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This is an implementation guide, not a report of a tested personal setup. The available product documentation describes mechanisms and performance considerations, but does not establish a measured speedup, slowdown, or “no slowdown” result for a particular machine.
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What a shell sandbox controls—and what approvals control
A shell sandbox limits what a command and its child processes can reach. Depending on the implementation, that can include filesystem paths, network destinations, and other operating-system resources. Approval prompts answer a different question: whether an action runs automatically or waits for a person to confirm it. Visual Studio Code’s Agent Host documentation makes this distinction explicit: sandbox restrictions constrain terminal commands and child processes, while approvals govern whether actions run automatically.
A practical target is to let routine work run inside a bounded policy, while retaining a deliberate escalation path for sensitive or unsupported operations. Set the boundary first; then decide which actions can run without a prompt. Otherwise, removing prompts may simply give an agent broad access more conveniently.
#1 Best Overall
Choose the boundary that fits the workflow
A Linux process sandbox and a microVM are different isolation approaches, not interchangeable settings. A process sandbox applies operating-system restrictions to commands in the host environment. A microVM provides a separate virtualized environment and can add a workspace boundary of its own. In either case, inspect the actual policy: “sandboxed” does not tell you by itself whether the agent can write your project, reach the network, or read local secrets.
| Choice | What it means | Workflow trade-off |
|---|---|---|
| Linux process sandbox | Restricts a process through mechanisms such as filesystem rules and network filtering; the exact policy depends on the tool and host. | Can make routine commands operate against configured paths directly. Requires supported platform prerequisites and verification of the active rules. |
| MicroVM workspace | Runs the agent in a virtual machine with additional isolation layers. Docker documents a per-sandbox Docker Engine so the agent has no path to the host Docker daemon. | Workspace configuration determines whether edits are shared immediately or need to be fetched from a private clone. |
For a Linux Codex example, start with explicit filesystem roots
The Codex Linux sandbox README describes bubblewrap as its default filesystem sandbox. Its documented approach makes the root filesystem read-only, then layers configured writable roots on top. Protected subpaths inside those writable roots can be made read-only again, and more-specific filesystem rules determine how nested allow and deny carveouts behave. The helper also applies PR_SET_NO_NEW_PRIVS and a seccomp network filter. See the Codex Linux sandbox README for implementation details.
Think in terms of explicit roots and exceptions rather than assuming the agent can “only see the repo.” A writable project path can contain files and settings that are readable or mutable under that path, including scripts, hooks, or credentials. The exact reach depends on the configured mounts and policy; a project directory is not automatically a clean security boundary.
Check Linux prerequisites
The same README says filesystem-restricted execution requires bubblewrap because the legacy Landlock option cannot isolate app-server Unix sockets for those policies. It documents WSL2 using the normal Linux bubblewrap path and WSL1 as unsupported for this route because WSL1 cannot create the required user namespaces. These are implementation details from the current Linux sandbox README, not guarantees for every Codex version or operating system.
Pick how the agent writes to the workspace
Docker documents three workspace patterns for its Sandboxes: no host workspace, a direct mount, and clone mode. The choice determines whether edits are immediately shared or kept in a private working copy.
| Workspace mode | Agent access and write-back | Best understood as |
|---|---|---|
| Mountless | No host workspace is provided. | A way to run without mounting a project workspace. |
| Direct mount | The host working tree is mounted read-write. Changes made by the agent and host are immediately visible to each other. | Fast, direct collaboration with a larger host-write boundary. |
| Clone mode | The host Git repository is mounted read-only; the agent works in a private clone. Changes can be fetched and reviewed before integration. | A separate writable working copy with an explicit import/review step. |
Clone mode is a write boundary, not a confidentiality boundary. Docker says the Git root—including untracked and ignored files—is mounted read-only and readable in the VM. A local .env file inside that root may therefore be readable. Keep secrets outside the workspace or use the product’s separate credential-isolation features. See Docker’s isolation layers documentation.
Review files that can execute later
With a direct mount, an agent can modify more than application source. Docker warns that workspace changes may affect build files, Git hooks, CI configuration, IDE settings, and AI project configuration. Some changes may run later when a developer commits, builds, pushes, installs, or opens the project. Review modified workspace files as you would changes from an untrusted contributor; a sandbox does not make later execution of those files safe.
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Filesystem restrictions do not necessarily block outbound connections. Network policy is a separate control and should be checked independently: determine whether egress is blocked, limited to approved destinations, or broadly available. Consider what the agent could send to permitted destinations, including workspace content or credentials.
Defaults differ by product. In the documented Visual Studio Code Agent Host implementation, outbound network access is allowed by default, with destination allow and deny settings available. Microsoft warns that allowing destinations can permit actions and expose credentials or workspace content. Do not assume that this VS Code default—or any other tool’s default—applies to your agent. See Microsoft’s Agent Host sandbox documentation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep performance claims tied to the actual workload
Sandboxing does not have one universal performance cost. Workspace location and filesystem access patterns matter. Docker documents that access between its sandbox VM and workspace uses filesystem passthrough and warns that remote or network-attached workspaces add latency because each file read and write crosses the network.
For direct mounts, Docker says virtiofs caching is enabled by default and reduces host-side read round-trips for read-heavy operations such as git status and directory scans. That describes a mechanism, not a published benchmark or a guarantee that every build or shell task will be as fast as an unsandboxed run. The documentation reviewed does not quantify an overall speedup or slowdown.
If you want to claim a performance result for your own setup, compare against a clearly defined baseline and report the machine, operating system and kernel, sandbox configuration, workspace location, workload, number of repetitions, and results. Without that information, describe the configuration and its expected sources of overhead rather than promising “no slowdown.” Docker’s architecture documentation covers filesystem passthrough and caching.
Best Value
Verify the effective policy, then review the changes
Configuration is not proof that a restriction took effect. Check the policy through the interface supplied by the agent you actually use, especially after changing settings or moving to a different host. For Visual Studio Code Agent Host sessions, /sandbox policy displays the effective execution host, implementation, filesystem restrictions, and network policy. That command is specific to this product, not a general command for coding agents.
For that implementation, filesystem path rules include read/write, read-only, and denied access; denied rules take precedence over read-only, which takes precedence over read/write. Use the displayed effective policy to confirm what is active rather than relying only on a settings file or toggle.
Quick Recap
- Confirm which project and other paths the agent can read and write.
- Check whether outbound network access is blocked, allowlisted, or otherwise available.
- Verify that the host meets the sandbox’s platform prerequisites.
- Review the resulting diff, paying particular attention to hooks, build and CI files, IDE settings, and agent configuration.
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