The value of "additionalProperties" MUST be a valid JSON Schema.

[lp2jx0309]

The value of "additionalProperties" MUST be a valid JSON Schema.
This keyword has always been a schema in the spec.
https://json-schema.org/draft-07/draft-handrews-json-schema-validation-01#rfc.section.6.5.6
https://json-schema.org/draft/2020-12/draft-bhutton-json-schema-01#name-additionalproperties

[tzolov]

@lp2jx0309, please elaborate.
the JsonSchema has additionalProperties filed:

java-sdk/mcp/src/main/java/io/modelcontextprotocol/spec/McpSchema.java

Line 1206 in 110a8d1

@JsonProperty("additionalProperties") Boolean additionalProperties,

[topikachu]

@tzolov

additionalProperties in the current JsonSchema definition is too restrictive.

According to the JSON Schema spec, additionalProperties can be either a boolean or a schema object (e.g. { "type": "boolean" }).

Defining it only as Boolean makes it impossible to handle the schema form.
Please check the official JSON Schema test example.

[david-christ]

This is actively biting us via Notion's MCP server. Their tools/list response includes tools like:

{
  "name": "API-post-page",
  "inputSchema": {
    "type": "object",
    "properties": { ... },
    "additionalProperties": { "type": "string" }
  }
}

Workaround until this is fixed in the SDK: register a Jackson mixin that replaces the record's default deserialization for additionalProperties. Coerce any JSON object to null (or true, depending on your semantics) and pass booleans through as-is:

public abstract class JsonSchemaMixin {
    @JsonDeserialize(using = AdditionalPropertiesDeserializer.class)
    @JsonProperty("additionalProperties")
    public abstract Boolean additionalProperties();

    static class AdditionalPropertiesDeserializer extends StdDeserializer<Boolean> {
        AdditionalPropertiesDeserializer() { super(Boolean.class); }

        @Override
        public Boolean deserialize(JsonParser p, DeserializationContext ctx) throws IOException {
            if (p.currentToken() == JsonToken.START_OBJECT) {
                p.skipChildren(); // consume the schema object, treat as unconstrained
                return null;
            }
            return p.getBooleanValue();
        }
    }
}

Then register it on your ObjectMapper:

objectMapper.addMixIn(McpSchema.JsonSchema.class, JsonSchemaMixin.class);

This is a stopgap — ideally additionalProperties would be typed as Object (or a sealed BooleanOrSchema type) to faithfully represent the spec.

[DragonFSKY]

Thanks for the report and for the recent Notion MCP example.

I took a look at the current main branch, and it seems that the main Tool.inputSchema path now uses Map<String, Object> rather than the deprecated JsonSchema record. With that shape, schema-valued additionalProperties, for example:

{
  "additionalProperties": { "type": "string" }
}

should be preserved when deserializing and serializing a tool schema.

The deprecated McpSchema.JsonSchema helper still has additionalProperties typed as Boolean, so that helper cannot represent the schema-object form. Since JsonSchema is now deprecated in favor of Map<String, Object>, I am not sure whether the intended fix should be to change the deprecated helper API, or simply add a regression test for the current Tool.inputSchema map path.

If this is still reproducible on current main, or on a version that includes #749, I would be happy to help with a focused regression test or a narrow compatibility fix.