4xx Client error
HTTP 400 Bad Request
The server can’t or won’t process the request because it sees something as a client error: malformed syntax, invalid framing, an invalid header or cookie, or a body it can’t parse. It is a catch-all for problems with the request itself.
Defined in RFC 9110, section 15.5.1(opens in a new tab)
//_01 Uptime checks
Is 400 up or down for a monitor?
Uptimecheck HTTP monitor
Default settings
Down. It is outside the default 200–399 range, so a monitor reports an outage once a second check confirms it. If 400 is the answer you expect from that URL, add it to the monitor’s expected status codes.
Is it down? checker
Free, at /is-it-down/
Up. The checker counts any answer below 500 as the site answering: a 400 comes from a server that is running.
//_02 Causes and fixes
Why you see 400 and what to do
Typical causes
- Cookies that grew too large or got corrupted (nginx says “Request Header Or Cookie Too Large”).
- Malformed JSON or missing fields sent to an API.
- Invalid characters in the URL, or a client or plugin bug.
If you run the site
- Log why requests are rejected, and say why in the response; APIs can use the problem details format (RFC 9457).
- Check the header size limits of every proxy and load balancer in the chain.
If you’re a visitor
- Clear the site’s cookies and reload.
- Check the URL for typos or odd characters.
Know the moment your site stops answering as it should
An Uptimecheck monitor requests your page as often as every minute, checks the status code against the ones you expect, confirms a failure with a second check and then alerts you.
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Questions
What does HTTP 400 mean?
The server can’t or won’t process the request because it sees something as a client error: malformed syntax, invalid framing, an invalid header or cookie, or a body it can’t parse. It is a catch-all for problems with the request itself.
Does an uptime monitor count 400 as down?
Down. It is outside the default 200–399 range, so a monitor reports an outage once a second check confirms it. If 400 is the answer you expect from that URL, add it to the monitor’s expected status codes.
Downtime is obsolete.
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