⚒ anvil

A throwaway Docker sandbox for agents.

Run code or shell commands in a resource-limited, network-isolated, single-use container and get back a structured result — stdout, stderr, exit_code, timed_out, duration_ms. So an agent can verify its work, reproduce a bug, or check output without ever touching the host.

Part of tools-for-agents. Zero npm dependencies — drives the docker CLI directly.

Safe by default

Every run is launched with:

• --network none — no network unless you opt in (network: "on")

• --memory 512m --cpus 1 --pids-limit 512 — bounded resources

• --cap-drop ALL --security-opt no-new-privileges — minimal privileges

• --rm ephemeral container, work dir is a fresh temp mount, removed after

• a hard timeout (default 30s, max 300s) that docker kills the container

• optional secure: true → read-only rootfs + tmpfs /tmp

Related MCP server: agent-jail-mcp

CLI

node src/cli.js check                          # docker version + presets
node src/cli.js run python 'print(2**10)'      # → 1024
node src/cli.js run node 'console.log(6*7)'    # → 42
echo 'print("hi")' | node src/cli.js run python -
node src/cli.js sh 'apk info 2>/dev/null | head'

Run history (anvil serve)

anvil is stateless by default. Set ANVIL_DB to record every run, then browse them in a dashboard:

export ANVIL_DB=./.anvil/runs.db          # opt in to the run log
node src/cli.js run python 'print(2**10)'  # …runs are now recorded
node src/cli.js serve                       # → http://localhost:7930  (--port to change)

A zero-dependency forge log of what the sandbox executed:

• Run list — every execution with its language, status (ok / failed / timed out), exit code and duration, colour-coded at a glance.

• Run detail — the exact code or command, full stdout and stderr, and the resource limits it ran under (network, memory, cpus, timeout). Each block has a one-click ⧉ copy so you can lift the code or its output straight to the clipboard.

• Compare two runs — hit ⇄ compare, pick any two runs, and see a side-by-side line diff of their code, stdout and stderr, plus every limit that changed (took 258ms → 559ms, mem 512m → 256m).

• New run — hit ＋ new run to type a snippet (bash / node / python), execute it in a fresh sandbox right from the dashboard, and watch the result open in the detail. Runs go through the same guarded, network-off container as the CLI (⌘/Ctrl+Enter to run). Pick a resource profile in one click — 🔒 strict (network off · 256m · 0.5 cpu · 15s), ⚙ default (512m · 1 cpu · 30s), or 🌐 networked (network on · 60s) — and the sandbox's memory, CPU, timeout and network are set for you (recorded with the run).

• Run code against something — run() has always taken { path: content } files and written them into /work beside your snippet (agents use it), and the form never offered it. Now ＋ file adds fixture files — a module and its test, a parser and a sample payload — and ＋ stdin pipes input to the process. A snippet on its own can only prove syntax; a snippet with a fixture can prove behaviour. Paths that try to climb out of the sandbox are refused before a container starts.

• Run a real repo, from the browser — the new-run form used to do snippets only, so the dashboard couldn't do the thing anvil is for. Switch it to command in an image, point mount at a directory, and it runs any command in any image with your repo mounted read-only at /repo — cd /repo && npm test — without the repo ever leaving your disk or being writable by the container (touch /repo/x → Read-only file system). There's a read-only rootfs switch too. A mount that doesn't exist is refused in the form, before docker gets a chance to fail at you.

• Watch it run — the sandbox writes as it goes, so anvil now says so as it goes: hit ▶ run and the container's stdout/stderr stream into a live console (with an elapsed timer) instead of leaving you staring at a spinner until it returns. A 30-second run no longer looks identical to a hung one. Streaming is opt-in on the API (POST /api/exec?stream=1 → an event stream ending with the logged run), and run({ onData }) gives the same live chunks to any caller — the CLI, an agent, anything.

• Re-run — hit ↻ re-run on any run to re-execute its exact code and limits in a fresh sandbox; the new run is logged and opened, so you can compare it against the original.

• Keep a run in cortex — a run is evidence: this code, in this sandbox, produced this output. Hit 🧠 → cortex and it becomes a note in your second brain — status, exit code, duration and the resource limits it ran under, the code as a fenced block, and its stdout/stderr — carrying a #run=<id> link straight back to the run in the forge log. anvil never writes: your browser POSTs to cortex's own /api/capture (point it elsewhere with ANVIL_CORTEX_URL).

• Link to a run — opening a run puts it in the URL (…/#run=<id>), so any run in the log can be linked, bookmarked or handed to someone else.

• Prune the log — the forge log grows with every run, so ✕ delete drops a single noisy run and 🗑 clear empties the whole log. Both arm on the first click and only fire on the second (clicking away, or waiting, cancels), and both live behind DELETE on the API — a stray GET can never prune anything.

• Search — a live search box filters the log by code, command, language or captured output as you type (matches highlighted, with a running 3 / 7 count); Esc clears it. Composes with the status filters, so you can find "that curl run that failed" in a long history.

• Filters + stats — narrow to failures, see ok/failed counts, average duration, and a by-language breakdown of everything the sandbox has run (python 3 · node 1 · bash 1).

• Keyboard navigation — press j/k (or ↑/↓) to move a cursor through the run list and open each run as you go — browse the whole forge log from the keyboard without touching the mouse (it stands down while you're typing in the search box or in compare mode).

• Keyboard-accessible — every control has a visible focus ring, and the run rows open with Tab + Enter (not just the mouse), with aria-labels throughout.

Logging is opt-in, fire-and-forget and fully guarded — it never slows or breaks a run. Try the demo without Docker: ANVIL_DB=./.anvil/runs.db node scripts/seed.js then anvil serve.

MCP server (for agents)

{
  "mcpServers": {
    "anvil": { "command": "node", "args": ["/abs/path/to/anvil/mcp/mcp-server.js"] }
  }
}

Tools

Example: verify a repo's syntax without copying it

{ "name": "anvil_run_command",
  "arguments": { "image": "node:22-alpine", "mount": "/path/to/repo/src",
                 "cmd": "node --check /repo/core.js && echo OK" } }

Result shape

{ "ok": true, "exit_code": 0, "timed_out": false, "duration_ms": 178,
  "image": "python:3.12-alpine", "stdout": "...", "stderr": "" }

The agent toolkit

anvil is the run safely leg of tools-for-agents — an operating system for agents. Seven zero-dependency, MCP-native tools that form one loop:

Reading this as an agent? /llms.txt is the map, and /tools.json hands you all 70 MCP tools — every name, every description, every install command — in one fetch, without cloning anything.

MIT licensed.