HTTP 422 vs 400: Use 422 for Validation Failures

APIs return 400 Bad Request for all client errors including schema validation failures, making it hard for clients to distinguish malformed requests from business-rule violations.

Use 400 for structurally malformed requests (unparseable JSON, wrong content-type). Use 422 Unprocessable Entity for syntactically valid requests that fail semantic validation (missing required field, invalid enum value).

// Express example
app.post('/users', (req, res) => {
  let body;
  try { body = JSON.parse(rawBody); }
  catch { return res.status(400).json({ error: 'Invalid JSON' }); }

  const errors = validateUser(body);
  if (errors.length) return res.status(422).json({ errors });

  createUser(body).then(u => res.status(201).json(u));
});

RFC 4918 defines 422 specifically for content that is well-formed but semantically incorrect. Clients can write separate error-handling branches for parse errors vs. field validation errors.