fix(router): align fallback semantics and oauth provider behavior

src/models/router.test.ts

@@ -47,6 +47,57 @@ describe('ModelRouter', () => {
     expect(fallbackClient.chat).toHaveBeenCalled();
   });
 
+  it('skips duplicate fallback clients that already failed as primary', async () => {
+    const failingPrimary = createMockClient('primary', true);
+    const fallbackClient = createMockClient('fallback');
+
+    const router = new ModelRouter({
+      default: failingPrimary,
+      fallbackChain: [failingPrimary, fallbackClient],
+    });
+
+    const response = await router.chat({ messages: [{ role: 'user', content: 'Hi' }] });
+
+    expect(response.content).toBe('Response from fallback');
+    expect(failingPrimary.chat).toHaveBeenCalledTimes(1);
+    expect(fallbackClient.chat).toHaveBeenCalledTimes(1);
+  });
+
+  it('applies retry policy to fallback clients', async () => {
+    const failingPrimary = createMockClient('primary', true);
+    let attempts = 0;
+    const flakyFallback: ModelClient = {
+      chat: vi.fn().mockImplementation(async () => {
+        attempts += 1;
+        if (attempts === 1) {
+          throw new Error('transient');
+        }
+        return {
+          content: 'Recovered fallback',
+          stopReason: 'end_turn',
+          usage: { inputTokens: 1, outputTokens: 1 },
+        } satisfies ChatResponse;
+      }),
+    };
+
+    const router = new ModelRouter({
+      default: failingPrimary,
+      fallbackChain: [flakyFallback],
+      retryConfig: {
+        maxRetries: 1,
+        initialDelayMs: 1,
+        backoffMultiplier: 1,
+        maxDelayMs: 1,
+        nonRetryablePatterns: [],
+      },
+    });
+
+    const response = await router.chat({ messages: [{ role: 'user', content: 'retry fallback' }] });
+
+    expect(response.content).toBe('Recovered fallback');
+    expect(flakyFallback.chat).toHaveBeenCalledTimes(2);
+  });
+
   it('throws when all providers fail', async () => {
     const failing1 = createMockClient('primary', true);
     const failing2 = createMockClient('fallback', true);