additionalProperties is forced to false when missing, but default should be allowed (JSON Schema 2020-12)

[topikachu]

Bug description

When validating object schemas, if the schema does not explicitly contain the additionalProperties keyword, the implementation automatically injects "additionalProperties": false.

According to the JSON Schema specification (including draft 2020-12), omitting additionalProperties is equivalent to using an empty schema, which means additional properties are allowed by default.

This behavior breaks all schema auto-generated by tools, such as victools/jsonschema-generator, because schemas for Map types typically rely on the default behavior of allowing additional properties.

Environment

• MCP Java SDK 0.13.1 (Server component)

Steps to reproduce

1. Define a simple schema without additionalProperties, for example:

   {
     "type": "object"
   }

2. Return a Map result with a message key in structuredContent.
3. The SDK returns:

   Validation failed: structuredContent does not match tool outputSchema. Validation errors: [$: property 'message' is not defined in the schema and the schema does not allow additional properties]
   

Expected behavior

The SDK should allow the result if the schema has no additionalProperties keyword, to comply with the JSON Schema standard.

See official test suite reference:
JSON-Schema-Test-Suite – draft2020-12/additionalProperties.json#L112

Minimal Complete Reproducible example

As described above.

Suspected cause

This behavior seems to have been introduced here:
DefaultJsonSchemaValidator.java#L131