diff --git a/packages/sdk/src/session.test.ts b/packages/sdk/src/session.test.ts
new file mode 100644
index 0000000000..52230055e2
--- /dev/null
+++ b/packages/sdk/src/session.test.ts
@@ -0,0 +1,331 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { GeminiCliSession } from './session.js';
+import type { GeminiCliAgent } from './agent.js';
+import type { GeminiCliAgentOptions } from './types.js';
+
+// Mutable mock client so individual tests can override sendMessageStream
+const mockClient = {
+  resumeChat: vi.fn().mockResolvedValue(undefined),
+  getHistory: vi.fn().mockReturnValue([]),
+  sendMessageStream: vi.fn().mockReturnValue((async function* () {})()),
+  updateSystemInstruction: vi.fn(),
+};
+
+// Mutable mock config so individual tests can spy on setUserMemory etc.
+const mockConfig = {
+  initialize: vi.fn().mockResolvedValue(undefined),
+  refreshAuth: vi.fn().mockResolvedValue(undefined),
+  getSkillManager: vi.fn().mockReturnValue({
+    getSkills: vi.fn().mockReturnValue([]),
+    addSkills: vi.fn(),
+  }),
+  getToolRegistry: vi.fn().mockReturnValue({
+    getTool: vi.fn().mockReturnValue(null),
+    registerTool: vi.fn(),
+    unregisterTool: vi.fn(),
+  }),
+  getMessageBus: vi.fn().mockReturnValue({}),
+  getGeminiClient: vi.fn().mockReturnValue(mockClient),
+  getSessionId: vi.fn().mockReturnValue('mock-session-id'),
+  getWorkingDir: vi.fn().mockReturnValue('/tmp'),
+  setUserMemory: vi.fn(),
+};
+
+// Mock scheduleAgentTools at module level so tests can override it
+const mockScheduleAgentTools = vi.fn().mockResolvedValue([]);
+
+// Mock @google/gemini-cli-core to avoid heavy filesystem/auth/telemetry setup
+vi.mock('@google/gemini-cli-core', async (importOriginal) => {
+  const actual =
+    await importOriginal<typeof import('@google/gemini-cli-core')>();
+  return {
+    ...actual,
+    Config: vi.fn().mockImplementation(() => mockConfig),
+    getAuthTypeFromEnv: vi.fn().mockReturnValue(null),
+    scheduleAgentTools: (...args: unknown[]) => mockScheduleAgentTools(...args),
+    loadSkillsFromDir: vi.fn().mockResolvedValue([]),
+    ActivateSkillTool: class {
+      static Name = 'activate_skill';
+    },
+    PolicyDecision: actual.PolicyDecision,
+  };
+});
+
+const mockAgent = {} as unknown as GeminiCliAgent;
+
+const baseOptions: GeminiCliAgentOptions = {
+  instructions: 'You are a helpful assistant.',
+};
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  // Reset sendMessageStream to empty stream by default
+  mockClient.sendMessageStream.mockReturnValue((async function* () {})());
+  mockScheduleAgentTools.mockResolvedValue([]);
+});
+
+describe('GeminiCliSession constructor', () => {
+  it('accepts string instructions', () => {
+    expect(
+      () => new GeminiCliSession(baseOptions, 'session-1', mockAgent),
+    ).not.toThrow();
+  });
+
+  it('accepts function instructions', () => {
+    const options: GeminiCliAgentOptions = {
+      instructions: async () => 'dynamic instructions',
+    };
+    expect(
+      () => new GeminiCliSession(options, 'session-2', mockAgent),
+    ).not.toThrow();
+  });
+
+  it('throws when instructions is an object (not string or function)', () => {
+    const options = {
+      instructions: { invalid: true },
+    } as unknown as GeminiCliAgentOptions;
+    expect(() => new GeminiCliSession(options, 'session-3', mockAgent)).toThrow(
+      'Instructions must be a string or a function.',
+    );
+  });
+
+  it('throws when instructions is a number', () => {
+    const options = {
+      instructions: 42,
+    } as unknown as GeminiCliAgentOptions;
+    expect(() => new GeminiCliSession(options, 'session-4', mockAgent)).toThrow(
+      'Instructions must be a string or a function.',
+    );
+  });
+
+  it('throws when instructions is an array', () => {
+    const options = {
+      instructions: ['step1', 'step2'],
+    } as unknown as GeminiCliAgentOptions;
+    expect(() => new GeminiCliSession(options, 'session-5', mockAgent)).toThrow(
+      'Instructions must be a string or a function.',
+    );
+  });
+});
+
+describe('GeminiCliSession id getter', () => {
+  it('returns the sessionId passed to the constructor', () => {
+    const session = new GeminiCliSession(
+      baseOptions,
+      'my-session-id',
+      mockAgent,
+    );
+    expect(session.id).toBe('my-session-id');
+  });
+
+  it('returns different ids for different sessions', () => {
+    const s1 = new GeminiCliSession(baseOptions, 'session-a', mockAgent);
+    const s2 = new GeminiCliSession(baseOptions, 'session-b', mockAgent);
+    expect(s1.id).not.toBe(s2.id);
+  });
+});
+
+describe('GeminiCliSession initialize()', () => {
+  it('initializes successfully with string instructions', async () => {
+    const session = new GeminiCliSession(
+      baseOptions,
+      'session-init-1',
+      mockAgent,
+    );
+    await expect(session.initialize()).resolves.toBeUndefined();
+  });
+
+  it('is idempotent — calling initialize() twice does not throw', async () => {
+    const session = new GeminiCliSession(
+      baseOptions,
+      'session-init-2',
+      mockAgent,
+    );
+    await session.initialize();
+    await expect(session.initialize()).resolves.toBeUndefined();
+  });
+
+  it('initializes with empty tools array', async () => {
+    const options: GeminiCliAgentOptions = { ...baseOptions, tools: [] };
+    const session = new GeminiCliSession(options, 'session-init-3', mockAgent);
+    await expect(session.initialize()).resolves.toBeUndefined();
+  });
+
+  it('initializes with empty skills array', async () => {
+    const options: GeminiCliAgentOptions = { ...baseOptions, skills: [] };
+    const session = new GeminiCliSession(options, 'session-init-4', mockAgent);
+    await expect(session.initialize()).resolves.toBeUndefined();
+  });
+
+  it('initializes with custom model', async () => {
+    const options: GeminiCliAgentOptions = {
+      ...baseOptions,
+      model: 'gemini-2.0-flash',
+    };
+    const session = new GeminiCliSession(options, 'session-init-5', mockAgent);
+    await expect(session.initialize()).resolves.toBeUndefined();
+  });
+
+  it('initializes with custom cwd', async () => {
+    const options: GeminiCliAgentOptions = {
+      ...baseOptions,
+      cwd: '/custom/working/dir',
+    };
+    const session = new GeminiCliSession(options, 'session-init-6', mockAgent);
+    await expect(session.initialize()).resolves.toBeUndefined();
+  });
+});
+
+describe('GeminiCliSession sendStream()', () => {
+  it('auto-initializes if not yet initialized', async () => {
+    const session = new GeminiCliSession(
+      baseOptions,
+      'session-stream-1',
+      mockAgent,
+    );
+    const events = [];
+    for await (const event of session.sendStream('Hello')) {
+      events.push(event);
+    }
+    expect(events).toHaveLength(0);
+  });
+
+  it('completes cleanly when model returns no tool calls', async () => {
+    const session = new GeminiCliSession(
+      baseOptions,
+      'session-stream-2',
+      mockAgent,
+    );
+    await session.initialize();
+    const events = [];
+    for await (const event of session.sendStream('Hello')) {
+      events.push(event);
+    }
+    expect(events).toHaveLength(0);
+  });
+
+  it('accepts an AbortSignal without throwing', async () => {
+    const session = new GeminiCliSession(
+      baseOptions,
+      'session-stream-3',
+      mockAgent,
+    );
+    const controller = new AbortController();
+    const events = [];
+    for await (const event of session.sendStream('Hello', controller.signal)) {
+      events.push(event);
+    }
+    expect(events).toHaveLength(0);
+  });
+
+  it('executes tool call loop and sends function response back to model', async () => {
+    const { GeminiEventType } = await import('@google/gemini-cli-core');
+
+    // First call: yield a ToolCallRequest, then end
+    // Second call: empty stream (model is done after tool result)
+    let callCount = 0;
+    mockClient.sendMessageStream.mockImplementation(() => {
+      callCount++;
+      if (callCount === 1) {
+        return (async function* () {
+          yield {
+            type: GeminiEventType.ToolCallRequest,
+            value: {
+              callId: 'call-1',
+              name: 'testTool',
+              args: { input: 'value' },
+            },
+          };
+        })();
+      }
+      return (async function* () {})();
+    });
+
+    mockScheduleAgentTools.mockResolvedValue([
+      {
+        response: {
+          responseParts: [
+            {
+              functionResponse: {
+                name: 'testTool',
+                response: { result: 'done' },
+              },
+            },
+          ],
+        },
+      },
+    ]);
+
+    const session = new GeminiCliSession(
+      baseOptions,
+      'session-stream-4',
+      mockAgent,
+    );
+    const events = [];
+    for await (const event of session.sendStream('Use the tool')) {
+      events.push(event);
+    }
+
+    // The ToolCallRequest event should have been yielded to the caller
+    expect(events).toHaveLength(1);
+    expect(events[0].type).toBe(GeminiEventType.ToolCallRequest);
+
+    // scheduleAgentTools should have been called with the tool call
+    expect(mockScheduleAgentTools).toHaveBeenCalledOnce();
+
+    // sendMessageStream called twice: once for prompt, once with tool result
+    expect(mockClient.sendMessageStream).toHaveBeenCalledTimes(2);
+  });
+
+  it('calls setUserMemory and updateSystemInstruction when instructions is a function', async () => {
+    const dynamicInstructions = vi
+      .fn()
+      .mockResolvedValue('updated instructions');
+    const options: GeminiCliAgentOptions = {
+      instructions: dynamicInstructions,
+    };
+
+    const session = new GeminiCliSession(
+      options,
+      'session-stream-5',
+      mockAgent,
+    );
+    for await (const _event of session.sendStream('Hello')) {
+      // consume stream
+    }
+
+    // The instructions function should have been called with a SessionContext
+    expect(dynamicInstructions).toHaveBeenCalledOnce();
+    const context = dynamicInstructions.mock.calls[0][0];
+    expect(context).toHaveProperty('sessionId');
+    expect(context).toHaveProperty('transcript');
+    expect(context).toHaveProperty('cwd');
+    expect(context).toHaveProperty('timestamp');
+
+    // Config should have been updated with the new instructions
+    expect(mockConfig.setUserMemory).toHaveBeenCalledWith(
+      'updated instructions',
+    );
+
+    // Client system instruction should have been refreshed
+    expect(mockClient.updateSystemInstruction).toHaveBeenCalledOnce();
+  });
+
+  it('does not call setUserMemory when instructions is a string', async () => {
+    const session = new GeminiCliSession(
+      baseOptions,
+      'session-stream-6',
+      mockAgent,
+    );
+    for await (const _event of session.sendStream('Hello')) {
+      // consume stream
+    }
+    expect(mockConfig.setUserMemory).not.toHaveBeenCalled();
+    expect(mockClient.updateSystemInstruction).not.toHaveBeenCalled();
+  });
+});