🐛 BUG: MiniflareCoreError [ERR_RUNTIME_FAILURE]: The Workers runtime failed to start.

TL;DR

Downgrade wrangler to 3.57.1 as a workaround: the reporter confirmed that version cleared the runtime startup failure. The bug hits newer wrangler versions when starting the Workers runtime (e.g. creating a second application).

The error

MiniflareCoreError [ERR_RUNTIME_FAILURE]: The Workers runtime failed to start.

Fix it

1. Check your wrangler version:

   wrangler --version

   Success check: the version is newer than 3.57.1, confirming you are affected.

2. Downgrade wrangler to 3.57.1:

   npm i -g wrangler@3.57.1

   Success check: wrangler --version reports 3.57.1.

3. Retry the command that failed (e.g. creating the second application). Success check: the Workers runtime starts with no MiniflareCoreError.

When this applies

• MiniflareCoreError [ERR_RUNTIME_FAILURE] on wrangler newer than 3.57.1.
• The failure happens at runtime startup, e.g. when scaffolding a second application.

When it does not apply

• The runtime starts but your worker crashes; that is an application bug.
• A different Miniflare error code.
• You are already on 3.57.1 or older and still see the failure.

Compatibility

• wrangler 3.57.1 (workaround version). Affected: newer 3.x versions at the time of the report.
• miniflare 3.20231218.1, workerd 1.20231218.0, Node 16+.

Variant phrasings

• MiniflareCoreError ERRRUNTIMEFAILURE
• wrangler Workers runtime failed to start
• Miniflare runtime failed to start second application

Why it happens

A regression in newer wrangler 3.x broke Miniflare's runtime startup path in some configurations. Version 3.57.1 predates the regression, so downgrading restores the working startup sequence.

Edge cases

• Pin the version in CI so a fresh install does not pull the broken release.
• Watch the upstream issue for a fixed release, then upgrade past it.
• Source: https://github.com/cloudflare/workers-sdk/issues/4709

In the GitHub issue, the reporter asked: 🐛 BUG: MiniflareCoreError [ERRRUNTIMEFAILURE]: The Workers runtime failed to start.. The working answer that came out of the thread: If wrangler fails with 'MiniflareCoreError [ERRRUNTIMEFAILURE]: The Workers runtime failed to start' on a newer wrangler version (the reporter hit it when creating a second application), downgrade wrangler to 3.57.1 as a workaround. That version cleared the runtime startup failure for the reporter. Source: https://github.com/cloudflare/workers-sdk/issues/4709