The consumer contract for MCP request authentication: authenticate/1 validates the request's Bearer token and adds identity to conn.assigns.

Consuming applications implement this behaviour to validate Bearer tokens from the Authorization header. The implementation typically:

1. Extracts the Bearer token from the Authorization header
2. Validates it (e.g., looks up a hashed token in the database)
3. On success: adds identity information to conn.assigns and returns {:ok, conn}
4. On failure: returns {:error, message}

The auth module is declared in the mount module:

defmodule MyApp.Mcp do
  use Wymcp.Router,
    tools: [MyApp.Tools.Events],
    auth: MyApp.McpAuth
end

When no :auth option is provided, Wymcp.Auth.Noop is used (no authentication).

authenticate/1 runs once per request on every non-fallthrough route — POST, GET (the SSE stream), and DELETE. Authenticate from connection data such as the Authorization header, never from the request body: on GET and DELETE no body is parsed, so conn.body_params is unfetched. Auth is per-request only — an already-open SSE stream lives until its session ends, and a reconnect is a fresh GET that re-authenticates naturally.

Example implementation

defmodule MyApp.McpAuth do
  @behaviour Wymcp.Auth

  @impl Wymcp.Auth
  def authenticate(conn) do
    case Plug.Conn.get_req_header(conn, "authorization") do
      [header] ->
        with "Bearer " <> token <- header,
             {:ok, user} <- MyApp.Accounts.fetch_user_by_api_token(token) do
          {:ok, Plug.Conn.assign(conn, :current_user, user)}
        else
          _ -> {:error, "Invalid Bearer token"}
        end

      [] ->
        {:error, "Missing Authorization header"}

      [_, _ | _] ->
        {:error, "Duplicated Authorization header. Send exactly one Authorization header."}
    end
  end
end

The three-way read is the point. Plug.Conn.get_req_header/2 returns every value of a repeated header, so a two-way ["Bearer " <> token] match folds a duplicated Authorization into the same answer as a missing one — a wrong message, not merely a vague one, and it is the arm a copied-and-trimmed example loses first.

Authorization is the one singleton header (docs/glossary.md) wymcp cannot validate centrally: Wymcp.Plugs.SingletonHeaders runs after this callback, because a 401 must win over any answer that depends on reading the request further. A duplicated Authorization therefore reaches authenticate/1 untouched, and this example is the only leverage wymcp has on it.

MCP specification notes

Per the MCP 2025-11-25 spec, servers that require authentication MUST return 401 with a WWW-Authenticate: Bearer challenge when the token is missing or invalid. The Wymcp.Plugs.Auth plug handles this response format automatically when authenticate/1 returns {:error, _}.