refactor: use explicit extensions field instead of spreading kwargs

Address PR review feedback: Instead of using **capability_extensions to
spread arbitrary keys into ClientCapabilities, explicitly use the
'extensions' field. This prevents potential conflicts if new fields are
added to ClientCapabilities in the future.

Changes:
- Add 'extensions' field to ClientCapabilities type definition
- Update ClientSession to pass capability_extensions as 'extensions' kwarg
- Update test to pass extensions dict directly (without 'extensions' key wrapper)

Author: jerome3o-anthropic

src/mcp/client/session.py

@@ -145,8 +145,8 @@ def __init__(
         self._task_handlers = experimental_task_handlers or ExperimentalTaskHandlers()
 
         # Capability extensions to include in initialize request
-        # These are merged into ClientCapabilities using Pydantic's extra fields
-        self._capability_extensions = capability_extensions or {}
+        # These are passed as the 'extensions' field of ClientCapabilities
+        self._capability_extensions = capability_extensions
 
     async def initialize(self) -> types.InitializeResult:
         sampling = (
@@ -182,7 +182,7 @@ async def initialize(self) -> types.InitializeResult:
                             experimental=None,
                             roots=roots,
                             tasks=self._task_handlers.build_capability(),
-                            **self._capability_extensions,
+                            extensions=self._capability_extensions or None,
                         ),
                         client_info=self._client_info,
                     ),

src/mcp/types.py

@@ -377,6 +377,8 @@ class ClientCapabilities(MCPModel):
     """Present if the client supports listing roots."""
     tasks: ClientTasksCapability | None = None
     """Present if the client supports task-augmented requests."""
+    extensions: dict[str, Any] | None = None
+    """Protocol extensions advertised by the client."""
 
 
 class PromptsCapability(MCPModel):

tests/client/test_session.py

@@ -779,7 +779,8 @@ async def test_client_session_capability_extensions():
     received_capabilities = None
 
     # Define capability extensions (e.g., UI extension)
-    capability_extensions = {"extensions": {"io.modelcontextprotocol/ui": {"mimeTypes": ["text/html;profile=mcp-app"]}}}
+    # These will be placed under the 'extensions' key of ClientCapabilities
+    capability_extensions = {"io.modelcontextprotocol/ui": {"mimeTypes": ["text/html;profile=mcp-app"]}}
 
     async def mock_server():
         nonlocal received_capabilities
@@ -833,7 +834,7 @@ async def mock_server():
 
     # Assert that the capability extensions were included in the request
     assert received_capabilities is not None
-    # The extensions should be present via Pydantic's extra fields
+    # The extensions should be present under the 'extensions' key
     caps_dict = received_capabilities.model_dump()
     assert "extensions" in caps_dict
     assert "io.modelcontextprotocol/ui" in caps_dict["extensions"]