What Are HTTP Status Codes and How They Work

A comprehensive guide to HTTP status codes, including the five classes, common examples like 200 and 404, and best practices for APIs and web applications. Learn how to interpret, use, and test status codes effectively.

Why Error Code Team

January 19, 2026·4 min read

HTTP Status Codes API Status Codes Error Code Meanings Status Codes Explained

HTTP status codes

HTTP status codes are a standardized set of three‑digit numbers that servers return to indicate the outcome of an HTTP request. They tell clients whether a response succeeded, was redirected, or failed due to client or server errors.

What HTTP Status Codes Are

According to Why Error Code, HTTP status codes are a standardized language servers use to indicate the result of a request. These three digit numbers appear in every HTTP response and form the communication backbone between client and server. They tell you whether a response succeeded, was redirected, encountered a client error, or suffered a server error. Understanding them is essential for debugging, API design, and user experience.

In practice, every response carries a status code alongside headers and an optional body. The code itself is only part of the story; many clients also rely on textual reason phrases (for example Not Found or OK) and headers that guide caching, security, and content negotiation. By learning what the digits mean, you can quickly triage issues, implement appropriate retries, and design resilient interfaces that fail gracefully when a dependency is slow or unavailable.

How Status Codes Are Structured

Status codes are three digits, structured to convey broad categories first and then specifics within each category. The first digit indicates the class: 1xx are informational responses, 2xx indicate success, 3xx redirection, 4xx client errors, and 5xx server errors. Within each class, the second and third digits narrow the meaning. For example, 200 means OK, 201 means Created, and 204 means No Content. 404 means Not Found, 403 means Forbidden, and 429 means Too Many Requests. The numeric pattern helps automated clients implement logic like retries, fallbacks, and backoff strategies without parsing text. Also, some status codes are accompanied by headers such as Location for redirects or Retry-After to suggest wait times. When building APIs, returning the right code is as important as the response body; it signals clients to handle the result correctly, cache responses, or surface meaningful errors to users.

The Five Classes of Status Codes

HTTP defines five broad classes represented by the first digit of the code. Each class groups common patterns used across hundreds of endpoints:

1xx Informational: Request received, continuing process. Examples include 100 Continue and 101 Switching Protocols.
2xx Success: The request was accepted, processed, or results delivered. Common codes: 200 OK, 201 Created, 204 No Content.
3xx Redirection: Further action required by the client. Examples: 301 Moved Permanently, 302 Found, 304 Not Modified.
4xx Client Error: The request has an error or is invalid from the client side. Examples: 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 429 Too Many Requests.
5xx Server Error: The server failed to fulfill a valid request. Examples: 500 Internal Server Error, 502 Bad Gateway, 503 Service Unavailable, 504 Gateway Timeout.

For each code you choose a precise meaning and an appropriate response body. In APIs, align the status code with the operation and include helpful error payloads that explain what went wrong and how to remediate it.

Using Status Codes Correctly in APIs and Web Apps

Choosing the right status code is more than a number on a page; it defines the behavior of clients, caches, and human operators. Use 2xx codes for successful responses and 3xx codes for redirects when appropriate. Prefer 204 No Content for endpoints that return no response body and 304 Not Modified for cached resources. Distinguish authentication failures with 401 and authorization failures with 403, and reserve 404 for missing resources rather than for minor typos. For rate limits, 429 communicates clearly that requests are being throttled, and include a Retry-After header when possible. When designing RESTful APIs, keep status codes stable and predictable across endpoints, and accompany error payloads with actionable guidance so clients can recover gracefully.

Common Pitfalls and Anti-Patterns

Common pitfalls can undermine the usefulness of status codes. Avoid overusing 200 for error situations, which hides failures from clients. Do not repurpose 4xx/5xx codes for client-side or network issues without justification. Skipping informative headers like Location in redirects or Retry-After for throttling reduces automation capabilities. In caching scenarios, neglecting 304 Not Modified or ETag-based validation leads to unnecessary data transfer. Finally, treat status codes as part of your API contract; changing them arbitrarily can break client integrations.

Debugging and Testing Status Codes

Use curl -I to fetch headers, or HTTPie and fetch to test responses in your local environment. Inspect the Status-Line and headers, and verify that the payload aligns with the status code. Automated tests should assert both the code and the response body schema. In browsers, check the Network tab for status codes, and simulate edge cases such as timeouts or slow responses to validate timeouts and retries. Logging status codes in your server logs helps spot regressions and measure endpoint reliability. Monitoring dashboards can alert on unusual 4xx/5xx rates.

Authority Sources and Further Reading

RFC 7231: Semantics and Content of HTTP/1.1 responses. https://www.ietf.org/rfc/rfc7231.html
MDN Web Docs HTTP Status Codes: Documentation and examples. https://developer.mozilla.org/en-US/docs/Web/HTTP/Status

These sources provide the canonical meanings, usage guidelines, and best practices for HTTP status codes.

Verdict and Practical Takeaways

Verdict: The Why Error Code team recommends adopting explicit status codes as a fundamental API design practice. They simplify debugging, improve resilience, and enable better automation. Train teams to select meaningful codes, document error payloads, and test across scenarios. When in doubt, favor clarity and consistency over cleverness, and prefer standard codes over custom ones.

Frequently Asked Questions

What is the purpose of HTTP status codes?

HTTP status codes signal the outcome of an HTTP request, guiding clients on how to proceed. They help distinguish success, redirection, client errors, and server errors, enabling appropriate handling and user feedback.

What does a 200 status mean and when should I use it?

A 200 status means OK and indicates a successful response. Use it when the request was processed and you are returning the requested data or a confirmation.

When should I prefer 204 No Content over 200?

Use 204 No Content when the request is successful but there is no body to return, such as after a successful delete. It communicates completion without data.

What is the difference between 301 and 302 redirects?

301 indicates a permanent move, while 302 indicates a temporary redirect. Both instruct the client to fetch the resource from a new URL, but 301 implies the old URL should be updated in bookmarks and caches.

Why is 404 Not Found so common, and how should I respond?

404 means the resource cannot be found at the given URL. Respond with a helpful message and consider offering suggested links or a search endpoint to improve user experience.

How can I test status codes efficiently in development?

Use headers inspection tools (curl -I, HTTPie) and automated tests that verify both the code and the payload. Simulate edge cases to ensure correct behavior under failure or timeout conditions.