用于使用 [[T0] 传输的 MCP 服务器的可流式 HTTP 和 SSE 代理 。

[注]

默认启用了可配置选项的 CORS。 详情见 CORS 配置 。

[注]

对于倒数——给一个远程MCP服务器,包括交互式OAuth流,只提供stdio客户端访问权限——参见mcp-remote.

[注]

MCP代理是FastMCP用来使可流的HTTP和SSE能够被使用.

□ 议定书修订

MCP代理协议修订2026-07-28和2025-时代的修订 ([[T2]至[T3]从同一端出. 每一个职位到 可流的HTTP端点按自身内容分类——2026-07-28 修订后,每一份请求都有一个[T4]]信封,没有一份请求的要求 由2025年代的会话机制服务——如此新老客户共享一个URL 也不必知道对方的存在。

QQ 2025-时代客户QQ 2026-07-28客户QQ ————————————————————————————————————————.

startStdioServer =======================================================================================

MCP Proxy

A TypeScript streamable HTTP and SSE proxy for MCP servers that use stdio transport.

[!NOTE] CORS is enabled by default with configurable options. See CORS Configuration for details.

[!NOTE] For the inverse — giving a stdio-only client access to a remote MCP server, including the interactive OAuth flow — see mcp-remote.

[!NOTE] MCP Proxy is what FastMCP uses to enable streamable HTTP and SSE.

Protocol revisions

MCP Proxy serves protocol revision 2026-07-28 and the 2025-era revisions (2024-10-07 through 2025-11-25) from the same endpoint. Every POST to the streamable HTTP endpoint is classified by its own content — the 2026-07-28 revision puts a _meta envelope on every request, and requests without one are served by the 2025-era session machinery — so old and new clients share one URL and neither has to know the other exists.

Progress notifications and cancellation cross the proxy in both eras: a forwarded call carries the caller’s abort signal, and relayed progress is re-tagged with the token the caller sent.

Use --no-modern to serve only 2025-era clients; a 2026-07-28 client is then answered by the 2025-era machinery, which does not recognise its request. The modern leg lives on the streamable HTTP endpoint, so it is also absent when that endpoint is disabled (--server sse, or streamEndpoint: null).

The upstream connection — the server MCP Proxy spawns — negotiates its own revision independently, so any combination works. It uses the 2025 initialize handshake by default; pass --upstreamProtocol auto to probe with server/discover first (needed to front a 2026-07-28 server) or --upstreamProtocol modern to require it.

# front a 2026-07-28 stdio server, serving both eras downstream
npx mcp-proxy --upstreamProtocol auto -- my-modern-mcp-server

What does not cross the proxy

These are consequences of one design decision — the proxy multiplexes every downstream connection onto a single upstream connection — and are called out rather than half-solved.

• Anything that asks the client for input (elicitation, sampling, roots). A 2025-era server pushes its request out of band, carrying nothing that says which caller it belongs to, so relaying it means guessing whose screen to put the prompt on. A 2026-07-28 upstream is not ambiguous that way — the request