Require protocol_version to be a string

The MCP schema defines protocolVersion as a plain string. The int variant
only existed for compatibility with pre-release protocol negotiation
(before 2024-10-07 date-based versioning), which sent protocolVersion: 1.
Integer versions can never successfully negotiate against the supported
version list, so accepting them only changed the failure mode. This
matches the TypeScript SDK, which made the same change in November 2024.

Author: maxisbey

src/mcp/client/streamable_http.py

@@ -120,7 +120,7 @@ def _maybe_extract_protocol_version_from_message(self, message: JSONRPCMessage)
             try:
                 # Parse the result as InitializeResult for type safety
                 init_result = InitializeResult.model_validate(message.result, by_name=False)
-                self.protocol_version = str(init_result.protocol_version)
+                self.protocol_version = init_result.protocol_version
                 logger.info(f"Negotiated protocol version: {self.protocol_version}")
             except Exception:  # pragma: no cover
                 logger.warning("Failed to parse initialization response as InitializeResult", exc_info=True)

src/mcp/types/_types.py

@@ -545,7 +545,7 @@ class TaskStatusNotification(Notification[TaskStatusNotificationParams, Literal[
 class InitializeRequestParams(RequestParams):
     """Parameters for the initialize request."""
 
-    protocol_version: str | int
+    protocol_version: str
     """The latest version of the Model Context Protocol that the client supports."""
     capabilities: ClientCapabilities
     client_info: Implementation
@@ -563,7 +563,7 @@ class InitializeRequest(Request[InitializeRequestParams, Literal["initialize"]])
 class InitializeResult(Result):
     """After receiving an initialize request from the client, the server sends this."""
 
-    protocol_version: str | int
+    protocol_version: str
     """The version of the Model Context Protocol that the server wants to use."""
     capabilities: ServerCapabilities
     server_info: Implementation