HTTP status codes / 4xx — Client error
422 Unprocessable Content
What it means
The request is syntactically fine — valid JSON, correct content-type — but semantically wrong: validation failed.
What causes it
Required fields missing, values out of range, business rules violated. Rails and many REST APIs use 422 for all validation errors.
How to fix it
Read the response body — well-behaved APIs enumerate exactly which fields failed and why.