feat: add model router with fallback chain support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/models/router.test.ts

@@ -0,0 +1,80 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { ModelRouter } from './router.js';
+import type { ModelClient, ChatResponse } from './types.js';
+
+describe('ModelRouter', () => {
+  const createMockClient = (name: string, shouldFail = false): ModelClient => ({
+    chat: vi.fn().mockImplementation(async () => {
+      if (shouldFail) {
+        throw new Error(`${name} failed`);
+      }
+      return {
+        content: `Response from ${name}`,
+        stopReason: 'end_turn',
+        usage: { inputTokens: 10, outputTokens: 5 },
+      } satisfies ChatResponse;
+    }),
+  });
+
+  it('uses default client when available', async () => {
+    const defaultClient = createMockClient('default');
+    const router = new ModelRouter({
+      default: defaultClient,
+      fallbackChain: [],
+    });
+
+    const response = await router.chat({ messages: [{ role: 'user', content: 'Hi' }] });
+
+    expect(response.content).toBe('Response from default');
+    expect(defaultClient.chat).toHaveBeenCalled();
+  });
+
+  it('falls back to next provider on failure', async () => {
+    const failingClient = createMockClient('primary', true);
+    const fallbackClient = createMockClient('fallback');
+
+    const router = new ModelRouter({
+      default: failingClient,
+      fallbackChain: [fallbackClient],
+    });
+
+    const response = await router.chat({ messages: [{ role: 'user', content: 'Hi' }] });
+
+    expect(response.content).toBe('Response from fallback');
+    expect(failingClient.chat).toHaveBeenCalled();
+    expect(fallbackClient.chat).toHaveBeenCalled();
+  });
+
+  it('throws when all providers fail', async () => {
+    const failing1 = createMockClient('primary', true);
+    const failing2 = createMockClient('fallback', true);
+
+    const router = new ModelRouter({
+      default: failing1,
+      fallbackChain: [failing2],
+    });
+
+    await expect(router.chat({ messages: [{ role: 'user', content: 'Hi' }] }))
+      .rejects.toThrow('All model providers failed');
+  });
+
+  it('uses tier-specific client when specified', async () => {
+    const defaultClient = createMockClient('default');
+    const fastClient = createMockClient('fast');
+
+    const router = new ModelRouter({
+      default: defaultClient,
+      fast: fastClient,
+      fallbackChain: [],
+    });
+
+    const response = await router.chat(
+      { messages: [{ role: 'user', content: 'Hi' }] },
+      'fast'
+    );
+
+    expect(response.content).toBe('Response from fast');
+    expect(fastClient.chat).toHaveBeenCalled();
+    expect(defaultClient.chat).not.toHaveBeenCalled();
+  });
+});