Skip to main content
A session’s sandbox VM does not stop the moment an agent finishes. Tembo holds it open for a short keep-alive window so you can read the diff, run commands in the terminal, or drive the Computer desktop. When the window expires, Tembo stops the VM. Keep-alive is what you extend when you want more time in a live sandbox — and what to watch when a sandbox disappears sooner than you expected.

​
Default windows

Each time work completes in a session, Tembo pushes the stop time out based on where the work came from: These automatic extensions only ever move the stop time later. They never shorten a longer window you set yourself.
Self-hosted deployments can change the dashboard and Slack windows with the SANDBOX_KEEP_ALIVE_APP_MINUTES and SANDBOX_KEEP_ALIVE_SLACK_MINUTES environment variables. See Self-hosted.

​
An open session tab holds the VM open

While a session is the active tab and your browser window is focused, the dashboard sends a keep-alive heartbeat every 4 minutes that pushes the stop time 10 minutes out. You do not need to do anything to keep a sandbox alive while you are actively looking at it. Switching tabs, backgrounding the window, or closing the session stops the heartbeat, so the VM stops roughly 10 minutes later.

​
Extend from the composer

To hold a sandbox open for longer than the default window:
  1. Type / in the session composer.
  2. Select Keep alive.
  3. Choose 10 minutes, 1 day, or 1 week.
A checkmark marks the window that is currently in effect. If the session has no running VM, the option reads No active VM and cannot be selected.
Choosing a duration sets the window from now, so it can shorten an existing one. Selecting 10 minutes while a 1 week window is active leaves 10 minutes, not a week and 10 minutes.

​
Extend from inside the sandbox

Run the tembo CLI from a session terminal, a hook, or a script:
The argument is a positive whole number of seconds. The command prints the new window:
The command fails if the session has no VM or if its VM is no longer running. Agents running in a session can do the same thing through Tembo’s own MCP server, which exposes a setVmKeepAlive tool taking the same seconds argument. That is what an agent reaches for when you ask it to keep the sandbox up while you take a look.

​
When the window expires

Tembo does not discard the sandbox outright. It suspends the VM, captures a session snapshot, then stops it. When the session needs a VM again — for example because you send another message — Tembo resumes from that snapshot instead of cloning and installing from scratch, falling back to the project environment and then to a cold boot if the snapshot cannot be restored. Tembo also skips stopping a VM that still has active agent work, even if the window has technically expired.

​
Constraints

  • Keep-alive applies to VM sandboxes and requires the VM to be running.
  • A VM that stays alive keeps consuming compute for the whole window, billed at the hourly rate for its sandbox size. A 1 week keep-alive on a large sandbox is expensive — prefer the shortest window that covers what you need.
  • Only one window is in effect at a time. Setting a new one replaces the previous value rather than adding to it.
  • The keep-alive window controls how long the VM stays up, not how long the session lasts. Sessions and their history remain available after the sandbox stops.