Complete HTTP Status Codes Guide

Understanding HTTP Status Codes

HTTP status codes are three-digit numbers returned by web servers to indicate the result of a client's request. They are essential for web development, API design, and troubleshooting. Each code provides specific information about what happened when processing the request.

1xx - Informational

Request received, continuing process

2xx - Success

Request successfully received and processed

3xx - Redirection

Further action needed to complete request

4xx - Client Error

Request contains bad syntax or cannot be fulfilled

5xx - Server Error

Server failed to fulfill valid request

Status Code	Meaning	Description & Examples
100	Continue	Client should continue sending the request body. curl -X POST -H "Expect: 100-continue" -d "data" /api/upload Used when uploading large files to check if server can accept the request before sending data.
101	Switching Protocols	Server agrees to switch protocols as requested by client. WebSocket handshake: HTTP → WebSocket protocol Essential for real-time applications like chat systems, live updates, and gaming.
102	Processing	Server has received and is processing the request, but no response is available yet. Long-running operations like data processing or file conversion Prevents client timeout during lengthy server operations.
200	OK	Standard response for successful HTTP requests. GET /api/users → Returns user list successfully Most common success response for GET, POST, PUT requests with content.
201	Created	Request succeeded and a new resource has been created. POST /api/users → New user account created RESTful APIs when creating new resources like users, posts, or records.
202	Accepted	Request accepted for processing, but processing not completed. POST /api/batch-process → Job queued for background processing Asynchronous operations, batch processing, email sending queues.
204	No Content	Request succeeded, but no content to return. DELETE /api/users/123 → User deleted successfully DELETE operations, PUT updates that don't return data, successful form submissions.
206	Partial Content	Server is delivering only part of the resource due to a range header. Video streaming: bytes 200-1023/2048 Video streaming, file downloads with resume capability, pagination.
301	Moved Permanently	Resource has moved permanently to a new URL. example.com/old-page → example.com/new-page Website restructuring, domain changes, SEO URL optimization.
302	Found (Temporary Redirect)	Resource temporarily moved to a different URL. Maintenance page redirect, A/B testing redirects Temporary maintenance, load balancing, feature flags.
304	Not Modified	Cached version is still valid, no need to re-download. If-Modified-Since header check → Resource unchanged Browser caching, CDN optimization, bandwidth saving.
307	Temporary Redirect	Temporary redirect that preserves the request method. POST request redirected while maintaining POST method Maintaining HTTP method during temporary redirects.
308	Permanent Redirect	Permanent redirect that preserves the request method. API versioning: /v1/users → /v2/users (keeping POST) API versioning, permanent URL changes while preserving HTTP methods.
400	Bad Request	Server couldn't understand the request due to invalid syntax. Invalid JSON: {"name": "John", "age": } → Missing value Malformed JSON, missing required fields, invalid data types.
401	Unauthorized	Authentication is required but not provided or invalid. API call without Authorization header or invalid token Missing login credentials, expired tokens, invalid API keys.
403	Forbidden	User authenticated but doesn't have permission to access resource. Regular user trying to access admin panel Role-based access control, insufficient permissions, blocked IP addresses.
404	Not Found	Requested resource not found on the server. GET /api/users/999 → User ID 999 doesn't exist Broken links, deleted resources, typos in URLs, non-existent API endpoints.
405	Method Not Allowed	HTTP method not supported for the requested resource. DELETE /api/users → Endpoint only supports GET and POST API design restrictions, read-only endpoints, method-specific handlers.
409	Conflict	Request conflicts with current server state. Creating user with email that already exists Duplicate entries, version conflicts, concurrent modifications.
410	Gone	Resource was available previously but has been permanently removed. Deleted blog post, discontinued product page Permanently deleted content, discontinued services, expired promotions.
422	Unprocessable Entity	Request is well-formed but contains semantic errors. Valid JSON but email format is invalid: "email": "not-an-email" Form validation errors, business logic violations, data format issues.
429	Too Many Requests	User has sent too many requests in a given time period. API rate limit: 100 requests per hour exceeded API rate limiting, DDoS protection, spam prevention.
500	Internal Server Error	Generic error when server fails unexpectedly. Database connection failed, unhandled exception in code Programming errors, database issues, server misconfigurations.
501	Not Implemented	Server doesn't support the functionality required to fulfill the request. HTTP method not implemented: PATCH requests not supported Unsupported HTTP methods, missing features, incomplete API implementations.
502	Bad Gateway	Invalid response received from upstream server. Load balancer receives invalid response from backend server Proxy server issues, microservice communication failures, CDN problems.
503	Service Unavailable	Server is temporarily unable to handle requests. Server maintenance, overloaded with requests Scheduled maintenance, server overload, temporary outages.
504	Gateway Timeout	Upstream server didn't respond within the timeout period. API gateway timeout waiting for microservice response Slow database queries, network issues, overloaded backend services.
507	Insufficient Storage	Server cannot store the representation needed to complete the request. File upload fails due to disk space limitations Disk space issues, storage quota exceeded, large file uploads.