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and in another it was a context-window limit breaking the app's websocket connection to its sandbox, fixed per-app with unpublished work left intact.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
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preview server' sandbox symptom; resolved by a Retool-side fix for a sandbox websocket/context-window issue with unpublished draft work preserved.
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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