Apps Builder Preview stuck — BackendAgent sandbox provisioning fails with HttpError

💬 App Building
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  1. My goal:Restore the draft Preview environment for our production Apps Builder application without publishing the currently untested draft.

  2. Issue:Our production application works normally, but the draft Preview no longer starts. It remains stuck at “Reconnecting to preview / Almost there…”.

    We traced the failure to Retool’s BackendAgent sandbox provisioning request:

    /api/ai/retool-next-agent/start

    which repeatedly fails with an HttpError. The sandbox health check then reports the sandbox as dead, and automatic recovery eventually reaches the maximum retry count. As a result, no new Vite/app Preview runtime starts.

    Organization: clavislogistic

    Apps Builder app:
    clavislogistic--containers-inventory-explorer.retool.app

    Production runtime observed: 4.71.0-f8dde02, build 395128.

    Production continues to work normally and has not been changed.

  3. Steps I've taken to troubleshoot:
    -Reloaded the Retool editor

  4. -Used the supported Reconnect flow

  5. Retried Preview startup multiple times

  6. Verified that the relevant draft source is saved and statically validates without a new file-specific TypeScript/build error

  7. Confirmed that production remains operational

  8. Investigated the documented Retool CLI path, but no CLI is currently installed and the app has no linked local checkout

  9. We deliberately did not publish the untested draft, push a new build, call undocumented internal endpoints, or perform an unsupported sandbox reset

Could you please investigate whether the Preview/BackendAgent sandbox for this application is stuck and advise how it can be safely re-provisioned or reset?

  1. Additional info: (Cloud or Self-hosted, Screenshots): Retool Cloud / Apps Builder.
    I can provide screenshots or additional diagnostic information if required.
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Summary

On Retool Cloud using the new App Builder, the draft Preview for a production React app is stuck at 'Reconnecting to preview / Almost there…' because the BackendAgent sandbox provisioning call (/api/ai/retool-next-agent/start) repeatedly fails with an HttpError and the sandbox is reported dead after max retries, while the published production app continues to work; the goal is to safely re-provision the Preview sandbox without publishing the untested draft.

AI Response

This symptom matches a known pattern in the new App Builder where the in-editor/authoring sandbox fails to provision or start while the published app keeps running normally, and recovery has required a Retool-side fix rather than any self-service or unsupported sandbox reset. In reported cases the cause has been server-side — in one instance Retool rolled back a code change that was breaking generation-worker startup, and in another it was a context-window limit breaking the app's websocket connection to its sandbox, fixed per-app with unpublished work left intact. Both resolutions preserved draft work, so avoiding a 'Reset branch' is appropriate here since on Retool-managed git that resets to a baseline snapshot rather than merging changes. As a first step, try reopening the app in a fresh editor thread/tab to confirm an older thread isn't pinned to a stale sandbox state and re-triggering the failure. Because these sandbox fixes have been app-specific, this scenario warrants a dedicated report so the specific sandbox can be inspected and re-provisioned.

Sources

:bookmark: My original Retool React App Builder app is frozen at “Starting preview server”
A solved new App Builder report with the same 'frozen at Starting preview server' sandbox symptom; resolved by a Retool-side fix for a sandbox websocket/context-window issue with unpublished draft work preserved.
:bookmark: Production React app authoring agent silently fails to start
A solved report where the production React app's authoring/generation sandbox silently failed to start while the published app kept working; resolved server-side when Retool stopped rolling out the breaking code change.

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