DevArbor
4xx Client Error

412 Precondition Failed

What it means

The server evaluated a conditional header (If-Match, If-None-Match, If-Modified-Since, If-Unmodified-Since) and found the condition wasn't met. Commonly used for optimistic locking — if the resource changed since you last read it, the update is rejected.

Site Visitor

What can I do?

  • Check the URL for typos — a single wrong character causes most 4xx errors.
  • Try navigating to the site's homepage and searching from there.
  • If the problem persists after retrying, contact the site owner.
Developer

How to debug & fix

  1. Return the current ETag in the 412 response so the client can refresh and retry
  2. Implement optimistic locking: clients read with ETag, update with If-Match
  3. Provide the user with a conflict resolution UI
  4. Review the RFC for this status code for precise semantics before implementation.

Code Example

Node.js / Express
app.put('/docs/:id', async (req, res) => {
  const doc = await Doc.findById(req.params.id);
  const clientEtag = req.headers['if-match'];
  if (clientEtag && clientEtag !== doc.etag) {
    return res.status(412).set('ETag', doc.etag)
      .json({ error: 'Precondition Failed — document was modified' });
  }
  // proceed with update...
});

Related Status Codes

409 Conflict 304 Not Modified

How HTTP Status Codes Work

Every HTTP response carries a three-digit status code that tells the client — browser, API consumer, or search-engine crawler — exactly what happened. The first digit defines the class: 1xx informational (request in progress), 2xx success, 3xx redirection, 4xx client error (bad request, missing auth, not found), and 5xx server failure.

Status codes are standardised in RFC 9110 (HTTP Semantics, 2022). Extensions like WebDAV (RFC 4918) and rate-limit headers (RFC 6585) added codes beyond the core set. When a client receives an unrecognised code, the rule is to treat it as the generic x00 of its class.

Why the Right Code Matters

Semantically correct codes help search engines index accurately (301 passes link equity; 410 removes pages faster than 404), allow API clients to implement correct retry logic (429 + Retry-After, 503 + Retry-After), and let monitoring systems distinguish bugs (500) from load issues (503) from auth failures (401/403).

Looking up a different status code? The full reference covers all HTTP codes with causes, fix guides, and copyable code examples for Node.js and Python.

Browse the full HTTP Status Code reference →

Frequently Asked Questions

What does HTTP 412 Precondition Failed mean?
The server evaluated a conditional header (If-Match, If-None-Match, If-Modified-Since, If-Unmodified-Since) and found the condition wasn't met. Commonly used for optimistic locking — if the resource changed since you last read it, the update is rejected.
Is HTTP 412 the visitor's fault?
HTTP 412 Precondition Failed is generally a client-side error, meaning the request itself has an issue. However, many causes — such as a broken link on the site or a misconfigured redirect — are the website owner's responsibility, not the visitor's.
How do I fix HTTP 412 Precondition Failed?
As a visitor: check the URL for typos, go to the homepage, or search for the content. As a developer: return the current ETag in the 412 response so the client can refresh and retry.