fix(agent): inject tool inventory note when tools change mid-session

Stale session history can cause the model to follow old "I can't do
that" patterns even when new tools are available. NativeAgent now tracks
a tool fingerprint and appends a system prompt note listing current
tools when the inventory changes, resetting on session reset.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/backends/native/agent.test.ts

@@ -218,6 +218,169 @@ describe('NativeAgent tool loop', () => {
     }));
   });
 
+  it('injects tool inventory note when history exists and fingerprint changes', async () => {
+    let callCount = 0;
+    const mockClient: ModelClient = {
+      chat: vi.fn().mockImplementation(() => {
+        callCount++;
+        return {
+          content: `Response ${callCount}`,
+          stopReason: 'end_turn',
+          usage: { inputTokens: 10, outputTokens: 5 },
+        };
+      }),
+    };
+
+    const registry = new ToolRegistry();
+    registry.register(echoTool);
+    const hooks = new HookEngine({ confirm: [], log: [], silent: [] });
+    const executor = new ToolExecutor(registry, hooks);
+
+    const agent = new NativeAgent({
+      modelClient: mockClient,
+      systemPrompt: 'You are helpful.',
+      toolRegistry: registry,
+      toolExecutor: executor,
+    });
+
+    // First message — no prior history, so no inventory note
+    await agent.process('Hi');
+    const firstCall = (mockClient.chat as ReturnType<typeof vi.fn>).mock.calls[0][0];
+    expect(firstCall.system).toBe('You are helpful.');
+
+    // Second message — history exists but fingerprint hasn't changed, no note
+    await agent.process('Hello again');
+    const secondCall = (mockClient.chat as ReturnType<typeof vi.fn>).mock.calls[1][0];
+    expect(secondCall.system).toBe('You are helpful.');
+
+    // Now add a new tool to change the fingerprint
+    const newTool: Tool = {
+      name: 'test.greet',
+      description: 'Greet',
+      inputSchema: { type: 'object', properties: {} },
+      execute: async () => ({ success: true, output: 'hi' }),
+    };
+    registry.register(newTool);
+
+    // Third message — history exists AND fingerprint changed, should inject note
+    await agent.process('What can you do?');
+    const thirdCall = (mockClient.chat as ReturnType<typeof vi.fn>).mock.calls[2][0];
+    expect(thirdCall.system).toContain('[Tool inventory updated');
+    expect(thirdCall.system).toContain('test_echo');
+    expect(thirdCall.system).toContain('test_greet');
+  });
+
+  it('does not inject tool inventory note on fresh session', async () => {
+    const mockClient: ModelClient = {
+      chat: vi.fn().mockResolvedValue({
+        content: 'Hello!',
+        stopReason: 'end_turn',
+        usage: { inputTokens: 10, outputTokens: 5 },
+      }),
+    };
+
+    const registry = new ToolRegistry();
+    registry.register(echoTool);
+    const hooks = new HookEngine({ confirm: [], log: [], silent: [] });
+    const executor = new ToolExecutor(registry, hooks);
+
+    const agent = new NativeAgent({
+      modelClient: mockClient,
+      systemPrompt: 'You are helpful.',
+      toolRegistry: registry,
+      toolExecutor: executor,
+    });
+
+    // First message ever — only one message in history (the user message just added)
+    await agent.process('Hi');
+    const call = (mockClient.chat as ReturnType<typeof vi.fn>).mock.calls[0][0];
+    expect(call.system).toBe('You are helpful.');
+    expect(call.system).not.toContain('Tool inventory updated');
+  });
+
+  it('only injects tool inventory note once per fingerprint change', async () => {
+    let callCount = 0;
+    const mockClient: ModelClient = {
+      chat: vi.fn().mockImplementation(() => {
+        callCount++;
+        return {
+          content: `Response ${callCount}`,
+          stopReason: 'end_turn',
+          usage: { inputTokens: 10, outputTokens: 5 },
+        };
+      }),
+    };
+
+    const registry = new ToolRegistry();
+    registry.register(echoTool);
+    const hooks = new HookEngine({ confirm: [], log: [], silent: [] });
+    const executor = new ToolExecutor(registry, hooks);
+
+    const agent = new NativeAgent({
+      modelClient: mockClient,
+      systemPrompt: 'You are helpful.',
+      toolRegistry: registry,
+      toolExecutor: executor,
+    });
+
+    // Build up history
+    await agent.process('Hi');
+    await agent.process('Hello');
+
+    // Add a new tool
+    const newTool: Tool = {
+      name: 'test.greet',
+      description: 'Greet',
+      inputSchema: { type: 'object', properties: {} },
+      execute: async () => ({ success: true, output: 'hi' }),
+    };
+    registry.register(newTool);
+
+    // First call after change — note injected
+    await agent.process('What tools?');
+    const thirdCall = (mockClient.chat as ReturnType<typeof vi.fn>).mock.calls[2][0];
+    expect(thirdCall.system).toContain('[Tool inventory updated');
+
+    // Second call with same tools — no note (fingerprint matches)
+    await agent.process('Anything else?');
+    const fourthCall = (mockClient.chat as ReturnType<typeof vi.fn>).mock.calls[3][0];
+    expect(fourthCall.system).toBe('You are helpful.');
+  });
+
+  it('resets tool fingerprint on reset()', async () => {
+    const mockClient: ModelClient = {
+      chat: vi.fn().mockResolvedValue({
+        content: 'Hello!',
+        stopReason: 'end_turn',
+        usage: { inputTokens: 10, outputTokens: 5 },
+      }),
+    };
+
+    const registry = new ToolRegistry();
+    registry.register(echoTool);
+    const hooks = new HookEngine({ confirm: [], log: [], silent: [] });
+    const executor = new ToolExecutor(registry, hooks);
+
+    const agent = new NativeAgent({
+      modelClient: mockClient,
+      systemPrompt: 'You are helpful.',
+      toolRegistry: registry,
+      toolExecutor: executor,
+    });
+
+    // Build history and establish fingerprint
+    await agent.process('Hi');
+    await agent.process('Hello');
+
+    // Reset clears fingerprint
+    agent.reset();
+
+    // After reset, first message has no history so no note
+    await agent.process('Hi again');
+    const call = (mockClient.chat as ReturnType<typeof vi.fn>).mock.calls[2][0];
+    expect(call.system).not.toContain('Tool inventory updated');
+  });
+
   it('handles multiple tool calls in single response', async () => {
     let callCount = 0;
     const mockClient: ModelClient = {