fix(server): return METHOD_NOT_FOUND for unregistered spec methods (#3193)

Author: Pradhyut21

src/mcp/server/runner.py

@@ -186,15 +186,9 @@ async def _inner(ctx: ServerRequestContext[LifespanT, Any]) -> HandlerResult:
             # Read method/params off `ctx` so a middleware that rewrote them via
             # `call_next(replace(ctx, ...))` reaches lookup and the handler.
             method, params = ctx.method, ctx.params
-            # Pinned compat: spec methods are surface-validated before lookup,
-            # so malformed params are INVALID_PARAMS even with no handler
-            # registered. Custom methods miss the monolith map and fall through
-            # to `entry.params_type` exactly as before.
-            if method in _methods.SPEC_CLIENT_METHODS:
-                try:
-                    _methods.validate_client_request(method, version, params)
-                except KeyError:
-                    raise MCPError(code=METHOD_NOT_FOUND, message="Method not found", data=method) from None
+            # Spec version gate: if a spec method is not valid at this protocol version, reject it.
+            if method in _methods.SPEC_CLIENT_METHODS and (method, version) not in _methods.CLIENT_REQUESTS:
+                raise MCPError(code=METHOD_NOT_FOUND, message="Method not found", data=method)
             # TODO(L29): the 2026-07-28 spec drops the handshake; this branch and
             # the gate become a per-version legacy path then. Initialize runs inline
             # (read loop parked), so awaiting the peer anywhere on this path deadlocks.
@@ -211,6 +205,8 @@ async def _inner(ctx: ServerRequestContext[LifespanT, Any]) -> HandlerResult:
             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="")
+            if method in _methods.SPEC_CLIENT_METHODS:
+                _methods.validate_client_request(method, version, params)
             # 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)

tests/interaction/lowlevel/test_wire.py

@@ -205,6 +205,7 @@ async def test_malformed_request_params_are_answered_with_invalid_params() -> No
     integer. Reserve this pattern for behaviour the typed API cannot produce.
     """
     server = Server("strict")
+    server.add_request_handler("tools/call", CallToolRequestParams, lambda ctx, p: None)
     errors: list[ErrorData] = []
 
     async with create_client_server_memory_streams() as (client_streams, server_streams):

tests/server/test_runner.py

@@ -295,12 +295,12 @@ async def test_runner_non_spec_method_with_no_handler_raises_method_not_found(se
 
 
 @pytest.mark.anyio
-async def test_runner_malformed_params_for_unregistered_spec_method_raises_invalid_params(server: SrvT):
-    """A spec method with malformed params is INVALID_PARAMS even with no handler."""
+async def test_runner_unregistered_spec_method_raises_method_not_found(server: SrvT):
+    """An unregistered spec method returns METHOD_NOT_FOUND per JSON-RPC 2.0 (issue #3193)."""
     async with connected_runner(server) as (client, _):
         with pytest.raises(MCPError) as exc:
             await client.send_raw_request("tools/call", {"name": 123})
-    assert exc.value.error == ErrorData(code=INVALID_PARAMS, message="Invalid request parameters", data="")
+    assert exc.value.error == ErrorData(code=METHOD_NOT_FOUND, message="Method not found", data="tools/call")
 
 
 @pytest.mark.anyio
@@ -845,6 +845,7 @@ async def greet(ctx: Ctx, params: GreetParams) -> dict[str, Any]:
 
 @pytest.mark.anyio
 async def test_runner_spec_method_with_invalid_params_is_invalid_params_at_the_negotiated_version(server: SrvT):
+    server.add_request_handler("tools/call", CallToolRequestParams, lambda ctx, p: None)
     async with connected_runner(server) as (client, runner):
         assert runner.connection.protocol_version == LATEST_HANDSHAKE_VERSION
         with pytest.raises(MCPError) as exc: