Does FastAPI Work With Vitest?
FastAPI and Vitest don't directly integrate since FastAPI is Python-based and Vitest is JavaScript/TypeScript, but you can use Vitest to test a FastAPI backend via HTTP requests.
Quick Facts
How FastAPI Works With Vitest
FastAPI runs on Python while Vitest runs in Node.js, so they operate in completely separate runtime environments. The practical integration pattern is testing FastAPI's HTTP API from Vitest using HTTP client libraries like `axios` or `node-fetch`. You'll typically run FastAPI as a development server (via `uvicorn`) and have Vitest make requests to it during test execution. This works well for integration and end-to-end testing scenarios where you want to verify API behavior from a client perspective. The developer experience is smooth—Vitest's watch mode and fast feedback loop pair nicely with FastAPI's auto-reload development server. However, this approach is slower than unit testing because it requires the server to be running and adds network latency. For pure unit testing of API logic, you'd use Python testing frameworks like pytest instead.
Best Use Cases
Testing FastAPI with Vitest
npm install vitest axios --save-devimport { describe, it, expect, beforeAll } from 'vitest';
import axios from 'axios';
const API_URL = 'http://localhost:8000';
describe('FastAPI Integration', () => {
beforeAll(async () => {
// Wait for server to be ready
let ready = false;
for (let i = 0; i < 10; i++) {
try {
await axios.get(`${API_URL}/docs`);
ready = true;
break;
} catch (e) {
await new Promise(r => setTimeout(r, 500));
}
}
if (!ready) throw new Error('FastAPI server not ready');
});
it('GET /items returns list', async () => {
const response = await axios.get(`${API_URL}/items`);
expect(response.status).toBe(200);
expect(Array.isArray(response.data)).toBe(true);
});
it('POST /items creates item', async () => {
const response = await axios.post(`${API_URL}/items`, {
name: 'Test Item',
price: 9.99
});
expect(response.status).toBe(200);
expect(response.data.name).toBe('Test Item');
});
});Known Issues & Gotchas
FastAPI server must be running before Vitest executes tests, or tests will fail with connection errors
Fix: Use Vitest's `globalSetup` to programmatically start the FastAPI dev server via child process, or use Docker Compose to orchestrate services in CI
Async/await in Vitest tests works differently than Python async patterns, leading to race conditions when tests run before responses arrive
Fix: Always await HTTP calls explicitly and use Vitest's timeout option: `it('test', async () => { ... }, { timeout: 10000 })`
CORS headers from FastAPI may block requests if not properly configured for localhost testing
Fix: Configure FastAPI's CORSMiddleware to allow `http://localhost:3000` and `http://localhost:5173` in development
No IDE autocomplete for API response types when using fetch/axios—responses are untyped strings
Fix: Use FastAPI's auto-generated OpenAPI schema with code generation tools like OpenAPI Generator or use TypeScript types manually
Alternatives
- •pytest (Python) + pytest-asyncio for pure backend testing without involving frontend tooling
- •Jest + node-fetch for a JavaScript-based test runner with similar capabilities to Vitest
- •Playwright or Cypress for full end-to-end testing that exercises both FastAPI backend and frontend UI together
Resources
Related Compatibility Guides
Explore more compatibility guides