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Start with the first digit, then identify which system generated the response. HTTP status codes 4xx usually point to the request, credentials, permissions, routing, or rate limits; 5xx usually point to the application, an upstream dependency, or a gateway path. Before changing code or retrying, save the exact method, URL, headers, response, timestamp, and request ID. That evidence prevents a browser symptom from being mistaken for an origin-server failure.
This guide explains what the common codes mean, how to reproduce them safely, which layer to inspect, and the corrective action for each code.
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Read the status-code class first
RFC 9110 defines status codes as three-digit integers from 100 through 599. The first digit is the quickest triage signal: 1xx is informational, 2xx is successful, 3xx is redirection, 4xx is client error, and 5xx is server error. The class is a starting hypothesis, not proof of which machine is at fault: a reverse proxy, CDN, load balancer, or application gateway can generate or rewrite the response seen by the client.
| Class | What it signals | First investigation |
|---|---|---|
| 1xx | Informational response while the request continues | Check protocol negotiation and whether the final response follows. |
| 2xx | The request was processed successfully | Check the response body and application result if the user still sees a failure. |
| 3xx | Redirection or cache-related instruction | Inspect the Location header, redirect chain, and HTTP-to-HTTPS or host rules. |
| 4xx | The server rejected the request, identity, policy, or target | Validate syntax, authentication, authorization, route, resource state, and rate limits. |
| 5xx | The server or a server-side intermediary could not complete a valid request | Inspect application, dependency, capacity, gateway, and upstream logs. |
A page that looks like a server error may therefore have been produced at the edge rather than by your application. Provider-specific request IDs and response headers are useful clues.
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Preserve evidence before retrying
Capture one complete failed transaction before refreshing the page or running a retry loop. Record:
- Exact status code, reason phrase or provider message, and response body.
- HTTP method, complete URL including query string, host, scheme, and trailing slash.
- Request and response headers, especially
Authorization,Content-Type,Content-Length,Transfer-Encoding,Retry-After,Location, and correlation or request IDs. - Request-body shape, content type, and whether the body was accepted by the client library.
- UTC timestamp, local timezone, account or tenant, and whether the problem is reproducible.
- Whether the response came directly from the origin or from a proxy, CDN, firewall, or load balancer.
Redact passwords, API keys, cookies, bearer tokens, signed URLs, and personal data before sharing a capture. A screenshot alone is rarely enough: it hides the method, headers, redirect chain, and response metadata needed to locate the failing layer.
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- Copy the failing method and URL exactly. Preserve the query string and host rather than testing only the visible page address.
- Remove optional headers and body fields, then add them back one at a time. Keep required authentication and content negotiation headers.
- Use browser developer tools for an interactive page, or an API client and
curl -ifor a controlled request. Save the full response headers and body. - Compare the failing request with a known-good request using the same host, method, account, and environment. Differences in a trailing slash, URL encoding, token audience, or content type often explain the result.
- Do not blindly repeat a non-idempotent operation such as an order-creation
POST. Retry only when the API documents idempotency keys or safe retry behavior.
For temporary conditions, use bounded exponential backoff with jitter. A retry policy should have a maximum attempt count and a time limit; otherwise many clients can turn one outage or 429 response into a retry storm.
Identify the layer that generated the response
Inspect the Server, Via, provider-specific headers, body branding, TLS certificate, and request ID. Then compare the public endpoint with a controlled request to the origin when your architecture permits it. A response that includes an edge request ID but no application correlation ID may have stopped before the application. Conversely, an application error structure and an application request ID suggest that the request reached the origin.
- Client: browser, mobile app, SDK, proxy settings, clock, or request construction.
- Edge: CDN cache, WAF, bot rule, geofence, TLS termination, or edge rate limit.
- Gateway or load balancer: health checks, routing, connection pools, host rules, and timeout limits.
- Origin application: route handling, exceptions, permissions, deployment, and configuration.
- Dependencies: database, cache, queue, authentication provider, or third-party API.
Cloudflare documents separate origin and edge causes for 502 and 504 responses, so do not assume a branded error page proves that the origin generated it. Check the intermediary and origin logs at the same timestamp.
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Fix common 4xx errors
400 Bad Request
A 400 response means the server cannot or will not process the request because it appears to contain a client error. Malformed syntax, invalid framing, invalid URL encoding, and contradictory body-length headers are common causes.
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- Percent-encode reserved characters and validate the entire URL, including query parameters.
- Validate JSON or XML before sending it. For multipart uploads, ensure the boundary in the body matches the boundary in
Content-Type. - Set a content type that matches the actual body, such as
application/jsonfor JSON. - Remove duplicate or conflicting headers added by multiple HTTP libraries, proxies, or middleware.
- Check body framing. A contradictory
Content-LengthandTransfer-Encodingcan be rejected as malformed. - Compare the request with a documented example byte for byte where possible.
401 Unauthorized
In HTTP semantics, 401 means the request lacks valid authentication credentials for the target resource. The server must send a WWW-Authenticate challenge describing the required scheme.
- Confirm the
Authorizationscheme and token format, including the required spacing and prefix. - For signed tokens, verify signature, issuer, audience, expiry, not-before time, and acceptable clock skew.
- Check that the token is sent to the intended host and endpoint and is not removed during a redirect.
- Read
WWW-Authenticatefor the required scheme, realm, or scope. - Refresh an expired access token through the documented flow; do not substitute a token from another environment.
A 401 is an authentication problem. If the identity is valid but access is refused by policy, the more appropriate response is usually 403.
403 Forbidden
A 403 response means the server understood the request or identity but refuses to authorize it. The block may be in the application, an organization policy, a WAF, or another edge control.
- Verify account, organization, role, OAuth scope, and resource-level ownership.
- Check IP allowlists, geofencing, bot controls, firewall rules, and WAF events.
- Confirm that the authenticated principal may use the requested HTTP method, not merely view the resource.
- Check CSRF protection for browser form submissions and verify the expected origin or anti-CSRF token.
- Use response headers and provider logs to distinguish an application authorization decision from a CDN or firewall block.
404 Not Found
404 means the server cannot find the requested resource. It may also be used intentionally to hide a resource whose existence should not be disclosed.
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- Compare path, host, scheme, query string, URL encoding, and trailing slash exactly.
- Check case sensitivity; a route or file that works on a case-insensitive development machine may fail on a case-sensitive server.
- Confirm that the deployment contains the route or asset and that the HTTP method matches the route definition.
- Inspect URL-rewrite, reverse-proxy, base-path, and SPA fallback rules.
- Check whether the resource moved, was deleted, or is intentionally represented as 404 for unauthorized users.
408 Request Timeout
408 indicates that the server did not receive a complete request from the client within its allotted time. Investigate slow uploads, unstable connections, idle connection settings, and mismatched client or proxy timeouts. Compare the time at which the client sent headers and body with the server’s request-timeout setting. For large uploads, use supported chunking or increase the timeout only after confirming that the endpoint is designed for it.
429 Too Many Requests
429 indicates a rate or quota limit. Read the response body and headers for the limit, remaining quota, reset time, or Retry-After. Reduce concurrency, honor the server’s retry time, and apply exponential backoff with jitter. Cache safe results and use bulk or batch endpoints where available. Never have every worker retry immediately at the same interval.
409 Conflict
409 means the request conflicts with the current state of a resource. Typical examples are an optimistic-lock version mismatch, a uniqueness constraint, or an operation that is invalid while a workflow is in another state. Fetch the current representation or version, resolve the business conflict, and resubmit deliberately. For concurrent updates, use the API’s version, ETag, or conditional-request mechanism instead of overwriting a newer change.
410 Gone
410 indicates intentional permanent removal. Update bookmarks, clients, integrations, and indexes rather than repeatedly retrying. If a replacement route exists, use the documented migration or redirect; do not mask a permanent removal as a transient network problem.
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Fix common 5xx errors
500 Internal Server Error
500 is a generic server failure. Possible causes include an unhandled exception, bad configuration, out-of-memory condition, file-permission problem, failed migration, or an unexpected input that the application did not handle.
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- Correlate the request ID and timestamp with application exception and access logs.
- Check the latest deployment, configuration change, secret rotation, database migration, and feature-flag update.
- Inspect memory, CPU, disk space, file permissions, worker counts, connection pools, and thread or event-loop saturation.
- Compare the failing input with a successful request and reproduce it in staging or a canary environment using the same configuration.
- Roll back a confirmed bad deployment or disable the offending feature flag, then fix the underlying exception and add a regression test.
A generic 500 body is not a diagnosis. The stack trace belongs in protected server logs, while the client-facing response should avoid exposing secrets or internal paths.
502 Bad Gateway
502 means a gateway received an invalid response from an upstream server. The upstream may be your origin, an internal service, or another intermediary. A crashed process, refused connection, TLS mismatch, malformed headers, or an invalid response body can all produce it.
- Compare the provider-branded response with a direct, authenticated origin request when safe.
- Verify DNS resolution, origin hostname, certificate chain, firewall allowlists, and network reachability from the gateway.
- Check origin crashes, overload, process restarts, dependency failures, and connection-pool exhaustion.
- Inspect load-balancer, proxy, CDN, and origin logs for the same request ID or timestamp.
- When the gateway reports malformed content, check compression and
Content-Lengthconsistency as well as header syntax.
Cloudflare notes that either Cloudflare or the origin can be responsible for a 502. Identify the source before changing application code.
503 Service Unavailable
503 means the server is temporarily unable to handle the request. Maintenance, overload, failed health checks, and unavailable dependencies are common causes. It is intended for temporary conditions.
- Check deployment and maintenance windows, health-check results, autoscaling events, and saturation.
- Verify database, cache, queue, authentication, and third-party dependency health.
- Inspect admission limits, worker queues, connection pools, and circuit breakers.
- Honor
Retry-Afterwhen supplied and retry with bounded backoff rather than flooding the service. - Escalate with the code, URL, timestamp and timezone, request ID, response headers, and intermediary logs.
504 Gateway Timeout
504 means a gateway did not receive an upstream response within its timeout. The application may still be running and may even complete the operation after the client has stopped waiting.
- Measure connect, TLS, time-to-first-byte, and total-read times separately.
- Check slow database queries, dependency calls, queue waits, lock contention, and overloaded workers.
- Compare gateway connect and read timeouts with application and dependency timeouts. A shorter outer timeout can fail a request that the origin would eventually complete.
- Check DNS, firewall, routing, and TLS reachability from the gateway to the origin.
- Make long operations asynchronous where possible: return a job identifier, then let the client poll a status endpoint.
As with 502, determine whether the edge or origin generated the response. A retry is safe only when the operation is idempotent or the API provides an idempotency key.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose the right diagnostic method
| Method | Best use | Limit |
|---|---|---|
| Browser developer tools | Quickly inspect page requests, redirects, cookies, timing, and response headers. | Harder to reproduce exact API bodies and to compare runs consistently. |
curl -i or an API client |
Control method, URL, headers, body, and retry behavior for a minimal reproduction. | Does not by itself show what happened inside proxies or the application. |
| Origin and intermediary logs | Correlate request IDs, routing, exceptions, health checks, and timeout events across layers. | Requires synchronized clocks, retained logs, and correctly propagated IDs. |
| Distributed tracing | Locate latency and failures across gateways, services, databases, and third-party calls. | Sampling or missing instrumentation can omit the failing transaction. |
For an interactive page, begin in the browser. For a reproducible API failure, use a minimal curl request. For a multi-service 5xx incident, centralized logs and tracing provide the strongest evidence. Evaluate every option by evidence quality, layer coverage, reproducibility, retry safety for the HTTP method, and the time needed to roll back or escalate.
A practical escalation checklist
Escalate when the request is valid, the failure is reproducible, and the responsible layer is outside your control. Include:
- Status code and complete response body, with secrets removed.
- Exact method, URL, query string, relevant request and response headers, and body schema.
- UTC timestamp plus timezone, frequency, affected account or region, and a known-good comparison.
- Request, trace, edge, and load-balancer IDs.
- Whether the response came from the browser, API client, CDN, gateway, or origin, and which logs were checked.
- Recent deployment, configuration, DNS, certificate, firewall, or dependency changes.
This information lets a hosting provider or site administrator match the event to infrastructure logs. Cloudflare’s support guidance likewise recommends contacting the hosting provider or site administrator for most 5xx investigations and supplying the code, time, and failing URL.
Prevent recurring HTTP failures
- Propagate a correlation ID from the edge through every service and include a safe identifier in error responses.
- Document timeout budgets and retry policies at each hop so an outer gateway does not time out first.
- Use idempotency keys for operations that clients may need to retry.
- Validate request schemas at the boundary and return precise, stable error bodies.
- Monitor error rates by route, status class, region, deployment, and dependency rather than relying on one aggregate percentage.
- Test health checks, graceful shutdown, rollback, rate limits, and dependency failure modes before production changes.
- Keep clocks synchronized so token validation and cross-service log correlation remain reliable.
There is no universal distribution of HTTP error codes that applies to every site. Provider-specific analytics may use a sample—for example, Cloudflare describes Error Analytics based on a 1% traffic sample—so such figures should not be generalized to all web traffic. Use your own route-level logs and traces to establish a baseline.
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