Streamable HTTP: malformed requests return raw pydantic errors to the client #3698

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)
Also applies to latest 1.x

Description

When the Streamable HTTP POST handler rejects a malformed body, it returns the raw exception text to the client (streamable_http.py L580-L596):

• No "jsonrpc" field → "Validation error: 6 validation errors for union[JSONRPCRequest,...] ... input_value={...} ... https://errors.pydantic.dev/2.14/v/missing"
• Not JSON → "Parse error: EOF while parsing a value at line 1 column 27"

A security audit of our MCP server flagged this: the errors reveal that the backend is Python, using pydantic (down to the exact version) and the MCP Python SDK, which helps an attacker enumerate the environment.

Expected: fixed messages (Parse error, Invalid JSON-RPC message), with detail in the server log only. The 2026-07-28 path already does this (_streamable_http_modern.py L415).

The same code is on v1.x, which we run. I’d like to fix both if assigned: a few lines plus tests, with no new API.

Related: #3557 (error code on these lines), #3572 (same pydantic text in tool-argument errors).

Disclosure: this issue was drafte by Claude Code and reviewed/iterated by myself and another senior developer. I ran the reproduction and verified the output myself.

Example Code

import json

from mcp.server import MCPServer
from mcp.server.transport_security import TransportSecuritySettings
from starlette.testclient import TestClient

server = MCPServer("repro")


@server.tool()
def echo(text: str) -> str:
    return text


app = server.streamable_http_app(
    stateless_http=True,
    transport_security=TransportSecuritySettings(enable_dns_rebinding_protection=False),
)
headers = {
    "Accept": "application/json, text/event-stream",
    "Content-Type": "application/json",
    "mcp-protocol-version": "2025-06-18",
}
with TestClient(app) as client:
    for body in (
        json.dumps({"id": 1, "method": "tools/call", "params": {"name": "echo", "arguments": {"text": "hi"}}}),
        json.dumps([{"jsonrpc": "2.0", "id": 1, "method": "tools/list"}]),
        '{"jsonrpc": "2.0", "id": 1,',
    ):
        response = client.post("/mcp", content=body, headers=headers)
        print(response.status_code, json.dumps(response.json(), indent=2))

Python & MCP Python SDK

Python 3.12.9
mcp 2.3.0
pydantic 2.14.0