Skip to main content

Gemini CLI Sandbox

Cosmonic Desktop is a local AI sandbox for Gemini CLI: the code Gemini CLI writes is compiled to WebAssembly and runs on your own machine with no access to your files, network, or keys until you grant it. Two settings wire it up, one for launching Gemini CLI from Builder with the sandbox playbook and one for installing Desktop's MCP server into Gemini CLI; use either or both.

Sandbox Gemini CLI with Cosmonic Desktop​

1. Settings → Agents: launch Gemini CLI from Builder​

Open Settings → Agents. The Detected agents card lists every coding agent Desktop found by its home directory (~/.gemini) or on your login shell's PATH, and says whether each one is launchable from Builder. In the Skills card, turn on Gemini CLI to install the cosmonic-sandbox skill at ~/.gemini/skills/cosmonic-sandbox/. This is the same install the one-time Set up your coding agents dialog offers on first run; nothing is installed silently.

The skill is the sandbox playbook: it teaches Gemini CLI to compile what it generates to a WebAssembly component and deploy it into the local sandbox instead of running it raw on your host. Once it is installed, Builder can launch Gemini CLI directly: open Builder, pick Gemini CLI as the agent, describe what you want, and its output lands in a sandboxed workload you can inspect before it runs.

Gemini CLI and Google Antigravity share the ~/.gemini/skills directory, so installing the skill for one covers both.

2. Settings → MCP Server: install Desktop's MCP server into Gemini CLI​

Open Settings → MCP Server. Under Install in your AI client, turn on Gemini CLI: Desktop runs gemini mcp add for you. This registers cosmonicd mcp serve as an MCP server in Gemini CLI's own configuration, and the cosmonic_* tools (observe, deploy, build) are available the next time you start it. Expand Instructions on the same card to copy the manual snippet instead.

The manual equivalent (using the daemon binary's path on your platform—macOS shown):

gemini mcp add cosmonic "/Applications/Cosmonic Desktop.app/Contents/Resources/cosmonicd" mcp serve

3. Verify the connection​

gemini mcp list   # the cosmonic server should be listed and enabled

Run gemini mcp list. If the server shows as Disabled (folder untrusted), trust the workspace (run gemini in the folder and accept the prompt), or register it machine-wide in ~/.gemini/settings.json instead.

Skills over MCP​

Desktop's MCP server also publishes its skill family—cosmonic-sandbox, cosmonic-go, cosmonic-nats, cosmonic-nats-tuning, and cosmonic-kafka—through the MCP Skills extension (io.modelcontextprotocol/skills). A client that supports Skills over MCP gets the playbooks the moment it connects, with nothing written to its skills directory, so the MCP registration above is enough on its own for those clients. The on-disk skill from Settings → Agents covers clients that read a skills directory but do not yet speak the extension, and it is what Builder relies on to launch Gemini CLI.

What Gemini CLI can do once connected​

Its full tool surface (observe, deploy, build) and the guardrails are covered in Connect Coding Agents; the tools work only while Cosmonic Desktop is running.

Frequently asked questions​

How do I sandbox Gemini CLI?

Install Cosmonic Desktop, then under Settings → Agents turn on Gemini CLI to install the cosmonic-sandbox skill, and under Settings → MCP Server register Desktop’s MCP server in Gemini CLI. From then on the code Gemini CLI writes is compiled to WebAssembly and runs in a local sandbox with no access to your files, network, or keys until you grant it.

Does Gemini CLI itself run inside the sandbox?

No. Gemini CLI keeps running in your terminal or IDE exactly as before. What changes is where its output executes: the programs and MCP servers it builds run as sandboxed WebAssembly workloads on your machine instead of directly on your host.

Do I need both the skill and the MCP server?

They do different jobs. The MCP server gives Gemini CLI the tools to build, deploy, and observe sandboxed workloads; the skill gives it the know-how to use them by default. Desktop also publishes its skills over the MCP Skills extension, so a client that supports Skills over MCP gets the playbook from the connection alone.

What can the code Gemini CLI writes reach inside the sandbox?

Nothing by default. Each workload starts with no filesystem, network, or credential access; you grant exactly the capabilities it needs in its manifest, and everything else stays denied. The same manifest deploys unchanged to Cosmonic Control on Kubernetes.

Why does Gemini CLI show the Cosmonic server as disabled?

Gemini CLI disables MCP servers in folders it has not been told to trust. Run `gemini` in the project folder and accept the trust prompt, or register the server machine-wide in `~/.gemini/settings.json`.

Next steps​

Last updated on by Liam Randall