Coding agent sandbox (mounted repo)

Run an agent (or any tool) inside an isolated Linux microVM, with your repo mounted read-write, then copy results back to the host.

Prerequisites

bash

grain up
grain image pull          # once
grain doctor              # QEMU + image OK
# Optional but recommended for grain x / grain cp without SSH:
just agent-linux          # from a source checkout

1. Create a sandbox with the repo mounted

From your project root:

bash

grain new \
  -n agent \
  -c 4 -m 4096 -d 20 \
  -v "$(pwd):/work" \
  --wait agent

Flag	Why
`-n agent`	stable name for scripts
`-c` / `-m`	room for builds and LLMs-in-guest
`-v …:/work`	virtio-9p share of the host checkout
`--wait agent`	ready when guest agent is healthy (falls back to SSH deploy if needed)

Persistent disk (keep state across stop/start):

bash

grain new -n agent -p -v "$(pwd):/work" --wait agent

2. Check the guest

bash

grain agent health agent
grain x agent -- uname -a
grain x agent -- ls -la /work

3. Run the agent (or your toolchain)

Examples:

bash

# Install toolchain once (ephemeral VM: re-do after rm)
grain x agent -- sudo apt-get update
grain x agent -- bash -lc 'cd /work && make test'

# Or invoke whatever agent binary/script you use:
grain x agent -- bash -lc 'cd /work && ./scripts/run-agent.sh'

Prefer a profile for repeated agent work (~/.grain/config.yaml):

yaml

profiles:
  coding-agent:
    cpus: 4
    memory_mb: 4096
    disk_gb: 20
    mounts:
      - {host: ".", guest: "/work"}

bash

grain new --profile coding-agent -n agent --wait agent

4. Copy results out

Agent-preferred copy (NAME:path):

bash

# guest → host
grain cp agent:/work/out/report.json ./report.json
grain cp agent:/tmp/agent-log.txt ./agent-log.txt

# host → guest
grain cp ./prompts/system.md agent:/work/prompts/system.md

Force agent or SSH:

bash

grain cp --agent agent:/work/out ./out
grain cp --ssh   agent:/work/out ./out

List guest paths without SSH:

bash

grain fs ls agent /work
grain fs stat agent /work/out

5. Tear down

bash

grain rm agent          # ephemeral: gone
# or keep disk:
grain stop agent
grain start agent       # later

6. Optional: allowlisted egress proxy

Keep API tokens on the host and deny other outbound HTTP(S) via the proxy:

bash

grain proxy up
grain proxy client create agent
grain secret set model-key --value '…'
grain proxy allow --host api.example.com --secret model-key

grain new --proxy -n agent -v "$(pwd):/work" --wait agent

Guests use HTTPS_PROXY=http://TOKEN@10.0.2.2:3128 (SLIRP gateway). See proxy.md for default-deny rules, listen address, and security notes.

Tips

9p mounts are great for source; heavy I/O (node_modules, caches) is happier on the guest disk under /home/ubuntu or /var/tmp.
First boot with package installs can take a few minutes — grain logs -f agent.
Golden image with agent baked in skips SSH deploy: see images.md and scripts/bake-golden.sh.
For CI-style one-shot create/exec/rm, see ci-ephemeral.md.
For host-side default-deny HTTP(S) egress, see proxy.md.