feat(models): add streaming and tier switching to ModelRouter

src/models/router.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest';
 import { ModelRouter } from './router.js';
-import type { ModelClient, ChatResponse } from './types.js';
+import type { ModelClient, ChatResponse, ChatStreamEvent } from './types.js';
 
 describe('ModelRouter', () => {
   const createMockClient = (name: string, shouldFail = false): ModelClient => ({
@@ -78,3 +78,66 @@ describe('ModelRouter', () => {
     expect(defaultClient.chat).not.toHaveBeenCalled();
   });
 });
+
+describe('ModelRouter streaming', () => {
+  it('streams from primary client', async () => {
+    const mockStream = async function* (): AsyncIterable<ChatStreamEvent> {
+      yield { type: 'content', content: 'Hello' };
+      yield { type: 'done', usage: { inputTokens: 5, outputTokens: 3 } };
+    };
+
+    const mockClient = {
+      chat: vi.fn(),
+      chatStream: vi.fn().mockReturnValue(mockStream()),
+    };
+
+    const router = new ModelRouter({
+      default: mockClient,
+      fallbackChain: [],
+    });
+
+    const chunks: string[] = [];
+    for await (const event of router.chatStream({ messages: [] })) {
+      if (event.type === 'content' && event.content) {
+        chunks.push(event.content);
+      }
+    }
+
+    expect(chunks).toEqual(['Hello']);
+  });
+
+  it('falls back when primary stream fails', async () => {
+    const failingStream = async function* (): AsyncIterable<ChatStreamEvent> {
+      yield { type: 'error', error: new Error('Primary failed') };
+    };
+
+    const fallbackStream = async function* (): AsyncIterable<ChatStreamEvent> {
+      yield { type: 'content', content: 'Fallback' };
+      yield { type: 'done', usage: { inputTokens: 5, outputTokens: 3 } };
+    };
+
+    const primaryClient = {
+      chat: vi.fn(),
+      chatStream: vi.fn().mockReturnValue(failingStream()),
+    };
+
+    const fallbackClient = {
+      chat: vi.fn(),
+      chatStream: vi.fn().mockReturnValue(fallbackStream()),
+    };
+
+    const router = new ModelRouter({
+      default: primaryClient,
+      fallbackChain: [fallbackClient],
+    });
+
+    const chunks: string[] = [];
+    for await (const event of router.chatStream({ messages: [] })) {
+      if (event.type === 'content' && event.content) {
+        chunks.push(event.content);
+      }
+    }
+
+    expect(chunks).toEqual(['Fallback']);
+  });
+});