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

docs/migration.md

@@ -1135,7 +1135,7 @@ from mcp.server import ServerRequestContext
 
 ### `ServerSession` is now a thin proxy (no longer a `BaseSession`)
 
-`ServerSession` no longer subclasses `BaseSession`. It is now a small connection-scoped proxy that exposes `send_request`, `send_notification`, the typed convenience helpers (`create_message`, `elicit_form`, `send_log_message`, `send_tool_list_changed`, ...), `client_params`, and `check_client_capability`. The receive loop, `initialize` handling, and per-request task isolation that previously lived in `ServerSession` have moved to `JSONRPCDispatcher` and `ServerRunner`.
+`ServerSession` no longer subclasses `BaseSession`. It is now a small connection-scoped proxy that exposes `send_request`, `send_notification`, the typed convenience helpers (`create_message`, `elicit_form`, `send_log_message`, `send_tool_list_changed`, ...), `client_params`, `protocol_version`, and `check_client_capability`. The receive loop, `initialize` handling, and per-request task isolation that previously lived in `ServerSession` have moved to `JSONRPCDispatcher` and `ServerRunner`.
 
 `ServerSession` is normally constructed for you by `Server.run()` and reached via `ctx.session` in handlers, so most servers are unaffected. If you were constructing or subclassing it directly:
 
@@ -1182,28 +1182,36 @@ Tasks are expected to return as a separate MCP extension in a future release.
 
 Previously, the lowlevel `Server` hardcoded `subscribe=False` in resource capabilities even when a `subscribe_resource()` handler was registered. The `subscribe` capability is now dynamically set to `True` when an `on_subscribe_resource` handler is provided. Clients that previously didn't see `subscribe: true` in capabilities will now see it when a handler is registered, which may change client behavior.
 
-### Extra fields no longer allowed on top-level MCP types
+### Unknown request methods now return `-32601` (Method not found)
 
-MCP protocol types no longer accept arbitrary extra fields at the top level. This matches the MCP specification which only allows extra fields within `_meta` objects, not on the types themselves.
+In v1, a request for a method the SDK didn't recognize failed request-union validation and was answered with `-32602` (`"Invalid request parameters"`, empty `data`). Any method the receiver doesn't serve — unrecognized, or a spec method with no registered handler — is now answered with the JSON-RPC-specified `-32601` (`"Method not found"`), with the method name in `data`, on both the server and the client side, in every initialization state. Update anything that matched on the old code for this case.
+
+### Extra fields on MCP types are no longer preserved
+
+In v1, MCP protocol types were configured with `extra="allow"`: unknown fields passed to a constructor or received from a peer were kept on the model and re-serialized on output.
+
+In v2, MCP types silently ignore extra fields. Unknown constructor keyword arguments and unknown keys in wire data are dropped during validation — no error is raised, and the values do not round-trip:
 
 ```python
-# This will now raise a validation error
 from mcp.types import CallToolRequestParams
 
 params = CallToolRequestParams(
     name="my_tool",
     arguments={},
-    unknown_field="value",  # ValidationError: extra fields not permitted
+    unknown_field="value",  # silently ignored, not stored
 )
+"unknown_field" in params.model_dump()  # False
 
-# Extra fields are still allowed in _meta
+# _meta remains the supported place for custom data, per the MCP spec
 params = CallToolRequestParams(
     name="my_tool",
     arguments={},
-    _meta={"my_custom_key": "value", "another": 123},  # OK
+    _meta={"my_custom_key": "value", "another": 123},  # OK, preserved
 )
 ```
 
+If you relied on extra fields round-tripping through MCP types, move that data into `_meta`.
+
 ## New Features
 
 ### `streamable_http_app()` available on lowlevel Server

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

src/mcp/server/connection.py

@@ -83,6 +83,10 @@ class Connection:
     """The full `initialize` request params; `None` before initialization."""
 
     protocol_version: str | None
+    """The protocol version negotiated during `initialize`; `None` before
+    initialization. Stateless connections don't require the handshake, so this
+    normally stays `None` there (a client that sends `initialize` anyway still
+    commits it). Handlers read this as `ServerSession.protocol_version`."""
 
     initialized: anyio.Event
     """Set when `notifications/initialized` arrives (matches TS `oninitialized`);

src/mcp/server/runner.py

@@ -243,12 +243,17 @@ class ServerRunner(Generic[LifespanT]):
             # (read loop parked), so awaiting the peer anywhere on this path deadlocks.
             if method == "initialize":
                 return self._handle_initialize(params)
+            # Methods without a handler are METHOD_NOT_FOUND regardless of
+            # initialization state: JSON-RPC 2.0 reserves -32601 for "not
+            # available on this server", and clients probing a server before
+            # the handshake key off that code. The init gate below therefore
+            # only ever applies to methods the server actually serves.
+            entry = self.server.get_request_handler(method)
+            if entry is None:
+                raise MCPError(code=METHOD_NOT_FOUND, message="Method not found", data=method)
             if not self.connection.initialize_accepted and method not in _INIT_EXEMPT:
                 # Pinned compat: the same error shape the union validation produced.
                 raise MCPError(code=INVALID_PARAMS, message="Invalid request parameters", data="")
-            entry = self.server.get_request_handler(method)
-            if entry is None:
-                raise MCPError(code=METHOD_NOT_FOUND, message="Method not found")
             # Absent params validate as {} (required fields still reject), so
             # the handler receives the model with its defaults, never None.
             typed_params = entry.params_type.model_validate({} if params is None else params, by_name=False)

src/mcp/server/session.py

@@ -50,6 +50,17 @@ class ServerSession:
         """The client's `initialize` request params; `None` before initialization."""
         return self._connection.client_params
 
+    @property
+    def protocol_version(self) -> str | None:
+        """The protocol version negotiated during `initialize`.
+
+        `None` before initialization completes. Stateless connections don't
+        require the handshake, so this is normally `None` there (on streamable
+        HTTP the per-request version is the `MCP-Protocol-Version` header,
+        available via `ctx.request.headers`).
+        """
+        return self._connection.protocol_version
+
     async def send_request(
         self,
         request: types.ServerRequest,

src/mcp/server/streamable_http.py

@@ -28,7 +28,7 @@ from mcp.server.transport_security import TransportSecurityMiddleware, Transport
 from mcp.shared._context_streams import ContextReceiveStream, ContextSendStream, create_context_streams
 from mcp.shared._stream_protocols import ReadStream, WriteStream
 from mcp.shared.message import ServerMessageMetadata, SessionMessage
-from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
+from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS, is_version_at_least
 from mcp.types import (
     DEFAULT_NEGOTIATED_VERSION,
     INTERNAL_ERROR,
@@ -238,7 +238,7 @@ class StreamableHTTPServerTransport:
         the stream is closed early because they didn't receive a priming event.
         """
         # Only provide close callbacks when client supports resumability
-        if self._event_store and protocol_version >= "2025-11-25":
+        if self._event_store and is_version_at_least(protocol_version, "2025-11-25"):
 
             async def close_stream_callback() -> None:
                 self.close_sse_stream(request_id)
@@ -271,7 +271,7 @@ class StreamableHTTPServerTransport:
         if not self._event_store:
             return
         # Priming events have empty data which older clients cannot handle.
-        if protocol_version < "2025-11-25":
+        if not is_version_at_least(protocol_version, "2025-11-25"):
             return
         priming_event_id = await self._event_store.store_event(
             str(request_id),  # Convert RequestId to StreamId (str)

src/mcp/shared/session.py

@@ -20,6 +20,7 @@ from mcp.shared.message import MessageMetadata, ServerMessageMetadata, SessionMe
 from mcp.types import (
     CONNECTION_CLOSED,
     INVALID_PARAMS,
+    METHOD_NOT_FOUND,
     REQUEST_TIMEOUT,
     CancelledNotification,
     ClientNotification,
@@ -286,6 +287,12 @@ class BaseSession(
         """Each subclass must provide its own request adapter."""
         raise NotImplementedError
 
+    @property
+    def _receive_request_methods(self) -> frozenset[str]:
+        """Method names in the receive-request union; anything else is
+        answered with METHOD_NOT_FOUND before validation is attempted."""
+        raise NotImplementedError
+
     @property
     def _receive_notification_adapter(self) -> TypeAdapter[ReceiveNotificationT]:
         raise NotImplementedError
@@ -297,6 +304,18 @@ class BaseSession(
                 async def _handle_session_message(message: SessionMessage) -> None:
                     sender_context: contextvars.Context | None = getattr(self._read_stream, "last_context", None)
                     if isinstance(message.message, JSONRPCRequest):
+                        if message.message.method not in self._receive_request_methods:
+                            # Unknown methods are METHOD_NOT_FOUND (-32601) per
+                            # JSON-RPC 2.0, not validation failures (-32602).
+                            error_response = JSONRPCError(
+                                jsonrpc="2.0",
+                                id=message.message.id,
+                                error=ErrorData(
+                                    code=METHOD_NOT_FOUND, message="Method not found", data=message.message.method
+                                ),
+                            )
+                            await self._write_stream.send(SessionMessage(message=error_response))
+                            return
                         try:
                             validated_request = self._receive_request_adapter.validate_python(
                                 message.message.model_dump(by_alias=True, mode="json", exclude_none=True),

src/mcp/shared/version.py

@@ -1,3 +1,37 @@
+"""Protocol-version registry and comparison helpers.
+
+Date-string protocol revisions happen to sort lexicographically, but versions
+are an enumerated set, not an ordered scalar: future identifiers are not
+guaranteed to be date-shaped, and unrecognized peer strings must compare
+conservatively instead of accidentally (e.g. "zzz" > "2025-11-25"). All
+ordering questions go through KNOWN_PROTOCOL_VERSIONS.
+"""
+
+from typing import Final
+
 from mcp.types import LATEST_PROTOCOL_VERSION
 
+KNOWN_PROTOCOL_VERSIONS: Final[tuple[str, ...]] = (
+    "2024-11-05",
+    "2025-03-26",
+    "2025-06-18",
+    "2025-11-25",
+)
+"""Every released protocol revision, oldest to newest."""
+
 SUPPORTED_PROTOCOL_VERSIONS: list[str] = ["2024-11-05", "2025-03-26", "2025-06-18", LATEST_PROTOCOL_VERSION]
+"""Protocol revisions this SDK can negotiate."""
+
+
+def is_version_at_least(version: str, minimum: str) -> bool:
+    """Return True if `version` is a known revision at least as new as `minimum`.
+
+    Unknown `version` strings return False (treat unrecognized peers
+    conservatively). `minimum` must be a member of KNOWN_PROTOCOL_VERSIONS;
+    passing anything else is programmer error and raises ValueError.
+    """
+    if minimum not in KNOWN_PROTOCOL_VERSIONS:
+        raise ValueError(f"minimum must be a known protocol version, got {minimum!r}")
+    if version not in KNOWN_PROTOCOL_VERSIONS:
+        return False
+    return KNOWN_PROTOCOL_VERSIONS.index(version) >= KNOWN_PROTOCOL_VERSIONS.index(minimum)

tests/client/test_auth.py

@@ -819,6 +819,24 @@ class TestProtectedResourceMetadata:
         assert "resource=" in content
 
 
+@pytest.mark.parametrize(
+    ("protocol_version", "expected"),
+    [
+        ("2025-03-26", False),
+        ("2025-06-18", True),
+        ("2025-11-25", True),
+        # Unrecognized strings gate conservatively, even ones sorting after 2025-06-18.
+        ("zzz", False),
+        ("9999-99-99", False),
+    ],
+)
+def test_should_include_resource_param_by_protocol_version(
+    oauth_provider: OAuthClientProvider, protocol_version: str, expected: bool
+) -> None:
+    """Resource param is included only for recognized versions >= 2025-06-18."""
+    assert oauth_provider.context.should_include_resource_param(protocol_version) is expected
+
+
 @pytest.mark.anyio
 async def test_validate_resource_rejects_mismatched_resource(
     client_metadata: OAuthClientMetadata, mock_storage: MockTokenStorage

tests/client/test_client.py

@@ -113,6 +113,12 @@ async def test_client_is_initialized(app: MCPServer):
         assert client.initialize_result.server_info.name == "test"
 
 
+async def test_client_initialize_result_exposes_negotiated_protocol_version(app: MCPServer):
+    """The negotiated protocol version is readable after initialization."""
+    async with Client(app) as client:
+        assert client.initialize_result.protocol_version == types.LATEST_PROTOCOL_VERSION
+
+
 async def test_client_with_simple_server(simple_server: Server):
     """Test that from_server works with a basic Server instance."""
     async with Client(simple_server) as client:

tests/interaction/lowlevel/test_completion.py

@@ -128,4 +128,6 @@ async def test_complete_without_handler_is_method_not_found(connect: Connect) ->
         with pytest.raises(MCPError) as exc_info:
             await client.complete(PromptReference(name="anything"), argument={"name": "topic", "value": ""})
 
-    assert exc_info.value.error == snapshot(ErrorData(code=METHOD_NOT_FOUND, message="Method not found"))
+    assert exc_info.value.error == snapshot(
+        ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="completion/complete")
+    )

tests/interaction/lowlevel/test_resources.py

@@ -236,7 +236,9 @@ async def test_subscribe_without_a_subscribe_handler_is_method_not_found(connect
         with pytest.raises(MCPError) as exc_info:
             await client.subscribe_resource("file:///watched.txt")
 
-    assert exc_info.value.error == snapshot(ErrorData(code=METHOD_NOT_FOUND, message="Method not found"))
+    assert exc_info.value.error == snapshot(
+        ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="resources/subscribe")
+    )
 
 
 @requirement("resources:unsubscribe")

tests/interaction/mcpserver/test_context.py

@@ -262,7 +262,9 @@ async def test_set_logging_level_is_rejected_and_messages_are_never_filtered(con
 
         await client.call_tool("chatter", {})
 
-    assert exc_info.value.error == snapshot(ErrorData(code=METHOD_NOT_FOUND, message="Method not found"))
+    assert exc_info.value.error == snapshot(
+        ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="logging/setLevel")
+    )
     assert received == snapshot(
         [
             LoggingMessageNotificationParams(level="debug", data="noise"),

tests/server/test_runner.py

@@ -168,6 +168,45 @@ async def test_runner_gates_requests_before_initialize(server: SrvT):
         assert await client.send_raw_request("ping", None) == {}
 
 
+@pytest.mark.anyio
+async def test_runner_unknown_method_before_initialize_raises_method_not_found(server: SrvT):
+    """An unknown method is METHOD_NOT_FOUND even before initialize: JSON-RPC
+    2.0 reserves -32601 for it, and clients probing a server before the
+    handshake key off that code. The init gate only applies to methods the
+    server actually serves."""
+    async with connected_runner(server, initialized=False) as (client, _):
+        with pytest.raises(MCPError) as exc:
+            await client.send_raw_request("x/unknown", None)
+        assert exc.value.error == ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="x/unknown")
+
+
+@pytest.mark.anyio
+async def test_runner_spec_method_without_handler_before_initialize_raises_method_not_found(server: SrvT):
+    """A spec method the server doesn't serve is METHOD_NOT_FOUND even before
+    initialize: -32601 means "not available on this server", so probing
+    clients get the same answer in every initialization state (the fixture
+    server registers no resources handlers)."""
+    async with connected_runner(server, initialized=False) as (client, _):
+        with pytest.raises(MCPError) as exc:
+            await client.send_raw_request("resources/list", None)
+        assert exc.value.error == ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="resources/list")
+
+
+@pytest.mark.anyio
+async def test_runner_custom_method_with_handler_is_still_gated_before_initialize(server: SrvT):
+    """A custom-registered method is a known method: before initialize it is
+    rejected by the init gate, not answered with METHOD_NOT_FOUND."""
+
+    async def greet(ctx: Ctx, params: RequestParams | None) -> Any:
+        raise NotImplementedError  # the gate rejects the request first
+
+    server.add_request_handler("custom/greet", RequestParams, greet)
+    async with connected_runner(server, initialized=False) as (client, _):
+        with pytest.raises(MCPError) as exc:
+            await client.send_raw_request("custom/greet", None)
+        assert exc.value.error == ErrorData(code=INVALID_PARAMS, message="Invalid request parameters", data="")
+
+
 @pytest.mark.anyio
 async def test_runner_routes_to_handler_and_builds_context(server: SrvT):
     async with connected_runner(server) as (client, runner):
@@ -186,7 +225,7 @@ async def test_runner_spec_method_with_no_handler_raises_method_not_found(server
     async with connected_runner(server) as (client, _):
         with pytest.raises(MCPError) as exc:
             await client.send_raw_request("resources/list", None)
-    assert exc.value.error.code == METHOD_NOT_FOUND
+    assert exc.value.error == ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="resources/list")
 
 
 @pytest.mark.anyio
@@ -196,7 +235,7 @@ async def test_runner_non_spec_method_with_no_handler_raises_method_not_found(se
     async with connected_runner(server) as (client, _):
         with pytest.raises(MCPError) as exc:
             await client.send_raw_request("nonexistent/method", None)
-    assert exc.value.error.code == METHOD_NOT_FOUND
+    assert exc.value.error == ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="nonexistent/method")
 
 
 @pytest.mark.anyio

tests/server/test_session.py

@@ -11,6 +11,7 @@ from typing import Any, cast
 import pytest
 
 from mcp import types
+from mcp.server import Server, ServerRequestContext
 from mcp.server.connection import Connection
 from mcp.server.session import ServerSession
 from mcp.shared.dispatcher import CallOptions
@@ -26,6 +27,8 @@ from mcp.types import (
     SamplingToolsCapability,
 )
 
+from .test_runner import connected_runner
+
 
 class StubDispatcher:
     """Records `send_raw_request` / `notify` calls and returns a canned result."""
@@ -158,3 +161,64 @@ def test_check_client_capability_delegates_to_connection():
     session = _make_session(dispatcher, capabilities=ClientCapabilities(sampling=SamplingCapability()))
     assert session.check_client_capability(ClientCapabilities(sampling=SamplingCapability())) is True
     assert session.check_client_capability(ClientCapabilities(experimental={"x": {}})) is False
+
+
+def _runner_server(seen_versions: list[str | None]) -> Server[dict[str, Any]]:
+    """A lowlevel Server whose tools/list handler records `ctx.session.protocol_version`."""
+
+    async def list_tools(
+        ctx: ServerRequestContext[dict[str, Any], Any], params: types.PaginatedRequestParams | None
+    ) -> types.ListToolsResult:
+        seen_versions.append(ctx.session.protocol_version)
+        return types.ListToolsResult(tools=[])
+
+    return Server(name="test-server", version="0.0.1", on_list_tools=list_tools)
+
+
+def _init_params(protocol_version: str) -> dict[str, Any]:
+    return InitializeRequestParams(
+        protocol_version=protocol_version,
+        capabilities=ClientCapabilities(),
+        client_info=Implementation(name="test-client", version="1.0"),
+    ).model_dump(by_alias=True, exclude_none=True)
+
+
+@pytest.mark.anyio
+async def test_protocol_version_is_none_before_initialize():
+    """No negotiated version is readable before the initialize handshake."""
+    async with connected_runner(_runner_server([]), initialized=False) as (_client, runner):
+        assert runner.session.protocol_version is None
+
+
+@pytest.mark.anyio
+async def test_protocol_version_is_negotiated_version_after_initialize():
+    """A supported requested version is echoed back and readable on the session,
+    both directly and from inside a handler via `ctx.session`."""
+    seen: list[str | None] = []
+    async with connected_runner(_runner_server(seen), initialized=False) as (client, runner):
+        result = await client.send_raw_request("initialize", _init_params("2025-03-26"))
+        assert result["protocolVersion"] == "2025-03-26"
+        assert runner.session.protocol_version == "2025-03-26"
+        await client.send_raw_request("tools/list", None)
+        assert seen == ["2025-03-26"]
+
+
+@pytest.mark.anyio
+async def test_protocol_version_reads_latest_when_requested_version_unsupported():
+    """An unsupported requested version negotiates down to LATEST_PROTOCOL_VERSION."""
+    async with connected_runner(_runner_server([]), initialized=False) as (client, runner):
+        result = await client.send_raw_request("initialize", _init_params("1999-01-01"))
+        assert result["protocolVersion"] == LATEST_PROTOCOL_VERSION
+        assert runner.session.protocol_version == LATEST_PROTOCOL_VERSION
+
+
+@pytest.mark.anyio
+async def test_protocol_version_is_none_on_stateless_connection():
+    """Stateless connections never see a handshake: requests flow, but the
+    negotiated version legitimately stays None."""
+    seen: list[str | None] = []
+    async with connected_runner(_runner_server(seen), initialized=False, stateless=True) as (client, runner):
+        result = await client.send_raw_request("tools/list", None)
+        assert result == {"tools": []}
+        assert seen == [None]
+        assert runner.session.protocol_version is None

tests/shared/test_session.py

@@ -9,6 +9,7 @@ from mcp.shared.memory import create_client_server_memory_streams
 from mcp.shared.message import SessionMessage
 from mcp.shared.session import RequestResponder
 from mcp.types import (
+    METHOD_NOT_FOUND,
     PARSE_ERROR,
     CancelledNotification,
     CancelledNotificationParams,
@@ -16,6 +17,7 @@ from mcp.types import (
     EmptyResult,
     ErrorData,
     JSONRPCError,
+    JSONRPCNotification,
     JSONRPCRequest,
     JSONRPCResponse,
     ServerNotification,
@@ -403,3 +405,43 @@ async def test_null_id_error_does_not_affect_pending_request():
     # Pending request completed successfully
     assert len(result_holder) == 1
     assert isinstance(result_holder[0], EmptyResult)
+
+
+@pytest.mark.anyio
+async def test_receive_loop_answers_unknown_request_method_with_method_not_found():
+    """A peer request whose method is not in the receive union gets -32601
+    (METHOD_NOT_FOUND) on the wire, not a validation failure (-32602)."""
+    async with create_client_server_memory_streams() as (client_streams, server_streams):
+        client_read, client_write = client_streams
+        server_read, server_write = server_streams
+
+        async with ClientSession(read_stream=client_read, write_stream=client_write):
+            await server_write.send(SessionMessage(message=JSONRPCRequest(jsonrpc="2.0", id=7, method="x/unknown")))
+            with anyio.fail_after(5):  # pragma: no branch
+                out = await server_read.receive()
+
+    assert isinstance(out, SessionMessage)
+    assert isinstance(out.message, JSONRPCError)
+    assert out.message.id == 7
+    assert out.message.error == ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="x/unknown")
+
+
+@pytest.mark.anyio
+async def test_receive_loop_drops_unknown_notification_method_without_response():
+    """An unknown notification method is dropped silently: JSON-RPC forbids
+    responses to notifications, and the receive loop keeps serving."""
+    async with create_client_server_memory_streams() as (client_streams, server_streams):
+        client_read, client_write = client_streams
+        server_read, server_write = server_streams
+
+        async with ClientSession(read_stream=client_read, write_stream=client_write):
+            await server_write.send(SessionMessage(message=JSONRPCNotification(jsonrpc="2.0", method="x/unknown")))
+            # The next wire output must be the answer to this follow-up ping,
+            # proving the notification produced no response and the loop survived.
+            await server_write.send(SessionMessage(message=JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")))
+            with anyio.fail_after(5):  # pragma: no branch
+                out = await server_read.receive()
+
+    assert isinstance(out, SessionMessage)
+    assert isinstance(out.message, JSONRPCResponse)
+    assert out.message.id == 1

tests/shared/test_streamable_http.py

@@ -1696,6 +1696,87 @@ async def test_close_sse_stream_callback_not_provided_for_old_protocol_version()
     assert session_msg_new.metadata.close_standalone_sse_stream is not None
 
 
+@pytest.mark.anyio
+async def test_priming_event_not_sent_for_unknown_protocol_version() -> None:
+    """_maybe_send_priming_event treats unrecognized version strings conservatively.
+
+    A garbage version must not be mistaken for a future one (lexicographically
+    "zzz" sorts after every date-shaped revision).
+    """
+    transport = StreamableHTTPServerTransport(
+        "/mcp",
+        event_store=SimpleEventStore(),
+    )
+
+    write_stream, read_stream = anyio.create_memory_object_stream[dict[str, Any]](1)
+
+    try:
+        await transport._maybe_send_priming_event("test-request-id", write_stream, "zzz")
+
+        # Nothing should have been written to the stream
+        assert write_stream.statistics().current_buffer_used == 0
+    finally:
+        await write_stream.aclose()
+        await read_stream.aclose()
+
+
+@pytest.mark.anyio
+async def test_close_sse_stream_callback_not_provided_for_unknown_protocol_version() -> None:
+    """close_sse_stream callbacks are withheld when the client's version is unrecognized."""
+    transport = StreamableHTTPServerTransport(
+        "/mcp",
+        event_store=SimpleEventStore(),
+    )
+
+    mock_message = JSONRPCRequest(jsonrpc="2.0", id="test-1", method="tools/list")
+    mock_request = MagicMock()
+
+    session_msg = transport._create_session_message(mock_message, mock_request, "test-request-id", "zzz")
+
+    assert session_msg.metadata is not None
+    assert isinstance(session_msg.metadata, ServerMessageMetadata)
+    assert session_msg.metadata.close_sse_stream is None
+    assert session_msg.metadata.close_standalone_sse_stream is None
+
+
+@pytest.mark.anyio
+async def test_initialize_with_unknown_protocol_version_gets_no_priming_event(
+    event_app: tuple[SimpleEventStore, Starlette],
+) -> None:
+    """A garbage protocolVersion in initialize params must not trigger priming.
+
+    The priming decision reads the raw body params before any validation, so an
+    unrecognized string must gate conservatively (old-client behavior), not
+    compare lexicographically past "2025-11-25".
+    """
+    event_store, app = event_app
+    init_request = {
+        "jsonrpc": "2.0",
+        "method": "initialize",
+        "params": {
+            "clientInfo": {"name": "test-client", "version": "1.0"},
+            "protocolVersion": "zzz",
+            "capabilities": {},
+        },
+        "id": "init-1",
+    }
+    async with make_client(app) as client:
+        response = await client.post(
+            "/mcp",
+            headers={
+                "Accept": "application/json, text/event-stream",
+                "Content-Type": "application/json",
+            },
+            json=init_request,
+        )
+        assert response.status_code == 200
+
+    # The store must have seen traffic (the initialize response), but no
+    # priming event — priming events are stored with a None payload.
+    assert event_store._events
+    assert all(message is not None for _, _, message in event_store._events)
+
+
 @pytest.mark.anyio
 async def test_streamable_http_client_receives_priming_event(
     event_app: tuple[SimpleEventStore, Starlette],

tests/shared/test_version.py

@@ -0,0 +1,59 @@
+"""Tests for the protocol-version registry and comparison helpers."""
+
+import pytest
+
+from mcp.shared.version import (
+    KNOWN_PROTOCOL_VERSIONS,
+    SUPPORTED_PROTOCOL_VERSIONS,
+    is_version_at_least,
+)
+
+
+@pytest.mark.parametrize(
+    ("version", "minimum", "expected"),
+    [
+        # equal
+        ("2025-11-25", "2025-11-25", True),
+        ("2024-11-05", "2024-11-05", True),
+        # above
+        ("2025-11-25", "2025-06-18", True),
+        ("2025-06-18", "2024-11-05", True),
+        # below
+        ("2025-06-18", "2025-11-25", False),
+        ("2024-11-05", "2025-03-26", False),
+    ],
+)
+def test_is_version_at_least_ordering(version: str, minimum: str, expected: bool) -> None:
+    """Known revisions order by registry position: equal, newer, and older pairs."""
+    assert is_version_at_least(version, minimum) is expected
+
+
+@pytest.mark.parametrize("version", ["zzz", "", "2025-11-26", "draft", "9999-99-99"])
+def test_is_version_at_least_unknown_version_is_false(version: str) -> None:
+    """Unrecognized peer strings compare conservatively, never accidentally."""
+    assert is_version_at_least(version, "2024-11-05") is False
+
+
+def test_is_version_at_least_unknown_minimum_raises() -> None:
+    """An unknown minimum is programmer error, not peer input."""
+    with pytest.raises(ValueError, match="zzz"):
+        is_version_at_least("2025-11-25", "zzz")
+
+
+@pytest.mark.parametrize(
+    ("version", "minimum"), [(v, m) for v in KNOWN_PROTOCOL_VERSIONS for m in KNOWN_PROTOCOL_VERSIONS]
+)
+def test_is_version_at_least_matches_lexicographic_for_known_versions(version: str, minimum: str) -> None:
+    """Drop-in equivalence: for every known (date-shaped) revision pair the helper
+    agrees with the string comparison it replaced."""
+    assert is_version_at_least(version, minimum) is (version >= minimum)
+
+
+def test_supported_versions_are_known() -> None:
+    """Every negotiable revision must be in the ordering registry."""
+    assert set(SUPPORTED_PROTOCOL_VERSIONS) <= set(KNOWN_PROTOCOL_VERSIONS)
+
+
+def test_known_versions_are_strictly_ordered() -> None:
+    """The registry tuple is the ordering source of truth: ascending, no duplicates."""
+    assert list(KNOWN_PROTOCOL_VERSIONS) == sorted(set(KNOWN_PROTOCOL_VERSIONS))