[Server] Decode JSON-RPC objects as stdClass, not associative arrays

[steffans]

Is your feature request related to a problem? Please describe.

The current implementation uses associative array for the decoding the JSON-RPC message. Which will break the Opis\JsonSchema\Validator since it needs the stdClass for a object as described in the docs. Currently the empty array/object ambiguity exists and the validation fails.

Describe the solution you'd like

Preserve JSON object/array semantics on inbound MCP messages and set associative parameter to false. Update all internal methods to work with stdClass objects instead of associative arrays.

json_decode($input, false, JSON_THROW_ON_ERROR)

Additional context

This feature would allow to work with fully validated object structures with #Schema attribute on input parameters in tool calls. Here is an example how a tool call failed and the model confusion about the parameter typing. The tool method is using this signature: McpServer::updateDocument(array $document) So i asked the model why did it fail, this is the answer:

The document parameter type should be object (not array), and the description should clarify the expected root structure:

{
  "document": {
    "type": "object",
    "description": "Complete document (root object with type, name, version properties)",
    "properties": {
      "type": { "type": "string" },
      "name": { "type": "string" },
      "version": { "type": "string" },
    },
    "required": ["type", "name", "version"]
  }
}

The key fix: changing "type": "array" to "type": "object" and adding a required array so it's explicit what the root document needs.

[steffans]

After some debugging i noticed that the tool call parameters will be converted with SchemaValidator::convertDataForValidator before the validation. It will convert all associative arrays to stdClass and keeps all empty arrays as array.

Caveat: that means an empty object can't be distinguished from an empty array therefore can't be validated.

The following tool definition works for me, though a little contradicting with object/array type for the same parameter:

#[McpTool('update_document')]
public function updateDocument(
    #[Schema(type: 'object', description: 'Complete document.')] // needs to be present for any object type
    array $document, // needs to be array because the JSON-RPC message is using json_decode($input, associative: true)
)

I hope this helps anyone troubleshooting the same issue.