Box Mount now runs in Vercel Sandbox
By Steven Van ·
Box Mount is in private preview, with access requested from Box, and syncs a Box folder to a sandbox directory so agent output survives sandbox deletion.
Agents running in Vercel Sandbox can now read and write Box files through standard filesystem operations, using Vercel's Box Mount integration. Box Mount syncs a Box folder with a directory inside the sandbox, and it is in private preview.
How the sync works
The agent reads and writes files at the mounted directory, and Box Mount syncs those changes to Box while the sandbox runs. Unmount before stopping the sandbox. Everything the agent wrote is then in Box after the sandbox is deleted.
The sync process and the agent run as separate sandbox users. This keeps the Box credentials apart from the agent's workspace: Box Mount holds the credentials, and the agent can access only the shared folder.
Contract-review example
Vercel provides an example that uses AI Gateway to review a synthetic contract inside a sandbox. It writes Reviewed/Acme-MSA-review.md to the Box folder and checks that the file is still there after the sandbox is deleted.
- Setup needs a Box developer token with file read and write access, an empty Box folder and its ID, the Box Mount 0.5.0 Linux x86_64 archive, an AI Gateway API key, and a Vercel project with Sandbox access. Node.js 20 or later is required.
- The example is cloned from the vercel-labs/vercel-box-mount repository and configured through a .env file.
- Its commands are npm run doctor, seed, demo, status and teardown. Doctor checks the setup and verifies that the agent user cannot read the Box credentials.
- Teardown unmounts the folder, verifies the review in Box and removes the sandbox. It should be run before the example's 30-minute session ends.
Limits and availability
If sync or unmount fails, follow the recovery steps before stopping the sandbox. Stopping it first can discard files that have not reached Box.
Box developer tokens expire after 60 minutes and cannot be refreshed. The preview also has workload and file-save limits, which matter for tools that replace files through an atomic rename.
Access is by request from Box, which supplies the Linux binary. After that, the Box Mount documentation covers setup in Vercel Sandbox.