additionalProperty

Additional property handling in JSON Schema refers to how a schema validates properties of an object that are not explicitly listed in the schema’s properties or patternProperties keywords. The keyword is named additionalProperties and can influence whether extra keys are allowed and how they are validated.

The value of additionalProperties can be a boolean or a schema. If it is true, extra properties beyond those defined in properties are allowed without further validation. If it is false, extra properties are not allowed. If it is a schema (an object), each extra property must satisfy that schema, meaning additional properties are validated just like defined properties, according to the specified constraints (for example, type, format, or other subschema rules).

AdditionalProperties works in conjunction with other object-constraining keywords. Typically, it applies to properties not covered by properties or patternProperties. If a property name matches a pattern in patternProperties, that property is validated against the corresponding pattern’s schema rather than the general additionalProperties schema. In practice, this means you can finely control which extra keys are permitted and how they must look, while still enumerating fixed fields.

Common use cases include enforcing a fixed set of known fields while allowing a controlled set of dynamic fields, or preventing any unexpected keys in strict configurations. For open-ended data, additionalProperties set to true or to a permissive schema can be convenient; for strict schemas, false or a narrowly defined schema helps guard against invalid input.

Notes: additionalProperties is defined in the JSON Schema specification and has evolved alongside other keywords such as patternProperties and unevaluatedProperties across different drafts.