420 Enhance Your Calm
loreA non-standard HTTP status code once returned by Twitter's API for rate limiting, named Enhance Your Calm.
420 has never been registered with IANA and does not appear in RFC 9110. Twitter used it for rate-limited clients before moving to the standard 429 Too Many Requests, and a few frameworks have since borrowed it for their own purposes. Encountering it in the wild tells you something about the service's age rather than about the request: it is a fossil of an era when large APIs invented status codes rather than waiting for the registry.
Also known as: HTTP 420, Enhance Your Calm, 420 status code