Why does my API work in Postman but fail in the browser?

Postman is a developer tool that does not enforce CORS. The browser sends an OPTIONS preflight that your server is not handling. Add an OPTIONS route handler that returns 204 with CORS headers.

What status code should OPTIONS return?

Return 204 (No Content) or 200 with the CORS headers and no body.

What is Access-Control-Max-Age?

It tells the browser how long to cache the preflight result in seconds. Set it to 86400 (24 hours) to avoid a preflight on every request.

CORS

CORS Preflight Failing — OPTIONS Returns 404 or 403

Postman works. curl works. The browser fails. That is almost always a missing preflight handler. Browsers send an OPTIONS request before POST, PUT, or DELETE — and your server is not responding to it correctly.

Browser Console Error

Access to fetch at 'https://api.example.com/submit' from origin 'https://app.example.com' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status.

What the browser actually sends first

OPTIONS /api/submit HTTP/1.1
Host: api.example.com
Origin: https://app.example.com
Access-Control-Request-Method: POST
Access-Control-Request-Headers: Content-Type, Authorization

Your server must respond to this OPTIONS request with a 200 or 204 and the correct CORS headers. If it returns 404, the browser stops and shows a CORS error — even though your actual POST endpoint works fine.

What triggers a preflight

Not all requests get a preflight. Simple requests (GET, HEAD, POST with only simple headers like Content-Type: application/x-www-form-urlencoded) skip it. You get a preflight when you use:

Methods: PUT, DELETE, PATCH
Headers: Authorization, Content-Type: application/json, or any custom header
Credentials: any request with credentials: 'include'

Fix by framework

Express

app.options('*', cors()); // Handle all OPTIONS preflights
app.use(cors({ origin: 'https://app.example.com' }));

Nginx

if ($request_method = OPTIONS) {
    add_header Access-Control-Allow-Origin "https://app.example.com";
    add_header Access-Control-Allow-Methods "POST, GET, PUT, DELETE, OPTIONS";
    add_header Access-Control-Allow-Headers "Content-Type, Authorization";
    add_header Access-Control-Max-Age 86400;
    add_header Content-Length 0;
    return 204;
}

FastAPI

# CORSMiddleware handles OPTIONS automatically — no extra config needed
app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://app.example.com"],
    allow_methods=["*"],
    allow_headers=["*"],
)

Django

# Install django-cors-headers
pip install django-cors-headers

# settings.py
INSTALLED_APPS = [..., 'corsheaders']
MIDDLEWARE = ['corsheaders.middleware.CorsMiddleware', ...]
CORS_ALLOWED_ORIGINS = ['https://app.example.com']
CORS_ALLOW_CREDENTIALS = True

Cache the preflight

Access-Control-Max-Age: 86400 tells the browser to cache the preflight result for 24 hours. Without it, the browser sends OPTIONS before every single request — visible as extra network calls in DevTools.

Send a live OPTIONS preflight to your API →