HTTP 400 vs 422: Bad Request vs Unprocessable Content

Quick Decision

• -Return 400 when the server cannot process the request because it is malformed at request syntax/framing level.
• -Return 422 when content type and syntax are understood, but the contained instructions are semantically invalid.
• -If deserialization/parsing fails, prefer 400; if parsing succeeds and business/schema rules fail, prefer 422.

Key Differences

• -400 means malformed request processing at protocol/request layer; 422 means semantic validation failure after successful interpretation.
• -400 remediation focuses on syntax, encoding, headers, and request framing.
• -422 remediation focuses on field constraints, state invariants, and domain rules.
• -Consistent 400/422 boundaries improve client error handling, retries, and SDK behavior.

Step-by-Step Diagnosis

1. 1Verify whether parser/deserializer succeeded before entering validation logic.
2. 2Capture exact failing stage: transport/parsing (400) vs validation/business rule (422).
3. 3Ensure error payloads use machine-readable fields so clients can correct input deterministically.
4. 4Test contract examples in CI to enforce stable status-code mapping for identical failure classes.

Common Misclassifications

• -Returning 400 for every validation error, which erases semantic error classification.
• -Returning 422 when payload cannot be parsed at all, which misstates failure stage.
• -Mixing 400 and 422 inconsistently across equivalent endpoints and confusing client behavior.

Response Examples

HTTP/1.1 400 Bad Request
Content-Type: application/problem+json

{
  "type": "https://api.example.com/problems/malformed-json",
  "title": "Bad Request",
  "status": 400,
  "detail": "Malformed JSON at line 3, column 15."
}

HTTP/1.1 422 Unprocessable Content
Content-Type: application/problem+json

{
  "type": "https://api.example.com/problems/validation-error",
  "title": "Unprocessable Content",
  "status": 422,
  "detail": "Validation failed for one or more fields.",
  "errors": [
    { "field": "status", "message": "Cannot transition from shipped to pending." }
  ]
}

Pro Tip

Frequently Asked Questions