Templates

Every sandbox is customizable — agents install packages, pull images, and configure tools as they work, and those changes persist for the sandbox's lifetime. Templates capture a configured environment into a reusable image so you don't have to set it up again every time.

Custom templates

Custom templates are reusable sandbox images that extend one of the built-in agent environments with additional tools and configuration baked in. Instead of asking the agent to install packages every time, build a template once and reuse it across sandboxes and team members.

Templates make sense when multiple people need the same environment, when setup involves steps that are tedious to repeat, or when you need pinned versions of specific tools. For one-off work, the default image is fine — ask the agent to install what's needed.

Note

Custom templates customize an existing agent's environment — they don't create new agent runtimes. The agent that launches inside the sandbox is determined by the base image variant you extend and the agent you specify in the sbx run command, not by binaries installed in the template. To define a new agent from scratch, see Kits.

Base images

All sandbox templates are published as docker/sandbox-templates:<variant>. They are based on Ubuntu and run as a non-root agent user with sudo access. Most variants include Git, Docker CLI, and common development tools like Node.js, Python, Go, and Java.

Each variant also has a -docker version (for example, claude-code-docker) that includes a full Docker Engine running inside the sandbox — no local Docker daemon required. When you pick a built-in agent without specifying a custom template, sbx run and sbx create use the -docker template variants by default.

The agent containers created from the -docker templates run in privileged mode inside the microVM (not on your host), with a dedicated block volume at /var/lib/docker, and dockerd starts automatically inside the sandbox. The block volume defaults to 50 GB and uses a sparse file, so it only consumes disk space as Docker writes to it.

To override the volume size, set the DOCKER_SANDBOXES_DOCKER_SIZE environment variable to a size string before starting the sandbox:

$ DOCKER_SANDBOXES_DOCKER_SIZE=10g sbx run claude

Use the non-Docker variant if you don't need to build or run containers inside the sandbox and want a lighter, non-privileged environment. Specify it explicitly with --template:

$ sbx run claude --template docker.io/docker/sandbox-templates:claude-code

Build a custom template

Building a custom template requires Docker Desktop.

Write a Dockerfile that extends one of the base images. Pick the variant that matches the agent you plan to run. For example, extend claude-code to customize a Claude Code environment, or codex to customize an OpenAI Codex environment.

The following example creates a Claude Code template with Rust and protocol buffer tools pre-installed:

FROM docker/sandbox-templates:claude-code
USER root
RUN apt-get update && apt-get install -y protobuf-compiler
USER agent
RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y

Use root for system-level package installations (apt-get), and switch back to agent before installing user-level tools. Tools that install into the home directory, such as rustup, nvm, or pyenv, must run as agent — otherwise they install under /root/ and aren't available in the sandbox.

Build the image and push it to an OCI registry, such as Docker Hub:

$ docker build -t my-org/my-template:v1 --push .

Note

The Docker daemon used by Docker Sandboxes pulls templates from a registry directly; it doesn't share the image store of your local Docker daemon on the host.

Unless you use the permissive allow-all network policy, you may also need to allow-list any domains that your custom tools depend on:

$ sbx policy allow network "*.example.com:443,example.com:443"

Then run a sandbox with your template. The agent you specify must match the base image variant your template extends:

$ sbx run --template docker.io/my-org/my-template:v1 claude

Because this template extends the claude-code base image, you run it with claude. If you extend codex, use codex; if you extend shell, use shell (which drops you into a Bash shell with no agent).

Note

Unlike Docker commands, sbx does not automatically resolve the Docker Hub domain (docker.io) in image references.

Template caching

Template images are cached locally. The first use pulls from the registry; subsequent sandboxes reuse the cache. Cached images persist across sandbox creation and deletion, and are cleared when you run sbx reset.

Saving a sandbox as a template

Instead of writing a Dockerfile, you can save a running sandbox's state as a template. This captures installed packages, configuration changes, and files into a reusable image — useful when you've set up an environment interactively and want to preserve it.

Save and reuse

Stop the sandbox (or let the CLI prompt you), then save it with a name and tag:

$ sbx template save my-sandbox my-template:v1

The image is stored in the sandbox runtime's local image store. Create a new sandbox from it with the -t flag:

$ sbx run -t my-template:v1 claude

List and remove templates

List all saved templates:

$ sbx template ls

Remove a template you no longer need:

$ sbx template rm my-template:v1

Export and import

To share a saved template or move it to another machine, export it as a tar file:

$ sbx template save my-sandbox my-template:v1 --output my-template.tar

On the other machine, load the tar file and use it:

$ sbx template load my-template.tar
$ sbx run -t my-template:v1 claude

Limitations

Agent configuration files are always recreated when a sandbox is created. Changes to user-level agent configuration files, such as /home/agent/.claude/settings.json and /home/agent/.claude.json, do not persist in saved templates.

If the saved template was built for a different agent than the one you specify in sbx run, you get a warning. For example, saving a Claude sandbox and running it with codex produces:

⚠ WARNING: template "my-template:v1" was built for the "claude" agent but you are using "codex".
  The sandbox may not work correctly. Consider using: sbx run -t my-template:v1 claude