ErrorFindly
4004xx Client ErrorMedium

HTTP 400 Bad Request

Defined in RFC 7231

What Does HTTP 400 Mean?

The server cannot process the request due to something perceived to be a client error, such as malformed syntax.

Common Causes

  1. 1Malformed JSON or XML in request body
  2. 2Missing required request parameters
  3. 3Invalid query string format
  4. 4Request body exceeds expected format

Impact

  • ⚠Form submission fails
  • ⚠API request is rejected
  • ⚠Page displays an error message

Developer Fix

For web developers and application engineers

  1. 1Validate request data before sending
  2. 2Check Content-Type header matches body format
  3. 3Ensure all required parameters are included
  4. 4Return descriptive error messages in 400 responses

Server Admin Fix

For system administrators and DevOps engineers

  1. 1Review server logs for patterns of 400 errors
  2. 2Ensure request parsing middleware is configured correctly
  3. 3Check for overly strict input validation

Frequently Asked Questions

What's the difference between 400 and 422?
400 means the request is malformed (syntax error). 422 means the request is well-formed but semantically invalid (e.g., valid JSON but with values that don't make sense). Use 400 for syntax issues and 422 for validation errors.
How do I debug a 400 error?
Check the response body for error details, verify your request headers (especially Content-Type), validate your request body format, and ensure all required parameters are present and correctly formatted.

Related HTTP Status Codes

422Unprocessable Entity415Unsupported Media Type405Method Not Allowed

About the Author

Web Infrastructure Team

Verified against official RFC specifications and real-world server configurations. HTTP status code behavior confirmed across Apache, Nginx, and Cloudflare.