4xx Client Errors
These status codes indicate that the client seems to have made an error. The server is unable or unwilling to process the request due to something perceived to be a client error — such as malformed syntax, invalid parameters, missing authentication, or trying to access a resource that doesn't exist.
400
Bad RequestThe server cannot process the request due to malformed syntax, invalid parameters, or other client‑side issues.
401
UnauthorizedThe request lacks valid authentication credentials. The client must authenticate to get the requested response.
402
Payment RequiredReserved for future use. Originally intended for digital payment systems, it is rarely used.
403
ForbiddenThe server understood the request but refuses to authorize it. Authentication will not help; the client is not allowed to access the resource.
404
Not FoundThe server cannot find the requested resource. This is the most well‑known client error.
405
Method Not AllowedThe HTTP method used (e.g., POST, PUT) is not supported for the requested resource.
406
Not AcceptableThe server cannot produce a response matching the list of acceptable values defined in the Accept‑* headers.
407
Proxy Authentication RequiredThe client must authenticate with a proxy before the request can be fulfilled.
408
Request TimeoutThe server timed out waiting for the client to send the full request.
409
ConflictThe request conflicts with the current state of the server (e.g., trying to update a resource that has been modified by another client).
410
GoneThe requested resource is no longer available and will not be available again. This is a permanent condition.
411
Length RequiredThe request did not specify the length of its content (Content‑Length header), which is required.
412
Precondition FailedOne or more conditions given in the request headers (e.g., If‑Match) evaluated to false.
413
Payload Too LargeThe request payload is larger than the server is willing or able to process.
414
URI Too LongThe URI provided is too long for the server to process.
415
Unsupported Media TypeThe request entity has a media type that the server does not support.
416
Range Not SatisfiableThe range specified in the Range header cannot be satisfied (e.g., out‑of‑bounds).
417
Expectation FailedThe server cannot meet the requirements of the Expect header (e.g., Expect: 100‑continue).
418
I'm a teapotAn April Fools' joke (RFC 2324). Not a real status code, but still in the registry.
421
Misdirected RequestThe request was directed at a server that is not able to produce a response (e.g., wrong virtual host).
422
Unprocessable EntityThe server understands the content type but cannot process the contained instructions (validation errors).
423
LockedThe resource is locked (WebDAV). The client must unlock it before making the request.
424
Failed DependencyThe request failed because a previous request in a batch failed (WebDAV).
425
Too EarlyThe server is unwilling to process a request that might be replayed later (early data).
426
Upgrade RequiredThe client should upgrade to a different protocol (e.g., from HTTP/1.1 to HTTP/2).
428
Precondition RequiredThe server requires the request to be conditional (e.g., If‑Match header) to avoid conflicts.
429
Too Many RequestsThe client has sent too many requests in a given amount of time (rate limiting).
431
Request Header Fields Too LargeThe client sent too many or too large headers.
451
Unavailable For Legal ReasonsThe resource is not accessible due to legal reasons (e.g., censorship).
* This list includes the most common 4xx codes. Some less common codes (e.g., 402, 418) are included for reference, but are rarely used in practice.