React 19 + Cloudflare Adapter -> "Bug: Uncaught ReferenceError: MessageChannel is not defined"

[ethanniser]

Astro Info

Astro                    v5.1.1
Node                     v22.6.0
System                   macOS (arm64)
Package Manager          bun
Output                   server
Adapter                  @astrojs/cloudflare
Integrations             @astrojs/mdx
                         @astrojs/sitemap
                         @astrojs/tailwind
                         @astrojs/react

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

originally shared facebook/react#31827 but I think it makes sense to be an issue here as I doubt its a react issue

It doesn't happen during build but instead during the "Deploying to Cloudflare's global network" step

What's the expected result?

The error should not happen

Link to Minimal Reproducible Example

https://github.com/phase/test-astro-react19-cf-workers (thank you @phase)

Participation

• I am willing to submit a pull request for this issue.

[phase]

here's a workaround withastro/adapters#436 (comment)

astro.config.mjs

// @ts-check
import { defineConfig } from "astro/config";
import cloudflare from "@astrojs/cloudflare";

import react from "@astrojs/react";

// https://astro.build/config
export default defineConfig({
  output: "server",
  adapter: cloudflare(),
  integrations: [react()],
  vite: {
    resolve: {
      // Use react-dom/server.edge instead of react-dom/server.browser for React 19.
      // Without this, MessageChannel from node:worker_threads needs to be polyfilled.
      alias: import.meta.env.PROD && {
        "react-dom/server": "react-dom/server.edge",
      },
    },
  },
});