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SSourav Saha

Building scalable backend systems, distributed infrastructure, and cloud-native applications.

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HTTP Status Lookup

A searchable reference for standard HTTP status codes. Quickly find definitions, common causes, and specification details.

Description

The initial part of a request has been received and has not yet been rejected by the server. The server intends to send a final response after the request has been fully received and acted upon.

Common Causes

Client sent an Expect: 100-continue header and the server is ready to receive the request body.

Specification

RFC 7231, Section 6.2.1

Description

The server understands and is willing to comply with the client's request, via the Upgrade header field, for a change in the application protocol being used on this connection.

Common Causes

Often seen when upgrading an HTTP connection to a WebSocket connection.

Specification

RFC 7231, Section 6.2.2

Description

An interim response used to inform the client that the server has accepted the complete request, but has not yet completed it.

Common Causes

Used in WebDAV to prevent the client from timing out while a complex request is being processed.

Specification

RFC 2518, Section 10.1

Description

Used to return some response headers before final HTTP message.

Common Causes

Used to allow the user agent to start preloading resources while the server prepares the main response.

Specification

RFC 8297

Description

The request has succeeded. The information returned with the response is dependent on the method used in the request.

Common Causes

Standard response for successful HTTP requests (GET, POST, etc.).

Specification

RFC 7231, Section 6.3.1

Description

The request has been fulfilled and has resulted in one or more new resources being created.

Common Causes

Typically the response to a successful POST or PUT request.

Specification

RFC 7231, Section 6.3.2

Description

The request has been accepted for processing, but the processing has not been completed.

Common Causes

Used for asynchronous processing where a background worker picks up the task.

Specification

RFC 7231, Section 6.3.3

Description

The request was successful but the enclosed payload has been modified from that of the origin server's 200 (OK) response by a transforming proxy.

Common Causes

A proxy server modified the response headers before passing it to the client.

Specification

RFC 7231, Section 6.3.4

Description

The server has successfully fulfilled the request and that there is no additional content to send in the response payload body.

Common Causes

Often returned after a successful DELETE or PUT request where the client doesn't need to navigate away.

Specification

RFC 7231, Section 6.3.5

Description

The server successfully fulfilled the request and desires that the user agent reset the "document view", which caused the request to be sent, to its original state.

Common Causes

Used after submitting a form so the browser clears the form fields.

Specification

RFC 7231, Section 6.3.6

Description

The server is successfully fulfilling a range request for the target resource.

Common Causes

Used for resumable downloads or video streaming using the Range header.

Specification

RFC 7233, Section 4.1

Description

The target resource has more than one representation, each with its own more specific identifier.

Common Causes

Used in content negotiation when multiple options are available (e.g., multiple video formats).

Specification

RFC 7231, Section 6.4.1

Description

The target resource has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs.

Common Causes

URL restructuring, migrating to HTTPS, or changing domains. Browsers will cache this redirect.

Specification

RFC 7231, Section 6.4.2

Description

The target resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client ought to continue to use the effective request URI for future requests.

Common Causes

Temporary redirects. Sometimes historically referred to as 'Moved Temporarily'.

Specification

RFC 7231, Section 6.4.3

Description

The server is redirecting the user agent to a different resource, as indicated by a URI in the Location header field, which is intended to provide an indirect response to the original request.

Common Causes

Used after a POST request (Post/Redirect/Get pattern) to prevent duplicate submissions on refresh.

Specification

RFC 7231, Section 6.4.4

Description

A conditional GET or HEAD request has been received and would have resulted in a 200 (OK) response if it were not for the fact that the condition evaluated to false.

Common Causes

Browser caching. The client already has the latest version of the resource.

Specification

RFC 7232, Section 4.1

Description

The target resource resides temporarily under a different URI and the user agent MUST NOT change the request method if it performs an automatic redirection to that URI.

Common Causes

Strict temporary redirect that preserves the HTTP method (unlike 302 which often changes POST to GET).

Specification

RFC 7231, Section 6.4.7

Description

The target resource has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs. The HTTP method MUST NOT change.

Common Causes

Strict permanent redirect that preserves the HTTP method (unlike 301).

Specification

RFC 7538

Description

The server cannot or will not process the request due to something that is perceived to be a client error.

Common Causes

Malformed request syntax, invalid request message framing, or deceptive request routing.

Specification

RFC 7231, Section 6.5.1

Description

The request has not been applied because it lacks valid authentication credentials for the target resource.

Common Causes

Missing, expired, or invalid authentication tokens or credentials.

Specification

RFC 7235, Section 3.1

Description

Reserved for future use.

Common Causes

Rarely used, but originally intended for digital cash or micro-payment systems. Sometimes used by APIs when usage limits are exceeded.

Specification

RFC 7231, Section 6.5.2

Description

The server understood the request but refuses to authorize it.

Common Causes

The client is authenticated, but does not have the necessary permissions (e.g., trying to access admin resources as a normal user).

Specification

RFC 7231, Section 6.5.3

Description

The origin server did not find a current representation for the target resource or is not willing to disclose that one exists.

Common Causes

Broken links, deleted content, or incorrect URLs.

Specification

RFC 7231, Section 6.5.4

Description

The method received in the request-line is known by the origin server but not supported by the target resource.

Common Causes

Sending a POST request to an endpoint that only accepts GET requests.

Specification

RFC 7231, Section 6.5.5

Description

The target resource does not have a current representation that would be acceptable to the user agent, according to the proactive negotiation header fields.

Common Causes

The client requested a specific content type (e.g., application/xml) via the Accept header, but the server can only return a different type (e.g., application/json).

Specification

RFC 7231, Section 6.5.6

Description

The server did not receive a complete request message within the time that it was prepared to wait.

Common Causes

Slow internet connections or stalled client processes.

Specification

RFC 7231, Section 6.5.7

Description

The request could not be completed due to a conflict with the current state of the target resource.

Common Causes

Trying to create a user with an email address that already exists, or edit conflicts in a collaborative app.

Specification

RFC 7231, Section 6.5.8

Description

The server is refusing to process a request because the request payload is larger than the server is willing or able to process.

Common Causes

Uploading a file that exceeds the server's maximum allowed size limit.

Specification

RFC 7231, Section 6.5.11

Description

The origin server is refusing to service the request because the payload is in a format not supported by this method on the target resource.

Common Causes

Sending XML to an API that only accepts JSON (Content-Type header mismatch).

Specification

RFC 7231, Section 6.5.13

Description

Any attempt to brew coffee with a teapot should result in the error code "418 I'm a teapot".

Common Causes

An April Fools' joke from 1998 (HTCPCP). Occasionally used as an easter egg.

Specification

RFC 2324

Description

The server understands the content type of the request entity, and the syntax of the request entity is correct, but was unable to process the contained instructions.

Common Causes

Validation errors, such as missing required fields or invalid data formats in a JSON body.

Specification

RFC 4918, Section 11.2

Description

The user has sent too many requests in a given amount of time ("rate limiting").

Common Causes

Exceeding API rate limits or DDoS protection mechanisms triggering.

Specification

RFC 6585, Section 4

Description

The server encountered an unexpected condition that prevented it from fulfilling the request.

Common Causes

Unhandled exceptions, bugs in the application code, or database connection failures.

Specification

RFC 7231, Section 6.6.1

Description

The server does not support the functionality required to fulfill the request.

Common Causes

The server does not recognize the HTTP method, or it lacks the ability to fulfill the request.

Specification

RFC 7231, Section 6.6.2

Description

The server, while acting as a gateway or proxy, received an invalid response from an inbound server it accessed while attempting to fulfill the request.

Common Causes

An upstream server (like an application server behind a reverse proxy/load balancer) returned an error or died.

Specification

RFC 7231, Section 6.6.3

Description

The server is currently unable to handle the request due to a temporary overload or scheduled maintenance, which will likely be alleviated after some delay.

Common Causes

Server maintenance, temporary overloading, or the backend service being down.

Specification

RFC 7231, Section 6.6.4

Description

The server, while acting as a gateway or proxy, did not receive a timely response from an upstream server it needed to access in order to complete the request.

Common Causes

The upstream server is taking too long to respond (e.g., a slow database query).

Specification

RFC 7231, Section 6.6.5