boxel

README

boxel — Tunnel MCP

A generic-operation MCP server that tunnels the Claude Code tool-call protocol to a remote sandbox VM.

Instead of re-declaring every sandbox capability as a typed MCP tool, boxel advertises one generic invoke operation whose body is a Claude Code tool call. Any Claude surface — CLI, desktop, or the phone app via a remote MCP connector — becomes a thin controller for a sandbox VM you own. The MCP layer is a transport tunnel; the tool semantics live at the far end.

See docs/prd-tunnel-mcp.md for the full product design.

Claude (phone / desktop / CLI)
        │  MCP (stdio, or streamable HTTP + bearer, behind a TLS tunnel)
        ▼
┌──────────────────────────────┐
│  tunnel-mcp server           │
│  ├─ MCP transport layer      │
│  ├─ Envelope parser/validator│
│  ├─ Permission engine ───────┼──► MCP elicitation → user approval
│  ├─ Harness (tool impls)     │
│  └─ Session manager          │
└───────────┬──────────────────┘
            ▼
      Sandbox VM filesystem + processes (workspace jail)

Quick start: join a VM to the fleet

On a VM that should join an existing boxel hub (once the hub's peer integration is attached to the VM — ssh exe.dev tag SOME_VM boxel):

curl -fsSL http://boxel.int.exe.xyz/install-agent | sudo bash

This installs a single agent process: a systemd unit running tunnel-mcp --hub-connect, which dials out to the hub and serves this VM's MCP in-process over the reverse channel — no local port, no separate forwarder. The service autodiscovers the hub and retries until it connects, so only the script fetch itself needs the integration attached. To front an arbitrary local HTTP server instead (or to bootstrap before the integration exists), use forward mode — see docs/pull-mode.md:

GOBIN=/usr/local/bin go install github.com/mkmik/boxel/cmd/boxel-agent@latest
sudo /usr/local/bin/boxel-agent setup

To set up the hub itself, see the setup runbook under Pull mode.

Advertised MCP surface

Tool	Purpose
invoke	Generic op. Body: {"tool": string, "input": object, "session": string?}, interpreted as a Claude Code tool call.
describe	Supported tool names + input schemas, active permission mode + redacted policy, sandbox metadata (hostname, OS, workspace root), sessions, limits.
session	Create / list / reset logical sessions (cwd, env, background shells, permission overlay).

Tunneled tools (v1)

Read, Write, Edit, Glob, Grep, Bash, BashOutput, KillShell — implemented natively with byte-exact Claude Code semantics (identical output formats and failure-mode strings), so the model's recovery behavior transfers unchanged. Use the exact input schemas you use natively; call describe if unsure. Unknown tool names return {"error": "unknown_tool", "supported": [...]}.

Running the server

Build:

go build ./cmd/tunnel-mcp

Run locally over stdio (for claude mcp add or local testing):

./tunnel-mcp --workspace /home/agent/work --permissions examples/permissions.json

Run over streamable HTTP (for a remote/phone connector; front with a TLS tunnel):

BOXEL_TOKEN=$(openssl rand -hex 32) \
  ./tunnel-mcp --http 127.0.0.1:8080 \
    --workspace /home/agent/work \
    --permissions examples/permissions.json \
    --audit-log /var/log/tunnel-mcp/audit.jsonl \
    --metrics-addr 127.0.0.1:9090

The MCP endpoint is POST /mcp (requires Authorization: Bearer <token>); GET /healthz is unauthenticated. Prometheus metrics are served separately on --metrics-addr at /metrics.

Flags

Flag	Default	Meaning
--http	(empty → stdio)	Serve streamable HTTP on this address.
--workspace	current dir	Workspace jail root; file ops outside it are hard-denied.
--permissions	(none)	Path to permissions.json (Claude Code-compatible rules).
--permission-mode	default	default | acceptEdits | bypassPermissions.
--audit-log	(disabled)	Append-only JSONL audit log path.
--metrics-addr	(disabled)	Serve Prometheus /metrics on this address.
--token / --token-file	$BOXEL_TOKEN	Static bearer token for HTTP (testing; front with OAuth for production).
--owner-email	(none)	Pin to one owner via the exe.dev edge: require the X-ExeDev-Email header to equal this address. Composes with --token. See docs/deployment.md.
--session-ttl	24h	Idle-session GC TTL (0 disables).
--hub-agent-owner-email	(none)	Enable the pull-mode hub (see below) with exe.dev identity registration: tokenless, names bound to the platform-verified caller VM.
--hub-agent-token / --hub-agent-token-file	$BOXEL_HUB_AGENT_TOKEN	Enable the pull-mode hub with token registration (non-exe.dev deployments; composes with the above).
--hub-agent-listen	(disabled)	Extra listener serving only the agent registration endpoint.
--hub-advertise-url	(reflection discovery / fetch URL)	Base URL agents dial; embedded in the /install-agent script.

For HTTP, at least one of --token / --owner-email must be set — the server refuses to listen unauthenticated.

Pull mode: one hub, many non-routable VMs

The model

One routable boxel instance — the hub — multiplexes MCP for boxel instances on VMs that expose no inbound HTTP port (their forwarded port stays free for the workload you're actually developing). On each such VM an agent dials out to the hub and registers under the VM's short hostname over a reverse HTTP/2 channel. The default, installer-provisioned agent is a single process — tunnel-mcp --hub-connect — that serves its own MCP in-process over that channel; alternatively the standalone boxel-agent forwarder fronts an arbitrary local HTTP server (default http://127.0.0.1:8080). The hub proxies the whole /vm/<name>/ base path over that channel, so for VM foobar:

• MCP endpoint: https://<hub-vm>.exe.xyz/vm/foobar/mcp
• any other path under /vm/foobar/ also reaches foobar (e.g. /vm/foobar/healthz hits the local instance's health check)

One connector origin and one credential cover the whole fleet: /vm/… and /agents (the JSON registry) sit behind the hub's normal client auth (--token / --owner-email), the same as its own /mcp.

On exe.dev there is no VM-to-VM network; agents reach the hub through a peer integration — an exe.dev-managed proxy at http://boxel.int.exe.xyz/ that authenticates the calling VM to the hub's edge and stamps the unforgeable caller VM name in X-Exedev-Source-Vm. Registration is therefore tokenless: the hub accepts a registration when the edge-injected X-ExeDev-Email equals --hub-agent-owner-email, and the agent's handle is taken from the verified source-VM header. Agents autodiscover the hub by querying the default reflection integration for an attached http-proxy integration named boxel. (Non-exe.dev deployments use --hub-agent-token instead; both methods can be enabled at once.)

Setup runbook (exe.dev)

Placeholders: HUB_VM = the hub's VM name, YOU@EXAMPLE.COM = your exe.dev account email (an agent can read it on any VM from curl -s https://reflection.int.exe.xyz/email).

Step 1 — on the hub VM: run tunnel-mcp with the hub enabled. --http requires client auth; with the VM kept private, edge SSO + --owner-email is enough. Bind to 127.0.0.1 so the edge is the only path in, and make sure the edge forwards to that port (ssh exe.dev share port HUB_VM 8080).

go build -o /usr/local/bin/tunnel-mcp ./cmd/tunnel-mcp   # or: GOBIN=/usr/local/bin go install github.com/mkmik/boxel/cmd/tunnel-mcp@latest
tunnel-mcp --http 127.0.0.1:8080 --workspace /home/agent/work \
  --owner-email YOU@EXAMPLE.COM \
  --hub-agent-owner-email YOU@EXAMPLE.COM

(For a production systemd unit and hardening, see docs/deployment.md; add the --hub-agent-owner-email flag to its ExecStart.)

Step 2 — once, from any shell with your exe.dev SSH key (laptop or the exe.dev web UI at /integrations; VMs themselves normally can't run ssh exe.dev account commands): create the fleet's peer integration, attached by tag:

ssh exe.dev integrations add http-proxy --name boxel \
  --target https://HUB_VM.exe.xyz/ --peer --attach tag:boxel

Step 3 — per fleet VM: attach the integration by tagging the VM (same place as step 2), and install the agent on the VM (needs a Go toolchain ≥ 1.25 and systemd). Steps 2/3a and 3b can happen in either order — the installer succeeds even while the integration is missing, and the agent service retries discovery every backoff cycle until it appears.

From your shell (or the web UI):

ssh exe.dev tag SOME_VM boxel

Then on SOME_VM, once the integration is attached:

curl -fsSL http://boxel.int.exe.xyz/install-agent | sudo bash

The script installs a single-process agent: it builds tunnel-mcp, creates a boxel-agent system user with a jailed workspace (/var/lib/boxel-agent/work by default, override with BOXEL_WORKSPACE), and enables a sandboxed boxel-agent.service running tunnel-mcp --hub-connect — one process that dials the hub and serves this VM's MCP in-process, with no local port and no separate forwarder — plus a boxel-agent-update.timer that polls the Go module proxy every 5 minutes and automatically installs newer releases. The install succeeds even while the hub is unreachable: the service autodiscovers and retries until the integration appears (an unattended agent must not retry or roll back), and the script prints the exact integrations add / tag commands that remain for the account owner.

To front an arbitrary local HTTP server instead — or for a hub-independent bootstrap that works before the integration exists — use the forward-mode boxel-agent (see docs/pull-mode.md):

GOBIN=/usr/local/bin go install github.com/mkmik/boxel/cmd/boxel-agent@latest
sudo /usr/local/bin/boxel-agent setup

VM-description snippet — paste this into a VM's description so a coding agent (e.g. Shelley) performs the install autonomously:

Set up the boxel fleet agent: ensure a Go toolchain is installed, then run curl -fsSL http://boxel.int.exe.xyz/install-agent | sudo bash and follow the script's output. If the URL is unreachable, the hub's peer integration isn't attached to this VM yet: relay ssh exe.dev tag <this-vm> boxel to the user and retry the same command after it's attached — the fetch is the only step that needs it. If the script's output says the hub is not reachable yet, the install still succeeded: relay the printed commands to the user, do not retry or uninstall, and check journalctl -u boxel-agent -n 3 later until it logs "registered with hub".

Step 4 — verify:

# on the fleet VM: the agent should log "registered with hub"
systemctl status boxel-agent && journalctl -u boxel-agent -n 5
# from an authorized client (e.g. your browser via edge SSO):
#   https://HUB_VM.exe.xyz/agents            → lists the VM
#   https://HUB_VM.exe.xyz/vm/SOME_VM/healthz → "ok" from the VM's local boxel

Step 5 — connect Claude: point the MCP connector at https://HUB_VM.exe.xyz/vm/SOME_VM/mcp with the hub's credentials (see docs/deployment.md for the connector auth options). The VM's own public hostname stays free: ssh exe.dev share port SOME_VM <your-app-port> gives it entirely to the app you're developing.

Troubleshooting

Symptom	Cause / fix
agent logs hub autodiscovery: no http-proxy integration named "boxel"	VM not tagged (step 3) or integration missing (step 2). Discovery retries every backoff cycle, so fixing the tag is enough.
agent logs hub refused registration: 401	Hub not started with --hub-agent-owner-email, or its value doesn't match the account that owns the VMs.
agent registers, then the channel drops immediately after the 101	An intermediary isn't passing the Upgrade: boxel-h2c handshake through — report it.
/vm/<name>/… returns 502 vm_not_connected (JSON body)	Agent not running/registered on that VM — check step 4.
/vm/<name>/… returns 502 agent_forward_failed (JSON body)	Forward-mode (boxel-agent) only — a --hub-connect agent serves MCP in-process and has no forward target. The agent is connected but can't reach its local forward target: nothing is listening on --target (default 127.0.0.1:8080), or it's on a different port. Start the local server, or fix --target.
/vm/<name>/… returns 502 with an empty body	An older agent (before the diagnostic endpoint) failing to forward — same cause as agent_forward_failed. Upgrade boxel-agent to get the explanatory body.
unsure whether it's the channel or the local target	Hit GET /vm/<name>/__boxel-agent — the agent answers this path itself (never forwarded), returning its name/version/--target and a live target_check reachability probe. A 200 here with target_check.reachable: false means the channel is up and the local target is down.
proxied requests get 401 from the local boxel (forward mode)	The agent isn't injecting the local token: ensure /etc/boxel-agent/target-token exists (rerun boxel-agent setup after creating /etc/tunnel-mcp/token) or set BOXEL_AGENT_TARGET_TOKEN_FILE in /etc/boxel-agent/env and restart boxel-agent.

Full details (generic token-based deployments, security model, design notes): docs/pull-mode.md.

Permissions

Rules use Claude Code's settings.json format. Precedence is deny > ask > allow, then mode defaults. See examples/permissions.json.

• Bash(git status:*) — prefix form: commands starting git status.
• Bash(rm *) — glob form: * spans any characters including spaces.
• Edit(/home/agent/work/**) — doublestar glob over the resolved absolute path.
• Read(**) — any path (still subject to the jail + credential hard denies).

Modes: default asks on any unmatched mutating call; acceptEdits auto-approves Write/Edit inside the jail; bypassPermissions is audit-only and server-flag only, never client-selectable.

Ask path: an "ask" decision issues an MCP elicitation — allow once / allow always / deny. "Allow always" appends a rule to a session-scoped overlay, never the persistent file.

Hard denies (always, even in bypassPermissions):

• Paths outside the workspace jail.
• Credential files — ~/.ssh/**, ~/.aws/**, ~/.config/gcloud/**, ~/.gnupg/**, ~/.kube/**, ~/.docker/config.json, ~/.netrc, ~/.git-credentials, /etc/shadow, /etc/sudoers, and anything under a .ssh/ directory — unless an explicit (non-catch-all) allow rule matches.

Security model

The generic invoke op is, by construction, an authenticated RCE endpoint — treat the whole design as "authenticated RCE with policy," not a typed API. Authentication is the primary boundary; the permission engine is defense-in-depth and UX. Deploy accordingly:

• Front the HTTP transport with a TLS-terminating tunnel and OAuth. The built-in bearer token is a second factor and a local-testing convenience, not the production auth story.
• Run the server as a dedicated unprivileged user, with the workspace on its own path and OS-level isolation (systemd sandboxing / bubblewrap / landlock).
• Deny-by-default egress from the sandbox user (e.g. nftables per-UID) with a registry/GitHub allowlist, to bound exfiltration if a prompt-injected session goes rogue.
• Every mutation is recorded in the audit log with an input digest and the permission decision; file contents are never logged, and Bash command lines flagged sensitive are redacted.

Known limitations of the policy layer

The permission engine is defense in depth, not the perimeter — deploy the OS-level isolation in docs/deployment.md. Specifically:

• Bash is unrestricted RCE by design. A Bash command carries no filesystem path for the engine to jail-check, so cat /etc/shadow is reachable subject only to Bash rule/mode gating (in default mode it prompts). The jail and credential hard-denies apply to the file tools (Read/Write/Edit/Glob/Grep), not to what a shell command can do. Egress deny + OS sandboxing are what actually contain a shell.
• Symlink resolution is best-effort. File-tool paths are resolved (symlinks followed) before the jail/credential check, so a symlinked parent inside the workspace can't disguise an outside target. This is not TOCTOU-proof against a path swapped between check and use; a landlock/bind-mount jail is.

See docs/deployment.md for a systemd unit and Cloudflare Tunnel walkthrough.

Development

go build ./...
go test ./...       # unit tests + end-to-end tunnel tests (in-memory MCP client)
go vet ./...

Package layout:

Package	Responsibility
internal/envelope	invoke envelope + typed Claude Code tool input schemas.
internal/policy	Permission engine: rule parsing, precedence, modes, jail + credential hard denies, session overlays.
internal/harness	Native tool implementations (Read/Write/Edit/Glob/Grep/Bash/BashOutput/KillShell).
internal/shellmgr	Bash execution: foreground with cwd persistence, background shell table.
internal/session	Session manager (cwd, env, shells, overlay) with TTL GC.
internal/audit	Append-only JSONL audit log.
internal/metrics	Prometheus instrumentation.
internal/tunnel	MCP server wiring: envelope → policy → elicitation → harness → audit/metrics.
internal/hub	Pull-mode multiplexer: agent registry, reverse HTTP/2 registration, /vm/<name>/ proxy, installer script.
internal/hubagent	Pull-mode agent runtime: dial-out, reconnect, forwarding to the local instance.
cmd/tunnel-mcp	Binary: stdio + streamable HTTP transports, bearer auth, hub mode, flags.
cmd/boxel-agent	Pull-mode agent binary for non-routable VMs.