modelcontextprotocol · lavish0000 · Mar 6, 2026 · Mar 6, 2026 · Mar 6, 2026 · Mar 8, 2026
diff --git a/src/mcp/client/session.py b/src/mcp/client/session.py
@@ -12,11 +12,17 @@
 from mcp.client.experimental.task_handlers import ExperimentalTaskHandlers
 from mcp.shared._context import RequestContext
 from mcp.shared.message import SessionMessage
-from mcp.shared.session import BaseSession, ProgressFnT, RequestResponder
+from mcp.shared.session import (
+    BaseSession,
+    ProgressFnT,
+    RequestResponder,
+    request_methods_for_union,
+)
 from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
 from mcp.types._types import RequestParamsMeta
 
 DEFAULT_CLIENT_INFO = types.Implementation(name="mcp", version="0.1.0")
+KNOWN_SERVER_REQUEST_METHODS = request_methods_for_union(types.ServerRequest)
 
 logger = logging.getLogger("client")
 
@@ -141,6 +147,10 @@ def __init__(
     def _receive_request_adapter(self) -> TypeAdapter[types.ServerRequest]:
         return types.server_request_adapter
 
+    @property
+    def _known_request_methods(self) -> frozenset[str]:
+        return KNOWN_SERVER_REQUEST_METHODS
+
     @property
     def _receive_notification_adapter(self) -> TypeAdapter[types.ServerNotification]:
         return types.server_notification_adapter

diff --git a/src/mcp/server/session.py b/src/mcp/server/session.py
@@ -47,6 +47,7 @@ async def handle_list_prompts(ctx: RequestContext, params) -> ListPromptsResult:
 from mcp.shared.session import (
     BaseSession,
     RequestResponder,
+    request_methods_for_union,
 )
 from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
 
@@ -63,6 +64,8 @@ class InitializationState(Enum):
     RequestResponder[types.ClientRequest, types.ServerResult] | types.ClientNotification | Exception
 )
 
+KNOWN_CLIENT_REQUEST_METHODS = request_methods_for_union(types.ClientRequest)
+
 
 class ServerSession(
     BaseSession[
@@ -100,6 +103,10 @@ def __init__(
     def _receive_request_adapter(self) -> TypeAdapter[types.ClientRequest]:
         return types.client_request_adapter
 
+    @property
+    def _known_request_methods(self) -> frozenset[str]:
+        return KNOWN_CLIENT_REQUEST_METHODS
+
     @property
     def _receive_notification_adapter(self) -> TypeAdapter[types.ClientNotification]:
         return types.client_notification_adapter

diff --git a/src/mcp/shared/session.py b/src/mcp/shared/session.py
@@ -4,7 +4,7 @@
 from collections.abc import Callable
 from contextlib import AsyncExitStack
 from types import TracebackType
-from typing import Any, Generic, Protocol, TypeVar
+from typing import Any, Generic, Protocol, TypeVar, get_args
 
 import anyio
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
@@ -17,6 +17,7 @@
 from mcp.types import (
     CONNECTION_CLOSED,
     INVALID_PARAMS,
+    METHOD_NOT_FOUND,
     REQUEST_TIMEOUT,
     CancelledNotification,
     ClientNotification,
@@ -45,6 +46,16 @@
 RequestId = str | int
 
 
+def request_methods_for_union(request_union: Any) -> frozenset[str]:
+    methods: set[str] = set()
+    for request_type in get_args(request_union):
+        field = getattr(request_type, "model_fields", {}).get("method")
+        default = getattr(field, "default", None)
+        if isinstance(default, str):
+            methods.add(default)
+    return frozenset(methods)
+
+
 class ProgressFnT(Protocol):
     """Protocol for progress notification callbacks."""
 
@@ -326,6 +337,10 @@ def _receive_request_adapter(self) -> TypeAdapter[ReceiveRequestT]:
         """Each subclass must provide its own request adapter."""
         raise NotImplementedError
 
+    @property
+    def _known_request_methods(self) -> frozenset[str]:
+        return frozenset()
+
     @property
     def _receive_notification_adapter(self) -> TypeAdapter[ReceiveNotificationT]:
         raise NotImplementedError
@@ -360,10 +375,18 @@ async def _receive_loop(self) -> None:
                             # response instead of crashing the server
                             logging.warning("Failed to validate request", exc_info=True)
                             logging.debug(f"Message that failed validation: {message.message}")
+                            if message.message.method not in self._known_request_methods:
+                                error = ErrorData(code=METHOD_NOT_FOUND, message="Method not found")
+                            else:
+                                error = ErrorData(
+                                    code=INVALID_PARAMS,
+                                    message="Invalid request parameters",
+                                    data="",
+                                )
                             error_response = JSONRPCError(
                                 jsonrpc="2.0",
                                 id=message.message.id,
-                                error=ErrorData(code=INVALID_PARAMS, message="Invalid request parameters", data=""),
+                                error=error,
                             )
                             session_message = SessionMessage(message=error_response)
                             await self._write_stream.send(session_message)

diff --git a/tests/issues/test_1561_invalid_method_code.py b/tests/issues/test_1561_invalid_method_code.py
@@ -0,0 +1,101 @@
+"""Test for issue #1561: unknown methods should return METHOD_NOT_FOUND."""
+
+import anyio
+import pytest
+from pydantic import BaseModel
+
+from mcp import types
+from mcp.client.session import KNOWN_SERVER_REQUEST_METHODS, ClientSession
+from mcp.server.models import InitializationOptions
+from mcp.server.session import ServerSession
+from mcp.shared.message import SessionMessage
+from mcp.shared.session import BaseSession, request_methods_for_union
+from mcp.types import METHOD_NOT_FOUND, JSONRPCError, JSONRPCRequest, ServerCapabilities
+
+
+@pytest.mark.anyio
+async def test_invalid_method_returns_method_not_found() -> None:
+    read_send_stream, read_receive_stream = anyio.create_memory_object_stream[SessionMessage | Exception](10)
+    write_send_stream, write_receive_stream = anyio.create_memory_object_stream[SessionMessage](10)
+
+    async with read_send_stream, read_receive_stream, write_send_stream, write_receive_stream:
+        async with ServerSession(
+            read_stream=read_receive_stream,
+            write_stream=write_send_stream,
+            init_options=InitializationOptions(
+                server_name="test_server",
+                server_version="1.0.0",
+                capabilities=ServerCapabilities(),
+            ),
+        ):
+            await read_send_stream.send(
+                SessionMessage(
+                    message=JSONRPCRequest(
+                        jsonrpc="2.0",
+                        id=1,
+                        method="invalid/method",
+                        params={},
+                    )
+                )
+            )
+
+            await anyio.sleep(0.1)
+
+            response_message = write_receive_stream.receive_nowait()
+            response = response_message.message
+
+            assert isinstance(response, JSONRPCError)
+            assert response.id == 1
+            assert response.error.code == METHOD_NOT_FOUND
+            assert response.error.message == "Method not found"
+
+
+class MissingDefaultMethodRequest(BaseModel):
+    jsonrpc: str = "2.0"
+    id: int = 1
+    method: str
+
+
+def test_request_methods_for_union_ignores_non_literal_defaults() -> None:
+    methods = request_methods_for_union(types.ServerRequest | MissingDefaultMethodRequest)
+    assert methods == KNOWN_SERVER_REQUEST_METHODS
+
+
+@pytest.mark.anyio
+async def test_client_session_known_request_methods_match_server_request_union() -> None:
+    read_send_stream, read_receive_stream = anyio.create_memory_object_stream[SessionMessage | Exception](10)
+    write_send_stream, write_receive_stream = anyio.create_memory_object_stream[SessionMessage](10)
+
+    async with read_send_stream, read_receive_stream, write_send_stream, write_receive_stream:
+        session = ClientSession(read_stream=read_receive_stream, write_stream=write_send_stream)
+        assert session._known_request_methods == KNOWN_SERVER_REQUEST_METHODS
+
+
+class DummyBaseSession(
+    BaseSession[
+        types.ClientRequest,
+        types.ClientNotification,
+        types.ClientResult,
+        types.ServerRequest,
+        types.ServerNotification,
+    ]
+):
+    @property
+    def _receive_request_adapter(self):
+        return types.server_request_adapter
+
+    @property
+    def _receive_notification_adapter(self):
+        return types.server_notification_adapter
+
+
+@pytest.mark.anyio
+async def test_base_session_known_request_methods_default_to_empty() -> None:
+    read_send_stream, read_receive_stream = anyio.create_memory_object_stream[SessionMessage | Exception](10)
+    write_send_stream, write_receive_stream = anyio.create_memory_object_stream[SessionMessage](10)
+
+    async with read_send_stream, read_receive_stream, write_send_stream, write_receive_stream:
+        session = DummyBaseSession(read_stream=read_receive_stream, write_stream=write_send_stream)
+        assert session._known_request_methods == frozenset()
+        assert session._receive_request_adapter is types.server_request_adapter
+        assert session._receive_notification_adapter is types.server_notification_adapter