Streamable HTTP server transport (MCP 2025-11-25): a single MCP endpoint handling POST/GET/DELETE, mountable in Phoenix or any Plug stack:

# Phoenix router
forward "/mcp", Noizu.MCP.Transport.StreamableHTTP.Plug, server: MyApp.MCP

# standalone with Bandit
{Bandit, plug: {Noizu.MCP.Transport.StreamableHTTP.Plug, server: MyApp.MCP}, port: 4040}

Behavior per spec: initialize POSTs create a session and return Mcp-Session-Id; requests answer as application/json when the handler produces only a response, upgrading to an SSE stream when progress, logging, or server-initiated requests flow first; GET opens the general SSE stream (with Last-Event-ID resumability backed by Noizu.MCP.Server.EventStore); DELETE terminates the session.

Options

• :server (required) — the use Noizu.MCP.Server module
• :origins — :localhost (default; allows non-browser clients and localhost origins), :mcp_clients (localhost plus the known browser MCP hosts, see mcp_client_origins/0), :any, or an explicit allowlist of origins. Origin validation guards against DNS-rebinding attacks.
• :idle_timeout — session idle expiry in ms (default 30 minutes)
• :request_timeout — max time to wait for a handler response (default 300_000)
• :keepalive — SSE keepalive comment interval in ms (default 25_000)
• :context — {module, function} invoked as fun.(conn) returning a map merged into session assigns at initialize (how plug-level auth reaches handlers)
• :auth — resource-server enforcement, see below
• :cors — false (default) or true/keyword to answer browser preflights, see below

Authorization (:auth)

• :verifier (required) — Noizu.MCP.Auth.TokenVerifier module or {module, opts}
• :resource_metadata — the RFC 9728 URL advertised in the 401 challenge. A binary, {module, function} (called with the conn), {module, function, args}, a 1-arity fun, or :derive (built from the request and the forward's mount path).
• :scope — scope advertised in the challenge, so a client that has never seen a token knows what to ask for

CORS (:cors)

A browser MCP client (claude.ai) cannot read the 401 challenge — and so cannot start OAuth — unless WWW-Authenticate is exposed, and cannot continue a session unless Mcp-Session-Id is. Enabling :cors answers OPTIONS with 204 and adds, to every response:

access-control-allow-origin: <origin>
access-control-expose-headers: WWW-Authenticate, Mcp-Session-Id, Mcp-Protocol-Version

Only origins that pass :origins are answered, so CORS never widens the DNS-rebinding guard. Options: allow_headers:, max_age:.