fix: allow opting out of Mcp-Param-* validation (avoids a full tools/list per tools/call)

src/mcp/server/_streamable_http_modern.py

@@ -342,7 +342,12 @@ async def _mcp_param_rejection(
     plain `application/json` 400 (the spec's MUST). With no `tools/list` handler
     the catalog is undiscoverable and there is no recognized header to validate.
     """
-    if req.method != "tools/call" or app.get_request_handler("tools/list") is None:
+    if (
+        not app.mcp_param_validation
+        or req.method != "tools/call"
+        or app.get_request_handler("tools/list") is None
+    ):
+        # Opted out (#3565): skip the schema-resolving tools/list walk entirely.
         return None
     params = req.params or {}
     name = params.get("name")

src/mcp/server/lowlevel/server.py

@@ -143,6 +143,12 @@ def __init__(
             [Server[LifespanResultT]],
             AbstractAsyncContextManager[LifespanResultT],
         ] = lifespan,
+        # Set to False to skip Mcp-Param-* header validation. Validation resolves
+        # the called tool's input schema by running the full tools/list handler
+        # on every argument-bearing tools/call; servers that never advertise
+        # x-mcp-header (e.g. aggregating gateways) can opt out of that per-call
+        # cost without changing any wire behavior.
+        mcp_param_validation: bool = True,
         # Request handlers
         on_list_tools: Callable[
             [ServerRequestContext[LifespanResultT], types.PaginatedRequestParams | None],
@@ -226,6 +232,12 @@ def __init__(
             [Server[LifespanResultT]],
             AbstractAsyncContextManager[LifespanResultT],
         ] = lifespan,
+        # Set to False to skip Mcp-Param-* header validation. Validation resolves
+        # the called tool's input schema by running the full tools/list handler
+        # on every argument-bearing tools/call; servers that never advertise
+        # x-mcp-header (e.g. aggregating gateways) can opt out of that per-call
+        # cost without changing any wire behavior.
+        mcp_param_validation: bool = True,
         # Request handlers
         on_list_tools: Callable[
             [ServerRequestContext[LifespanResultT], types.PaginatedRequestParams | None],
@@ -318,6 +330,12 @@ def __init__(
             [Server[LifespanResultT]],
             AbstractAsyncContextManager[LifespanResultT],
         ] = lifespan,
+        # Set to False to skip Mcp-Param-* header validation. Validation resolves
+        # the called tool's input schema by running the full tools/list handler
+        # on every argument-bearing tools/call; servers that never advertise
+        # x-mcp-header (e.g. aggregating gateways) can opt out of that per-call
+        # cost without changing any wire behavior.
+        mcp_param_validation: bool = True,
         # Request handlers
         on_list_tools: Callable[
             [ServerRequestContext[LifespanResultT], types.PaginatedRequestParams | None],
@@ -416,6 +434,7 @@ def __init__(
 
         self.name = name
         self.version = version
+        self.mcp_param_validation = mcp_param_validation
         self.title = title
         self.description = description
         self.instructions = instructions

tests/server/test_streamable_http_modern.py

@@ -1222,3 +1222,42 @@ async def post() -> None:
         "io.modelcontextprotocol/subscriptionId": 9,
         SERVER_INFO_META_KEY: {"name": "test", "version": "1.2.3"},
     }
+
+
+async def test_modern_tools_call_skips_tools_list_when_mcp_param_validation_opted_out() -> None:
+    """#3565: with `mcp_param_validation=False` an argument-bearing `tools/call` no
+    longer runs the schema-resolving `tools/list` handler before dispatch; the
+    call dispatches normally, so aggregating servers never advertise
+    `x-mcp-header` can drop the per-call listing cost without wire changes."""
+
+    list_calls = 0
+
+    async def list_tools(
+        ctx: ServerRequestContext, params: PaginatedRequestParams | None
+    ) -> ListToolsResult:
+        nonlocal list_calls
+        list_calls += 1
+        return ListToolsResult(tools=[_REGION_TOOL], ttl_ms=0, cache_scope="public")
+
+    server: Server[Any] = Server(
+        "test",
+        mcp_param_validation=False,
+        on_list_tools=list_tools,
+        on_call_tool=_ok_call_tool,
+    )
+    async with _asgi_client(server) as http:
+        response = await http.post("/mcp", json=_tool_call_body({"region": "east"}), headers=_TOOL_CALL_HEADERS)
+    assert response.status_code == 200
+    assert list_calls == 0
+
+
+async def test_modern_tools_call_runs_tools_list_by_default_for_validation() -> None:
+    """The default keeps validating: an argument-bearing `tools/call` against an
+    `x-mcp-header`-advertising server still resolves the schema (and rejects a
+    missing header), so opting out is strictly opt-in."""
+    async with _asgi_client(_x_mcp_server()) as http:
+        response = await http.post(
+            "/mcp", json=_tool_call_body({"region": "east"}), headers=_TOOL_CALL_HEADERS
+        )
+    assert response.status_code == 400
+    assert response.json()["error"]["code"] == HEADER_MISMATCH