529 Site Is Overloaded

Non-standard: the service is overloaded; used by some APIs instead of 503.

529 at a glance

Code: 529
Name: Site Is Overloaded
Class: 5xx Server error
Status: Non-standard used by Various APIs (e.g. Anthropic API, Qualys SSL Labs)
Documented by: Various APIs (e.g. Anthropic API, Qualys SSL Labs)
Cacheable by default: No by default. Caches only store responses with unrecognised status codes if explicit freshness (Cache-Control max-age) is given.

What 529 means

529 is not registered but is used by some services to say "overloaded, try again soon", distinguishing global capacity problems from per-client rate limits (429). The Anthropic API returns 529 with an overloaded_error, and Qualys SSL Labs' API used 529 when overloaded.

What typically causes a 529

Temporary capacity shortage on the provider's side, independent of your own usage.

Fix it on the server

If you design an API, prefer 503 with Retry-After; standard clients and proxies understand it.

Fix it as a client or visitor

Retry with exponential backoff and jitter; it is not a sign you exceeded your quota.

Headers that go with 529

Header	Role
`Retry-After`	May be included as a hint.

Example response

HTTP/1.1 529
Content-Type: application/json

{"type": "error", "error": {"type": "overloaded_error", "message": "Overloaded"}}