What each code means, and what it means for a monitor.
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| Code | Meaning | For monitoring |
|---|---|---|
| 200 | OK | Success |
| 201 | Created | Success |
| 204 | No Content | Success, but a body assertion will fail |
| 301 | Moved Permanently | Endpoint moved: your check may be measuring the old address |
| 302 | Found | Temporary redirect, often a login wall |
| 304 | Not Modified | Cache hit, treated as success |
| 400 | Bad Request | The check sends something the service rejects |
| 401 | Unauthorized | Usually the check lost its credentials, not an outage |
| 403 | Forbidden | Often a WAF or geo-block hitting the monitor |
| 404 | Not Found | Wrong path, or the route was removed in a deploy |
| 408 | Request Timeout | Server gave up waiting: usually a real symptom |
| 429 | Too Many Requests | Service is alive but rate-limiting your checks |
| 500 | Internal Server Error | Failure |
| 502 | Bad Gateway | Proxy up, application behind it down |
| 503 | Service Unavailable | Overload or maintenance mode |
| 504 | Gateway Timeout | Application too slow for the proxy timeout |
An uptime check normally treats 2xx as success and 5xx as failure. The interesting cases are in between: a 301 means the endpoint moved and your check may be measuring the wrong thing, a 401 usually means the check lost its credentials rather than the service being down, and a 429 means the service is alive but is refusing you specifically.