Transports

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FastestMCP supports streamable HTTP and stdio.

Streamable HTTP

The supported HTTP transport is streamable HTTP.

FastestMCP supports:

  • a single MCP endpoint at /mcp
  • stateful GET /mcp, POST /mcp, and DELETE /mcp
  • optional stateless mode
  • mcp-session-id session headers
  • event-stream framing for streamed responses and notifications
  • Plug-first embedding for Bandit, Phoenix, or custom Plug stacks

Deprecated standalone SSE transport is intentionally unsupported.

Plug Embedding

FastestMCP.http_app/2 returns a Plug-compatible app:

children = [
  {Bandit, plug: FastestMCP.http_app(MyApp.MCPServer, allowed_hosts: :localhost), port: 4100}
]

You can also run the transport child spec directly:

children = [
  FastestMCP.streamable_http_child_spec(MyApp.MCPServer,
    port: 4100,
    allowed_hosts: :localhost
  )
]

Phoenix forwarding uses the same transport module:

forward "/mcp", FastestMCP.Transport.HTTPApp,
  server_name: MyApp.MCPServer,
  path: "/mcp",
  allowed_hosts: :any

When the route sits behind your Plug or Phoenix auth pipeline, select assigns for auth with auth_assigns::

pipeline :mcp do
  plug :fetch_session
  plug MyAppWeb.UserAuth, :fetch_current_user
end

scope "/" do
  pipe_through :mcp

  forward "/mcp", FastestMCP.Transport.HTTPApp,
    server_name: MyApp.MCPServer,
    path: "/mcp",
    auth_assigns: [:current_user]
end

Selected assigns are copied into auth input under "assigns" and are not added to normal handler request metadata.

Stdio

The stdio transport is available for local tooling and process-owned workflows:

FastestMCP.stdio_dispatch(MyApp.MCPServer, request)

Use FastestMCP.Transport.Stdio when you want a long-lived stdio transport entrypoint instead of dispatching request by request.

Why This Shape

FastestMCP keeps the transport layer thin. HTTP and stdio both feed the same execution pipeline, so transport features do not become separate behavior forks with different auth, middleware, provider, or task semantics.