Unprocessable Content

Overview

422 Unprocessable Content (formerly Unprocessable Entity) means the request is syntactically well-formed (valid JSON, etc.) but the values it contains are semantically invalid, so the server cannot process it. It is used for validation failures such as a malformed email, a negative age, or a missing required field.

Where 400 Bad Request signals "the request itself is broken", 422 is the more specific "we parsed the request fine, but its contents failed validation or a business rule." Many REST/JSON APIs use it for form-validation failures.

When it happens

• The JSON is valid but a field value fails validation
• A required field is empty, or a format (email, date, range) is wrong
• A business rule is violated (e.g. a start date after the end date)

Request / Response example

POST /api/signup HTTP/1.1
Host: api.example.com
Content-Type: application/json

{"email":"not-an-email","age":-3}

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

{"errors":[{"field":"email","msg":"invalid format"},{"field":"age","msg":"must be >= 0"}]}

In code

app.post('/api/signup', (req, res) => {
  const errors = validate(req.body);
  if (errors.length) {
    return res.status(422).json({ errors });
  }
  // create the user...
});

@app.post('/api/signup')
def signup():
    data = request.get_json()
    errors = validate(data)
    if errors:
        return jsonify(errors=errors), 422
    # ...

Notes

• Some teams collapse both syntax and semantic errors into 400 — check the API docs for which convention it uses.
• 422 will not resolve by retrying; you must correct the input to pass (unlike 429/503, where retrying helps).

Related status codes

Related headers

Specification