HTTP 412 Precondition Failed

What does HTTP 412 mean?

A condition in the request headers (If-Match, If-Unmodified-Since) was not met. Used for safe concurrent updates: the client sends the ETag it last saw, and the server rejects the update with 412 if the resource changed since then.

4xx Client error: The request has a problem the client can fix: wrong URL, missing credentials, bad data or too many requests.

How to fix 412 Precondition Failed

• Re-fetch the resource and its current ETag, then retry.

412 and SEO

Not relevant for web pages.

Example response

HTTP/1.1 412 Precondition Failed

How to return a 412 response

Node.js (Express)

res.status(412).json({ error: 'Precondition Failed' });

PHP

http_response_code(412);

Python (Flask)

return {'error': 'Precondition Failed'}, 412

Go (net/http)

w.WriteHeader(412)

nginx

return 412;

Related status codes

• 409 Conflict — The request conflicts with the current state of the resource.
• 304 Not Modified — The cached copy is still valid, so the server sends no body.
• 428 Precondition Required — The server requires the request to be conditional.

Check which status code a URL returns with the HTTP header checker.