Wrangler bundles code that imports WorkerEntrypoint as service worker if no default export is provided

[irvinebroque]

What versions & operating system are you using?

MacOS Latest, Wrangler 4.125

Please provide a link to a minimal reproduction

https://github.com/irvinebroque/repro-esm-entrypoint

Describe the Bug

If you have code like this:

import { WorkerEntrypoint } from "cloudflare:workers";

// This is intentionally the only entrypoint. A named WorkerEntrypoint export is
// valid module-worker syntax and cannot run as a Service Worker.
export class NamedEntrypoint extends WorkerEntrypoint {
	fetch(): Response {
		return new Response("Hello from the named entrypoint");
	}
}

• Wrangler incorrectly selects Service Worker format and fails.
• The Vite plugin warns that default is missing, then adds an empty default export to the bundle.

So you have to do this if you are using Wrangler:

export default {
	fetch() {
		throw new Error("unimplemented");
	},
};

Please provide any relevant error logs

No response

[workers-devprod]

Automated triage

• Reproduction: ⚠️ Blocked.
• Actionability: ✅ Actionable.
• Type: Bug (high confidence).
• Labels:
  • package:wrangler (high)
  • package:vite-plugin (high)

Reproduction details

Automated reproduction failed before producing a result.

Actionability and missing information

The report is a high-confidence bug in Wrangler's module-worker entrypoint detection, with a minimal reproduction and version/environment noted; the Vite plugin is also affected. Exact error output was not provided.

Missing information:

• error output

Reproduction findings

⚠️ Blocked. Automated reproduction agent failed before recording a result.

[penalosa]

@irvinebroque I think we'd actually consider this a bug in the Vite plugin, because it doesn't match the control plane (which had an outstanding bug where it would reject Workers without a default export—that may be fixed, in which case I'm out of date).

That said, Wrangler should provide better error messaging here, what logs did you get? I think there should have been a warning in this case.

[irvinebroque]

Warning: ES module exports found without a default export; building using "service-worker" format.

then

  The uploaded script has no registered event handlers. Please visit the following website for more
  information: https://developers.cloudflare.com/workers/runtime-apis/handlers/. [code: 10068]

[irvinebroque]

do we need to change the edgeworker validator to allow for a Worker that has no default export, but has a named export?

[iruygloriani-dotcom]

Commitb

[NuroDev]

@workers-devprod help

[workers-devprod]

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[NuroDev]

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