Anubis.Server.Transport.StreamableHTTP.Plug (anubis_mcp v2.1.0)

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A Plug implementation for the Streamable HTTP transport.

This plug handles the MCP Streamable HTTP protocol as specified in MCP 2025-03-26. It provides a single endpoint that supports both GET and POST methods:

  • GET: Opens an SSE stream for server-to-client communication
  • POST: Handles JSON-RPC messages from client to server
  • DELETE: Closes a session

Usage in Phoenix Router

pipeline :mcp do
  plug :accepts, ["json"]
end

scope "/mcp" do
  pipe_through :mcp
  forward "/", to: Anubis.Server.Transport.StreamableHTTP.Plug, server: :your_server_name
end

Configuration Options

  • :server - The server process name (required)
  • :session_header - Custom header name for session ID (default: "mcp-session-id")
  • :request_timeout - Request timeout in milliseconds (default: 30000)
  • :subscriber_metadata - A 1-arity function (Plug.Conn.t() -> map()) called when an SSE stream is opened. Its return value is stored verbatim as the subscriber's opaque metadata (see Anubis.Server.Transport.StreamableHTTP.register_sse_handler/3) and can later be selected on with send_message_to_subscribers/4 and handler_count/2. Tag subscribers by tenant, user, feature scope, etc. derived from the request. Defaults to fn _conn -> %{} end. Use a remote function capture (&MyApp.sse_metadata/1) so it survives compile-time plug option escaping.

Summary

Functions

call(conn, opts)

Authorizes an HTTP request and dispatches it to its protocol-era binding.

init(opts)

Builds transport options from a keyword list with a required :server.

Functions

call(conn, opts)

Authorizes an HTTP request and dispatches it to its protocol-era binding.

Takes options returned by init/1 and returns the response connection. Serves OAuth resource metadata on its well-known path; other requests use the declared protocol versions, with duplicate version headers rejected when the server supports the stateless era.

init(opts)

Builds transport options from a keyword list with a required :server.

Applies the default session header, timeout and subscriber metadata callback. Raises KeyError when :server is absent.