What it means
The server understands the format (e.g. valid JSON), but the content breaks the rules: a required field is missing, an email is invalid or a date is in the past. The code is common in REST APIs (Rails, Laravel, FastAPI) for validation errors.
Most common causes
- A missing or empty required field.
- A value in the wrong format (email, phone, date).
- A value outside the allowed range.
- A business rule that doesn't allow the request.
What you can do as a visitor
- Check what you entered in the form; the field with the error is usually highlighted.
What the site owner or developer should do
- Return a list of fields and error messages in the response.
- Use 400 for an unreadable request and 422 for invalid content.
- Validate on the client side too for a better user experience.
Impact on SEO and Google
422s occur in forms and APIs and don't affect page indexing.
Frequently asked questions
400 or 422?
400 for a malformed request, 422 for a well-formed request with invalid data.
Why does FastAPI return 422?
Because the data doesn't match the expected model (types, required fields).