@immediately-run/worker-transport

The same-origin worker MessagePort transport for immediately.run: the WorkerMessageBus request/response protocol and the one-time { type: 'connect' } MessagePort handshake, extracted into a leaf package so a worker-owning service package can ship its own worker entry without depending on sandbox.

Why a dedicated leaf package (not the SDK)

The transport is imported by the worker-owning packages (today @immediately-run/transpiler's Babel worker; later the tsc/eslint/prettier authoring services per R3-107) and by sandbox. Routing it through the SDK would pull the host (immediately-run-site-main) into a build dependency on the app-facing client library — which the host↔SDK design deliberately avoids (the SDK is fetched at runtime, never a host build input; SDK_PACKAGING_SPEC §5). A leaf package keeps the transport entanglement-free. See SIMPLIFIED_DEPLOYMENT_SPEC §14 / roadmap R3-149.

API

• WorkerMessageBus — the channel-scoped request/response bus over any MessageEndpoint (a Worker, a MessagePort, or self in a worker).
• bindWorkerMessageBus(self, opts) — installs the { type: 'connect' } handshake on a worker's self: waits for the parent to transfer a MessagePort, starts it, and binds a WorkerMessageBus on it. This is the exact handshake sandbox's babel-worker.ts used, generalized so every worker entry installs it identically.

import { bindWorkerMessageBus } from '@immediately-run/worker-transport';

bindWorkerMessageBus(self, {
  channel: 'sandpack-babel',
  handleRequest: (method, data) =>
    method === 'transform' ? doTransform(data) : Promise.reject(new Error('Unknown method')),
});

Runtime topology

The worker is spawned by the parent page (not the sandboxed iframe) so the iframe can drop allow-same-origin. The parent transfers a MessagePort entangled with one handed into the iframe, so transform requests flow directly between the iframe and the worker (the MessagePort identity rule, HOST_ORIGIN_HARDENING_SPEC §2.4). This package only owns the wire protocol; where the bytes are served from is the consumer's concern.