Precondition Failed

4xx Client Error

Last updated: July 19, 2026

A precondition given in the request headers (e.g. If-Match) evaluated to false.

Description

Used with ETag-based concurrency control. If the client sends If-Match: 'v1' but the server's current ETag is 'v2', respond with 412.

Return 412 on optimistic locking failures where an If-Match / If-Unmodified-Since header didn't match.

RFC 9110 §15.5.13

HTTP response

HTTP/1.1 412 Precondition Failed
Content-Type: application/json

{"error":"Precondition Failed"}

Node.js (Express)

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

Go (net/http)

w.WriteHeader(http.Status412)

Python (Flask)

return jsonify({"error": "Precondition Failed"}), 412

Not Modified

The cached copy the client already has is still valid; no body is returned.

Conflict

The request conflicts with the current state of the resource.

Precondition Required

The server requires the request to be conditional (e.g. include If-Match).