HTTP 400 Bad Request

What it means

400 Bad Request means "there's something wrong with the request itself." Maybe the JSON didn't parse, a required field was missing, a query string parameter was the wrong type, or a header was malformed. The server is rejecting the request before any business logic runs.

400 is intentionally vague — it's a catch-all. APIs that care about developer experience usually return more specific 4xx codes (422 for semantic validation failures, 401/403 for auth, 415 for unsupported media types) and reserve 400 for syntactically invalid requests.

Common causes

• Malformed JSON in the request body
• Missing required headers (e.g. Content-Type)
• Query parameter type mismatch (string where number expected)
• Oversized URL or header
• Invalid character encoding

How to fix it

1. 1Inspect the request body — does it parse as the declared Content-Type?
2. 2Verify required parameters are present and correctly typed
3. 3Check for tooling that's mangling the payload (proxies, middlewares)

How Uptimera reports 400

Uptimera flags 400 as down by default since healthy production endpoints rarely return 400 for the same fixed monitor request. Adjust the expected status per monitor if your endpoint intentionally returns 400 for the monitor's payload.

Related codes