Function mcpDispatch

packages/hosts/mcp/src/envelope.ts:216–273

(resource: McpResource)

/** Dispatch an MCP request through authenticate -> store.dispatch -> transport. */
const mcpDispatch = (resource: McpResource) =>
  Effect.gen(function* () {
    const httpRequest = yield* HttpServerRequest.HttpServerRequest;
    const auth = yield* McpAuthProvider;
    const store = yield* McpSessionStore;
    const request = yield* toWebRequest(httpRequest);

    // CORS preflight: answer before auth so unauthenticated clients can probe.
    if (request.method === "OPTIONS") {
      return fromWebResponse(corsPreflightResponse());
    }

    // Streamable-HTTP only defines GET/POST/DELETE on the endpoint. Any other
    // method (PUT/PATCH/…) is rejected with a JSON-RPC 405 BEFORE auth/dispatch —
    // otherwise it would fall through and spin up a session engine for a method
    // the transport can't serve.
    if (!ALLOWED_MCP_METHODS.has(request.method)) {
      return fromWebResponse(jsonRpcResponse(405, -32001, "Method not allowed"));
    }

    const sessionId = request.headers.get("mcp-session-id");

    // Authenticate (and, for session-aware providers, authorize) on EVERY
    // request. Non-authenticated outcomes render directly. Session teardown is
    // only safe after the store can validate the authenticated principal and MCP
    // resource; an auth-level Forbidden may not carry either.
    const outcome = yield* auth.authenticate(request);
    if (!Predicate.isTagged(outcome, "Authenticated")) {
      return fromWebResponse(renderAuthError(auth, request, outcome));
    }
    const principal = outcome.principal;

    // No session id: per the streamable-HTTP transport contract, only POST opens
    // a session. A GET needs an existing id (400); a DELETE on nothing is a
    // no-op (204). Both short-circuit BEFORE dispatch so the store never spins up
    // an engine for a bare GET/DELETE.
    if (!sessionId) {
      if (request.method === "GET") {
        return fromWebResponse(
          jsonRpcResponse(400, -32000, "mcp-session-id header required for SSE"),
        );
      }
      if (request.method === "DELETE") {
        return fromWebResponse(
          new Response(null, { status: 204, headers: { "access-control-allow-origin": "*" } }),
        );
      }
    }

    const result: McpDispatchResult = yield* store.dispatch({
      request,
      principal,
      resource,
      sessionId,
      method: request.method,
    });
    return fromWebResponse(result instanceof Response ? result : renderDispatchError(result));
  });