Running harness-fe on the LAN

The daemon defaults to 127.0.0.1 — only your own dev machine can reach it. This is the right default: console_tail, network_tail, session.recordings.* and the dashboard expose console, network requests, and full DOM recordings of whatever's running. None of that should be on a public socket.

For real-device debugging (phone browser, second machine, RN device) you can bind the daemon to a routable IP. Token auth becomes mandatory the moment you do.

Three modes at a glance

Mode	Bind	Auth	Use it for
Local-only (default)	127.0.0.1	none	Day-to-day on your own machine
LAN with token	0.0.0.0 (or a specific LAN IP)	token	Mobile / second device testing
Remote MCP	LAN + --mcp-transport http	token	Sharing one daemon across machines/agents

Local-only

npx @harness-fe/mcp-server

That's it. stdio MCP to whatever spawned the process, WS bridge on 127.0.0.1:47729. Nothing else can connect.

LAN with token

npx @harness-fe/mcp-server --host 0.0.0.0 --token auto

The daemon prints:

[harness-fe] leader: WS bridge listening on ws://0.0.0.0:47729
[harness-fe] WARNING: bound to non-loopback host 0.0.0.0.
[harness-fe]   anyone reaching this host:port with the token can read console / network / recordings.
[harness-fe] dashboard: http://192.168.1.20:47729?token=<random>
[harness-fe] token:     <random>

Open the dashboard URL on your phone. The token in the query string is swapped for a cookie on first paint, so subsequent navigation Just Works.

To wire a plugin or runtime on a second device, pass the same token via the plugin config:

harnessFE({ mcpUrl: 'ws://192.168.1.20:47729', token: '<random>' })

Or, for the runtime client (when the script tag injects __HARNESS_FE__.mcpUrl for you, the URL already carries the token — no extra config needed).

Remote MCP (HTTP transport)

Want an agent on another machine to share the same daemon? Add --mcp-transport http:

npx @harness-fe/mcp-server \
  --host 0.0.0.0 \
  --token auto \
  --mcp-transport http \
  --mcp-path /mcp

The banner adds:

[harness-fe] mcp http:  http://192.168.1.20:47729/mcp?token=<random>
[harness-fe]   agent config: { "url": "http://192.168.1.20:47729/mcp", "headers": { "Authorization": "Bearer <random>" } }

Drop that JSON into Claude Code / Cursor / Kiro as an HTTP MCP server.

Auth flow detail

A request is authorized if any of these match:

1. Authorization: Bearer <token> header — for agents, CLIs, server-to-server.
2. Cookie: harness_fe_token=<token> — set by the login page on first visit.
3. URL query ?token=<token> — works for both HTTP and WS upgrade.
4. WS subprotocol harness-fe.token.<token> — the only way browsers can pass a token through the WebSocket API.

Loopback binds (127.*, localhost, ::1) skip the check entirely.

What's behind the auth wall

Every HTTP route and every WS upgrade. Concretely:

• The dashboard (GET /, GET /sessions/:id, …)
• The replay viewer (GET /replay/:exportId)
• The events ingest endpoint (POST /events)
• The MCP HTTP transport (POST /mcp) — when enabled
• The WS bridge that plugin/runtime use to talk to the daemon

Translation: an attacker on the same WiFi cannot watch your console, read your network requests, scrub through DOM recordings, or pivot via the MCP tool surface without your token.

Picking a public host

If you bind 0.0.0.0 the daemon picks the first non-internal IPv4 interface for the URLs it prints. On a multi-homed machine that might not be the right one. Override with --public-host 192.168.x.y (or HARNESS_FE_PUBLIC_HOST=...).

Environment variables

Every flag has an env var equivalent for package.json scripts / docker / CI:

Env	Equivalent flag
HARNESS_FE_HOST	--host
HARNESS_FE_TOKEN	--token (use auto to generate)
HARNESS_FE_MCP_TRANSPORT	--mcp-transport
HARNESS_FE_MCP_PATH	--mcp-path
HARNESS_FE_URL	legacy: full ws://host:port (overridden by flags)

Safety reminder

LAN mode is not a substitute for putting the daemon behind a proper reverse proxy if you're sharing it with a team. Token auth here is defence-in-depth for casual same-WiFi probing — not the access control layer of a production service. Don't expose it to the public internet.