HTTP Error Codes: Troubleshooting & Fixes
HTTP error status codes indicate why an HTTP request could not be completed successfully. Explore detailed guides for common 4xx client errors and 5xx server errors.
4xx Client errors
HTTP 400 Bad Request: Causes and Fixes
Understand HTTP 400 Bad Request, why servers reject malformed syntax or framing errors, and how to diagnose and resolve API client request issues.
HTTP 401 Unauthorized: Causes, Examples, and Fixes
Understand HTTP 401 Unauthorized, why it signals missing or invalid authentication credentials, how to inspect WWW-Authenticate, and how to fix it.
HTTP 403 Forbidden: Causes, Examples, and Fixes
Understand HTTP 403 Forbidden, why the server refuses an understood request, how permissions and access policies work, and how to debug it safely.
HTTP 404 Not Found: Causes, Examples, and Fixes
Understand HTTP 404 Not Found, why the server cannot find the requested resource, how to check routing and methods, and how to fix it.
HTTP 405 Method Not Allowed: Causes, Examples, and Fixes
Understand HTTP 405 Method Not Allowed, why servers reject unsupported request methods on valid targets, and how to verify the required Allow header.
HTTP 408 Request Timeout: Causes, Examples, and Fixes
Understand HTTP 408 Request Timeout, why origin servers drop idle client connections before receiving complete requests, and how to diagnose latency issues.
HTTP 413 Payload Too Large: Causes, Examples, and Fixes
Understand HTTP 413 Payload Too Large, why servers and proxies reject oversized request bodies, and how to configure upload and buffer limits safely.
HTTP 415 Unsupported Media Type: Causes, Examples, and Fixes
Understand HTTP 415 Unsupported Media Type, why servers reject unsupported payload formats, and how to configure Content-Type and encoding headers.
HTTP 429 Too Many Requests: Retry-After, Backoff, and Fixes
Understand HTTP 429 Too Many Requests, how rate limiting works, how to parse the Retry-After header, and how to implement exponential backoff.
Why Does My POST Request Return 403 Forbidden?
Diagnose why POST requests fail with 403 Forbidden while GET requests work, how CSRF tokens and write permissions operate, and how to fix it safely.
5xx Server errors
HTTP 500 Internal Server Error: Causes, Examples, and Fixes
Understand HTTP 500 Internal Server Error, why unhandled backend exceptions trigger generic server failures, and how to inspect application logs to isolate root causes.
HTTP 501 Not Implemented: Causes, Examples, and Fixes
Understand HTTP 501 Not Implemented, why servers reject unsupported HTTP methods or protocol features, and how to differentiate 501 from 405 Method Not Allowed.
HTTP 502 Bad Gateway: What It Means and How to Debug It
Understand HTTP 502 Bad Gateway, why reverse proxies and gateways return it, how upstream communication fails, and how to debug the issue safely.
HTTP 503 Service Unavailable: Causes and API Handling
Understand HTTP 503 Service Unavailable, why servers return it during maintenance or overload, how Retry-After works, and how to implement backoff.
HTTP 504 Gateway Timeout: Causes and Fixes
Understand HTTP 504 Gateway Timeout, why gateways and proxies time out while waiting for upstream responses, and how to investigate the request path safely.
HTTP 505 HTTP Version Not Supported: Causes, Examples, and Fixes
Understand HTTP 505 HTTP Version Not Supported, why servers reject outdated or unsupported major HTTP protocol versions, and how to configure HTTP version negotiation.
HTTP 508 Loop Detected: Causes, Examples, and Fixes
Understand HTTP 508 Loop Detected, why servers terminate requests encountering infinite loops, and how to fix cyclic redirects and WebDAV binding loops.
Investigating HTTP Errors
When troubleshooting HTTP error codes, always inspect the response status line, response headers, and response payload to isolate client configuration errors from origin server failures.