The value of "additionalProperties" MUST be a valid JSON Schema. #445

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1. tzolov commented on Aug 7, 2025

   

   tzolov

   on Aug 7, 2025

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   @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,
2. tzolov

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   on Aug 8, 2025
3. topikachu commented on Sep 29, 2025

   

   topikachu

   on Sep 29, 2025

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   @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.
4. chemicL

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   on Feb 19, 2026
5. david-christ commented on May 19, 2026

   

   david-christ

   on May 19, 2026

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   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.
6. DragonFSKY commented on May 21, 2026

   

   DragonFSKY

   on May 21, 2026

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   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.
7. tzolov

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8. tzolov commented on Aug 17, 2026

   

   tzolov

   on Aug 17, 2026

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   As pointed out by @DragonFSKY this should have been resolved by moving the input schema to Map<String, Object> in 2.0GA+.
   Closing as resolved.
9. tzolov

   closed this as completedon Aug 17, 2026