Fix unknown-method error code and add a protocol version registry (#2836)

src/mcp/client/auth/oauth2.py

@@ -49,6 +49,7 @@ from mcp.shared.auth_utils import (
     check_resource_allowed,
     resource_url_from_server_url,
 )
+from mcp.shared.version import is_version_at_least
 
 logger = logging.getLogger(__name__)
 
@@ -172,9 +173,7 @@ class OAuthContext:
         if not protocol_version:
             return False
 
-        # Check if protocol version is 2025-06-18 or later
-        # Version format is YYYY-MM-DD, so string comparison works
-        return protocol_version >= "2025-06-18"
+        return is_version_at_least(protocol_version, "2025-06-18")
 
     def prepare_token_auth(
         self, data: dict[str, str], headers: dict[str, str] | None = None

src/mcp/client/session.py

@@ -1,10 +1,10 @@
 from __future__ import annotations
 
 import logging
-from typing import Any, Protocol
+from typing import Any, Protocol, cast, get_args
 
 import anyio.lowlevel
-from pydantic import TypeAdapter
+from pydantic import BaseModel, TypeAdapter
 
 from mcp import types
 from mcp.client._transport import ReadStream, WriteStream
@@ -95,6 +95,14 @@ async def _default_logging_callback(
 
 ClientResponse: TypeAdapter[types.ClientResult | types.ErrorData] = TypeAdapter(types.ClientResult | types.ErrorData)
 
+_SERVER_REQUEST_METHODS: frozenset[str] = frozenset(
+    cast(type[BaseModel], arm).model_fields["method"].default for arm in get_args(types.ServerRequest)
+)
+"""Method names in the SDK's `ServerRequest` union, derived from the
+discriminator literal on each arm. Requests for any other method — including
+spec methods this SDK deliberately doesn't model, like `tasks/*` — are
+answered with METHOD_NOT_FOUND instead of failing union validation."""
+
 
 class ClientSession(
     BaseSession[
@@ -134,6 +142,10 @@ class ClientSession(
     def _receive_request_adapter(self) -> TypeAdapter[types.ServerRequest]:
         return types.server_request_adapter
 
+    @property
+    def _receive_request_methods(self) -> frozenset[str]:
+        return _SERVER_REQUEST_METHODS
+
     @property
     def _receive_notification_adapter(self) -> TypeAdapter[types.ServerNotification]:
         return types.server_notification_adapter