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Selecting the right status code improves client behavior, caching efficiency, SEO, and observability. Pick the status class flowchart TD A[Request handled successfully] A -->|Yes| S[2xx] A -->|No| Q{Client problem} Q -->|Yes| C[4xx] Q -->|No| E[5xx] 2xx: the server accepted and completed the request. 4xx: the request is invalid or not allowed. 5xx: the server or an upstream failed to process a valid request. Success details: 200 vs 201 vs 202 vs 204 flowchart TD S[2xx] --> K{Created new resource} K -->|Yes| C201[201 Created] K -->|No| P{Processing deferred} P -->|Yes| C202[202 Accepted] P -->|No| B{Response body present} B -->|Yes| C200[200 OK] B -->|No| C204[204 No Content] 201 Created: the request created one or more resources. Location can identify the primary resource; otherwise, the target URI identifies it. 202 Accepted: processing has not finished and might not occur. Describe the current status and provide a status monitor when available. 200 OK: the request succeeded. The meaning of its content depends on the request method, and the content can have zero length. 204 No Content: the action succeeded and there is no additional content or trailers to send. The flowchart is a starting point, not a complete account of method semantics. A successful response with no content is not automatically a 204. ...

The 1xx class indicates the server has received the request headers and the client should proceed. Bodies are not returned. Most clients handle these automatically. Table of 1xx Codes Code Name Summary 100 Continue Client may send the request body. 101 Switching Protocols Protocol upgrade agreed (for example, HTTP to WebSocket). 102 Processing Server has accepted the request, still working (WebDAV). 103 Early Hints Hints for preload before final response (RFC 8297). 100 Continue Used with Expect: 100-continue. Lets large uploads avoid sending a body if the server will reject the request. ...

The 2xx class means the request was successfully received, understood, and accepted. Table of 2xx Codes Code Name Summary 200 OK Standard success. 201 Created New resource created; Location may be set. 202 Accepted Processing deferred; result not yet available. 203 Non-Authoritative Information Metadata altered by a proxy. 204 No Content Success, no body. 205 Reset Content Client should reset the document view. 206 Partial Content Range response. 207 Multi-Status WebDAV multi-result. 208 Already Reported WebDAV deduplication of results. 226 IM Used Instance-manipulations applied. 200 OK General success. Caching: A 200 response is heuristically cacheable unless the method or explicit cache controls prohibit it. Use available validators such as ETag and Last-Modified to support validation. ...

The 3xx class indicates the client must take additional action to complete the request. Table of 3xx Codes Code Name Summary 300 Multiple Choices Several representations available. 301 Moved Permanently Permanent redirect. 302 Found Temporary redirect (legacy semantics). 303 See Other Redirect to a different URI; use GET. 304 Not Modified Cached representation is still valid. 305 Use Proxy Deprecated. 306 (Unused) Reserved. 307 Temporary Redirect Temporary; method preserved. 308 Permanent Redirect Permanent; method preserved. 300 Multiple Choices Rarely used on the public web; content negotiation is usually implicit. ...

The 4xx class signals a problem with the client’s request. Use specific codes to help clients recover. Table of 4xx Codes (selected) Code Name Summary 400 Bad Request Malformed syntax or invalid parameters. 401 Unauthorized Requires authentication (send WWW-Authenticate). 403 Forbidden Understood but refused. 404 Not Found Resource does not exist. 405 Method Not Allowed Method not supported; send Allow. 406 Not Acceptable No acceptable representation. 408 Request Timeout Client took too long to send. 409 Conflict State conflict; e.g., versioning. 410 Gone Resource intentionally removed. 412 Precondition Failed ETag/If-* precondition failed. 415 Unsupported Media Type Content type not supported. 418 I’m a teapot Non-standard; avoid in production. 422 Unprocessable Content Semantically invalid data. 425 Too Early Retry-unsafe with early data. 429 Too Many Requests Rate limit exceeded. 431 Request Header Fields Too Large Headers too large. 451 Unavailable For Legal Reasons Blocked due to legal demands. 400 Bad Request Use for malformed JSON, invalid parameters, or schema violations. Include machine-readable error details. ...

The 5xx class indicates the server failed to fulfil an apparently valid request. These drive SLOs and on-call paging. Table of 5xx Codes (selected) Code Name Summary 500 Internal Server Error Generic server failure. 501 Not Implemented Method not supported at server. 502 Bad Gateway Upstream error at proxy/gateway. 503 Service Unavailable Overloaded or maintenance. 504 Gateway Timeout Upstream timeout. 507 Insufficient Storage WebDAV storage failure. 508 Loop Detected WebDAV loop. 510 Not Extended Further extensions required. 511 Network Authentication Required Captive portal/auth needed. 500 Internal Server Error Catch-all for unhandled exceptions. Log with correlation IDs; prefer specific 5xx when possible. ...

Redirects inform clients that a resource has moved. The choice of status code affects method handling, cache persistence, and search engine behaviour. Redirect Decision Flow flowchart TD A[Is the move permanent?] -->|Yes| B[Must the HTTP method be preserved?] A -->|No| C[Must the HTTP method be preserved?] B -->|Yes| D[308 Permanent Redirect] B -->|No| E[301 Moved Permanently] C -->|Yes| F[307 Temporary Redirect] C -->|No| G[302 Found] %% Contextual hints D:::perm --> H[Set long TTL; expect SEO consolidation] E:::perm --> H F:::temp --> I[Short TTL; use for maintenance] G:::temp --> I classDef perm fill:#ddd,stroke:#333,color:#000; classDef temp fill:#eee,stroke:#333,color:#000; Overview Table Code Meaning Method Preserved Cacheable by Default SEO Signal 301 Permanent redirect Not guaranteed Yes Passes link equity 302 Temporary redirect (legacy) Not guaranteed No Limited 307 Temporary, method preserved Yes No Limited 308 Permanent, method preserved Yes Yes Passes link equity 301 Moved Permanently Permanent relocation. Clients may update bookmarks. Caching: Long-lived; set deliberate TTL. ...