In JSON Schema, additionalProperties controls whether object fields absent from properties are accepted. They are allowed by default; set it to false for strict validation. section 7.3
A database response with an unwanted field
A book response containing dummy_property passes a schema by default, even when title, isbn, and available are the only declared properties. Adding additionalProperties: false rejects it.
Choose the boundary deliberately:
- Allow extra request fields when client flexibility is more useful than strict rejection. section 7.3
- Disallow extra response fields when the server must guarantee exactly the documented shape. section 7.3
