fix: allow extra fields on RequestParams to match TypeScript SDK

Add model_config = ConfigDict(extra="allow") to RequestParams so that
extra top-level fields in request params are preserved, matching the
TypeScript SDK behavior where params is typed as { [key: string]: unknown }.

Author: Br1an67

src/mcp/types/_types.py

@@ -66,6 +66,8 @@ class TaskMetadata(MCPModel):
 
 
 class RequestParams(MCPModel):
+    model_config = ConfigDict(extra="allow")
+
     task: TaskMetadata | None = None
     """
     If specified, the caller is requesting task-augmented execution for this request.

tests/test_types.py

@@ -360,3 +360,22 @@ def test_list_tools_result_preserves_json_schema_2020_12_fields():
     assert tool.input_schema["$schema"] == "https://json-schema.org/draft/2020-12/schema"
     assert "$defs" in tool.input_schema
     assert tool.input_schema["additionalProperties"] is False
+
+
+def test_request_params_allows_extra_fields():
+    """RequestParams should accept extra fields at the top level.
+
+    This matches the TypeScript SDK's behavior where params is defined as
+    { _meta?: {...}; [key: string]: unknown }.
+
+    See: https://github.com/modelcontextprotocol/python-sdk/issues/913
+    """
+    from mcp.types import RequestParams
+
+    params = RequestParams(**{"custom_field": "value", "another": 42})
+    assert params.model_extra == {"custom_field": "value", "another": 42}
+
+    # Extra fields survive serialization round-trip
+    dumped = params.model_dump(mode="json", exclude_none=True)
+    assert dumped["custom_field"] == "value"
+    assert dumped["another"] == 42