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An Astro or Cloudflare preview can fail to start in a Carbon (Mark 3.1) sandbox when an older Miniflare version binds its runtime inspector to localhost. Upgrade the Cloudflare development dependencies or disable the inspector when you do not need it.

Symptoms

The preview process may hang at:
In applications that wait for the preview server, the visible error can instead be a secondary connection failure:
The refused connection does not necessarily mean that the application selected the wrong port. It can mean that Miniflare never finished starting the underlying workerd runtime.

Confirm the sandbox generation

Run the following command inside the sandbox:
Inspect the main non-loopback interface. A global inet6 address indicates that the sandbox is running on Carbon (Mark 3.1).
The presence of IPv6 identifies the sandbox generation during this rollout. It does not by itself prove that Miniflare is affected.

Check for the Miniflare issue

Check which Cloudflare packages the application resolved:
Then inspect localhost resolution and the IPv6 loopback interface:
The known failure occurs when localhost resolves to ::1, but the runtime cannot bind the IPv6 loopback address. Older Miniflare versions pass --inspector-addr=localhost:0 to workerd and wait for an inspector-ready event that never arrives. Confirm the inspector address while the preview process is stuck:
An affected process includes:

Upgrade Miniflare or Wrangler

The preferred fix is to update the package that provides Miniflare and regenerate the lockfile. The fix is included in:
  • Miniflare 4.20260601.0 and later
  • Wrangler 4.97.0 and later
For a project that uses Wrangler directly:
Run npm ls miniflare wrangler again to verify the resolved versions. The fixed Miniflare release uses 127.0.0.1 for the runtime inspector instead of relying on localhost resolution.

Disable the inspector

If you cannot upgrade immediately and do not need runtime debugging, disable the inspector in the Cloudflare integration. For an Astro project whose Cloudflare adapter supports inspectorPort, update astro.config.mjs:
For a project that configures the Cloudflare Vite plugin directly:
Restart the preview process after changing the configuration. This workaround disables local runtime debugging but does not affect the application preview. Cloudflare fixed the underlying address selection in workers-sdk pull request #14086. The change replaces localhost with 127.0.0.1 for the runtime inspector address.

Set up Astro with Blaxel Sandboxes

Configure and expose an Astro development server in a sandbox.

Infrastructure generations

Compare Carbon with earlier Blaxel infrastructure generations.
Last modified on October 5, 2026