HTTP status code reference

Look up a code quickly and know whether it is your application, your proxy or the client.

Filter by code or name

1xx Informational

100Continue

Meaning: The server received the request headers and the client should send the body.
For monitoring: Rarely seen by monitors; interim response.

101Switching Protocols

Meaning: The server agrees to switch protocols, typically to WebSocket.
For monitoring: Expected only on upgrade requests.

Code	Name	Meaning	For monitoring
100	Continue	The server received the request headers and the client should send the body.	Rarely seen by monitors; interim response.
101	Switching Protocols	The server agrees to switch protocols, typically to WebSocket.	Expected only on upgrade requests.

2xx Success

200OK

Meaning: The request succeeded.
For monitoring: Healthy by default. Combine with a content assertion to catch error pages served with 200.

201Created

Meaning: A resource was created.
For monitoring: Expected for POST on create endpoints in transaction checks.

202Accepted

Meaning: The request was accepted for asynchronous processing.
For monitoring: Healthy, but says nothing about whether the job completes.

204No Content

Meaning: Success with an empty body.
For monitoring: Healthy. Common for health endpoints and beacons.

206Partial Content

Meaning: A range request returned part of the resource.
For monitoring: Healthy for range requests.

Code	Name	Meaning	For monitoring
200	OK	The request succeeded.	Healthy by default. Combine with a content assertion to catch error pages served with 200.
201	Created	A resource was created.	Expected for POST on create endpoints in transaction checks.
202	Accepted	The request was accepted for asynchronous processing.	Healthy, but says nothing about whether the job completes.
204	No Content	Success with an empty body.	Healthy. Common for health endpoints and beacons.
206	Partial Content	A range request returned part of the resource.	Healthy for range requests.

3xx Redirection

301Moved Permanently

Meaning: The resource has a new permanent URL.
For monitoring: Followed automatically. A chain of 301s adds latency; monitor the final URL directly.

302Found

Meaning: Temporary redirect; the method may change to GET.
For monitoring: Followed automatically. Unexpected 302 to a login page often means a session or auth problem.

303See Other

Meaning: Redirect to a different resource with GET.
For monitoring: Followed automatically.

304Not Modified

Meaning: The cached copy is still valid.
For monitoring: Only on conditional requests. Monitors send none, so a 304 indicates an odd cache layer.

307Temporary Redirect

Meaning: Temporary redirect that keeps the method and body.
For monitoring: Followed automatically.

308Permanent Redirect

Meaning: Permanent redirect that keeps the method and body.
For monitoring: Followed automatically.

Code	Name	Meaning	For monitoring
301	Moved Permanently	The resource has a new permanent URL.	Followed automatically. A chain of 301s adds latency; monitor the final URL directly.
302	Found	Temporary redirect; the method may change to GET.	Followed automatically. Unexpected 302 to a login page often means a session or auth problem.
303	See Other	Redirect to a different resource with GET.	Followed automatically.
304	Not Modified	The cached copy is still valid.	Only on conditional requests. Monitors send none, so a 304 indicates an odd cache layer.
307	Temporary Redirect	Temporary redirect that keeps the method and body.	Followed automatically.
308	Permanent Redirect	Permanent redirect that keeps the method and body.	Followed automatically.

4xx Client errors

400Bad Request

Meaning: The request was malformed.
For monitoring: Failure. Check required headers or parameters the endpoint expects.

401Unauthorized

Meaning: Authentication is required or failed.
For monitoring: Failure unless expected. Expired tokens and rotated credentials are the usual cause.

403Forbidden

Meaning: The server understood but refuses.
For monitoring: Failure. WAFs and bot protection often block monitoring user agents; allowlist the probe.

404Not Found

Meaning: No resource at this URL.
For monitoring: Failure. Deployments that drop routes or change paths show up here first.

405Method Not Allowed

Meaning: The method is not supported for this URL.
For monitoring: Failure. HEAD is sometimes rejected where GET works; use GET.

408Request Timeout

Meaning: The server timed out waiting for the request.
For monitoring: Failure; usually a slow network path or an idle-connection policy.

409Conflict

Meaning: The request conflicts with current state.
For monitoring: Expected for some idempotency tests, otherwise a failure.

410Gone

Meaning: The resource is permanently gone.
For monitoring: Failure; remove the monitor if intentional.

413Content Too Large

Meaning: The request body exceeds server limits.
For monitoring: Failure on transaction steps with large bodies.

415Unsupported Media Type

Meaning: The body format is not supported.
For monitoring: Check the Content-Type header of the step.

422Unprocessable Content

Meaning: The syntax is fine but the content fails validation.
For monitoring: Failure; validation rules changed or test data is stale.

425Too Early

Meaning: The server refuses a request that might be replayed.
For monitoring: Rare; related to TLS early data.

429Too Many Requests

Meaning: Rate limited.
For monitoring: Failure by default. Lower the check frequency or allowlist the probe; honour Retry-After.

451Unavailable For Legal Reasons

Meaning: Access is denied for legal reasons.
For monitoring: Depends on the probe location; compare regions.

Code	Name	Meaning	For monitoring
400	Bad Request	The request was malformed.	Failure. Check required headers or parameters the endpoint expects.
401	Unauthorized	Authentication is required or failed.	Failure unless expected. Expired tokens and rotated credentials are the usual cause.
403	Forbidden	The server understood but refuses.	Failure. WAFs and bot protection often block monitoring user agents; allowlist the probe.
404	Not Found	No resource at this URL.	Failure. Deployments that drop routes or change paths show up here first.
405	Method Not Allowed	The method is not supported for this URL.	Failure. HEAD is sometimes rejected where GET works; use GET.
408	Request Timeout	The server timed out waiting for the request.	Failure; usually a slow network path or an idle-connection policy.
409	Conflict	The request conflicts with current state.	Expected for some idempotency tests, otherwise a failure.
410	Gone	The resource is permanently gone.	Failure; remove the monitor if intentional.
413	Content Too Large	The request body exceeds server limits.	Failure on transaction steps with large bodies.
415	Unsupported Media Type	The body format is not supported.	Check the Content-Type header of the step.
422	Unprocessable Content	The syntax is fine but the content fails validation.	Failure; validation rules changed or test data is stale.
425	Too Early	The server refuses a request that might be replayed.	Rare; related to TLS early data.
429	Too Many Requests	Rate limited.	Failure by default. Lower the check frequency or allowlist the probe; honour Retry-After.
451	Unavailable For Legal Reasons	Access is denied for legal reasons.	Depends on the probe location; compare regions.

5xx Server errors

500Internal Server Error

Meaning: The application crashed or raised an unhandled error.
For monitoring: Failure. Correlate with deployments and application logs.

501Not Implemented

Meaning: The method is not implemented.
For monitoring: Failure; configuration problem.

502Bad Gateway

Meaning: A proxy got an invalid response from upstream.
For monitoring: Failure. The load balancer is up but the application behind it is not.

503Service Unavailable

Meaning: Overloaded or down for maintenance.
For monitoring: Failure unless inside a maintenance window. Check capacity and health checks of upstreams.

504Gateway Timeout

Meaning: The upstream did not answer in time.
For monitoring: Failure. Slow database queries or stuck workers are typical causes.

505HTTP Version Not Supported

Meaning: The HTTP version is not supported.
For monitoring: Rare; protocol misconfiguration.

507Insufficient Storage

Meaning: The server is out of storage.
For monitoring: Failure; disk full.

508Loop Detected

Meaning: The server detected an infinite loop.
For monitoring: Failure.

511Network Authentication Required

Meaning: The network requires authentication (captive portal).
For monitoring: Indicates the probe network, not your site.

Code	Name	Meaning	For monitoring
500	Internal Server Error	The application crashed or raised an unhandled error.	Failure. Correlate with deployments and application logs.
501	Not Implemented	The method is not implemented.	Failure; configuration problem.
502	Bad Gateway	A proxy got an invalid response from upstream.	Failure. The load balancer is up but the application behind it is not.
503	Service Unavailable	Overloaded or down for maintenance.	Failure unless inside a maintenance window. Check capacity and health checks of upstreams.
504	Gateway Timeout	The upstream did not answer in time.	Failure. Slow database queries or stuck workers are typical causes.
505	HTTP Version Not Supported	The HTTP version is not supported.	Rare; protocol misconfiguration.
507	Insufficient Storage	The server is out of storage.	Failure; disk full.
508	Loop Detected	The server detected an infinite loop.	Failure.
511	Network Authentication Required	The network requires authentication (captive portal).	Indicates the probe network, not your site.

Alert on the status codes that matter to you, with expected-status rules. Free for 5 monitors, no credit card.

Start monitoring

How it measures

Definitions follow RFC 9110 and the IANA registry; the monitoring notes reflect how UptimeMonitor360 treats each class by default.

Limits

Non-standard codes used by some vendors (for example 520 to 530 from CDNs) are summarized by class only.