({
+      llmContent: [{ text: finalOutput }],
+      returnDisplay: finalProgress,
+    });
+  }
+
+  private async _getAuthHandler(): Promise<AuthenticationHandler | undefined> {
+    if (this.authHandler) return this.authHandler;
+    if (!this.definition.auth) return undefined;
+
+    const targetUrl = getRemoteAgentTargetUrl(this.definition);
+    const provider = await A2AAuthProviderFactory.create({
+      authConfig: this.definition.auth,
+      agentName: this.definition.name,
+      targetUrl,
+      agentCardUrl: this.definition.agentCardUrl,
+    });
+    if (!provider) {
+      throw new Error(
+        `Failed to create auth provider for agent '${this.definition.name}'`,
+      );
+    }
+    this.authHandler = provider;
+    return this.authHandler;
+  }
+
+  // ---------------------------------------------------------------------------
+  // Internal helpers
+  // ---------------------------------------------------------------------------
+
+  private _emit(events: AgentEvent[]): void {
+    if (events.length === 0) return;
+    const subscribers = [...this._subscribers];
+    for (const event of events) {
+      this._events.push(event);
+      if (event.type === 'agent_end') {
+        this._agentEndEmitted = true;
+      }
+      for (const sub of subscribers) {
+        sub(event);
+      }
+    }
+  }
+
+  private _clearActiveStream(): void {
+    this._activeStreamId = undefined;
+  }
+
+  private _ensureAgentStart(): void {
+    if (!this._agentStartEmitted) {
+      this._agentStartEmitted = true;
+      this._emit([this._makeEvent('agent_start', {})]);
+    }
+  }
+
+  private _ensureAgentEnd(reason: StreamEndReason = 'completed'): void {
+    if (!this._agentEndEmitted && this._agentStartEmitted) {
+      this._emit([this._makeEvent('agent_end', { reason })]);
+    }
+  }
+
+  private _finishStream(reason: StreamEndReason): void {
+    this._ensureAgentEnd(reason);
+    this._clearActiveStream();
+  }
+
+  private _emitErrorAndAgentEnd(err: unknown): void {
+    const message = this._formatError(err);
+    this._ensureAgentStart();
+
+    const meta: Record<string, unknown> = {};
+    if (err instanceof Error) {
+      meta['errorName'] = err.constructor.name;
+      meta['stack'] = err.stack;
+    }
+
+    this._emit([
+      this._makeEvent('error', {
+        status: 'INTERNAL',
+        message,
+        fatal: true,
+        ...(Object.keys(meta).length > 0 ? { _meta: meta } : {}),
+      }),
+    ]);
+    this._ensureAgentEnd('failed');
+  }
+
+  private _formatError(error: unknown): string {
+    if (error instanceof A2AAgentError) {
+      return error.userMessage;
+    }
+    return `Error calling remote agent: ${error instanceof Error ? error.message : String(error)}`;
+  }
+
+  private _nextEventFields() {
+    return {
+      id: `${this._streamId}-${this._eventCounter++}`,
+      timestamp: new Date().toISOString(),
+      streamId: this._streamId,
+    };
+  }
+
+  private _makeEvent<T extends AgentEvent['type']>(
+    type: T,
+    payload: Omit<AgentEvent<T>, 'id' | 'timestamp' | 'streamId' | 'type'>,
+  ): AgentEvent {
+    // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+    return {
+      ...this._nextEventFields(),
+      type,
+      ...payload,
+    } as AgentEvent;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// Public export
+// ---------------------------------------------------------------------------
+
+export class RemoteSubagentSession extends AgentSession {
+  private readonly _remoteProtocol: RemoteSubagentProtocol;
+
+  constructor(
+    definition: RemoteAgentDefinition,
+    context: AgentLoopContext,
+    messageBus: MessageBus,
+  ) {
+    const protocol = new RemoteSubagentProtocol(
+      definition,
+      context,
+      messageBus,
+    );
+    super(protocol);
+    this._remoteProtocol = protocol;
+  }
+
+  /**
+   * Returns the ToolResult once the remote agent stream completes.
+   * Used by RemoteAgentInvocation to return the result.
+   */
+  getResult(): Promise<ToolResult> {
+    return this._remoteProtocol.getResult();
+  }
+
+  /**
+   * Returns the most recent SubagentProgress snapshot, updated per streaming
+   * chunk. Useful for constructing error progress when getResult() rejects.
+   */
+  getLatestProgress(): SubagentProgress | undefined {
+    return this._remoteProtocol.getLatestProgress();
+  }
+
+  /**
+   * Convenience: start execution with a query string.
+   * Equivalent to send({message: {content: [{type:'text', text: query}]}}).
+   */
+  async startWithQuery(query: string): Promise<{ streamId: string | null }> {
+    return this.send({
+      message: { content: [{ type: 'text', text: query }] },
+    });
+  }
+}
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index cf0ba653c1..4ceabc37fe 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -277,9 +277,6 @@ export * from './hooks/index.js';
 // Export hook types
 export * from './hooks/types.js';
 
-// Export agent types
-export * from './agents/types.js';
-
 // Export stdio utils
 export * from './utils/stdio.js';
 export * from './utils/terminal.js';