Mcp-Param-* validation runs the full tools/list handler on every tools/call

[kratsg]

Description

On the 2026-07-28 HTTP path, every tools/call with arguments runs the server's full tools/list handler before dispatch, to find the called tool's inputSchema for Mcp-Param-* validation (_mcp_param_rejection → _tool_input_schema, added in #3033). There is no way to opt out, and it runs even when no tool declares x-mcp-header.

For a server whose tools/list is expensive this is a large per-call cost. We run a gateway that aggregates ~7 backend MCP servers; its tools/list fans out to every backend with per-user credentials, so each tools/call now lists every backend. #3033 notes this cost as "optimizable later"; a by-name schema lookup, or skipping the check when no tool declares x-mcp-header, would avoid it.

Expected: a tools/call doesn't invoke the tools/list handler (or there's a way to avoid it). Actual: one full listing per tools/call.

Example Code

import asyncio
import threading
import time

import uvicorn
from mcp import types
from mcp.client.client import Client
from mcp.server.lowlevel.server import Server

list_calls = 0


async def on_list_tools(ctx, params):
    global list_calls
    list_calls += 1
    return types.ListToolsResult(
        tools=[
            types.Tool(
                name="echo",
                inputSchema={"type": "object", "properties": {"text": {"type": "string"}}},
            )
        ]
    )


async def on_call_tool(ctx, params):
    # How many times tools/list ran before this call was dispatched.
    return types.CallToolResult(content=[types.TextContent(type="text", text=str(list_calls))])


server = Server("repro", on_list_tools=on_list_tools, on_call_tool=on_call_tool)
app = server.streamable_http_app(stateless_http=True, json_response=True)
threading.Thread(
    target=uvicorn.Server(uvicorn.Config(app, port=8765, log_level="warning")).run, daemon=True
).start()
time.sleep(1)


async def main():
    async with Client("http://127.0.0.1:8765/mcp", mode="2026-07-28") as client:
        result = await client.call_tool("echo", {"text": "hi"})
    print(f"tools/list calls before the tools/call was dispatched: {result.content[0].text}")


asyncio.run(main())

Prints 1. With mode="legacy" it prints 0.

Python & MCP Python SDK

Python 3.12.13, mcp 2.2.0

Related: #3513 (client-side analog), #3484.

Drafted with AI assistance (Claude Code); the reproduction above was run against mcp 2.2.0.

[0xamlab]

We hit this exact wall operating a 140-tool MCP server: a full tools/list per tools/call is ~19 ms of pure overhead per call at that registry size (measured on our side before we started memoizing schema conversion — see PR #2839 for the same class of fix). A by-name fast path (validate against the cached tool map built at registration) plus an opt-out for trusted servers would kill both costs without weakening SEP-2243 conformance. Happy to PR with a benchmark test if a maintainer assigns.

Affiliation: we build Algenta's MCP server and conformance-test against this SDK.