AgentUI

Interactive UIs for coding agents: an MCP server that lets any agent ask its human with sliders, forms, diffs and live previews instead of guessing.

When an agent is about to invent a number, describe a colour in prose, or write "let me know if you'd prefer…", it can instead open a real UI in your browser. You adjust it while the agent keeps working, and it collects the answer when it is ready.

Install

npx agentui install

The installer detects the agents on your machine — Claude Code, Claude Desktop, Cursor, Windsurf, VS Code (Copilot), Gemini CLI, Codex CLI — and registers the server in each one's own config format.

Related MCP server: Kontrol

The tools

Full usage guidance for agents lives in skills/agentui/SKILL.md.

How it works

One Node process launched by the agent over stdio, acting as both an MCP server and a local HTTP + WebSocket server bound to 127.0.0.1. The agent authors a zod-validated JSON interface spec, optionally alongside a single-file ESM preview module rendered in a sandboxed iframe on a second origin. All durable state is plain JSON/NDJSON under .agent-ui/, so the agent can also just read it off disk.

Agent ──stdio/MCP──> [agentui process] ──HTTP/WS──> Browser (React shell)
                            │                            │
                            └── .agent-ui/sessions/<id>/  └── sandboxed iframe (preview.mjs)

Opening a widget on your phone

Off by default, because binding to 127.0.0.1 is not a detail — it is the whole security model. There is no login anywhere in this server; loopback is what made one unnecessary.

Set AGENT_UI_HOST=lan in the env of your agent's MCP registration and restart it. On startup the server prints an address and a key:

Open on another device on this network:
  http://192.168.1.112:4608/?k=vckqvceqbzz7

Open that once on the phone; a cookie carries the key from then on, and the inbox lists whatever is waiting, so no one types a session id. If the address it picks is wrong — a VPN tunnel often outranks your real card — pin the right one with AGENT_UI_HOST=192.168.1.112.

What you are trading. Traffic is plain HTTP, because a self-signed certificate on a LAN address produces a full-page warning on exactly the phone this exists to serve. Anyone on that network holding the key can read what the agent attached — diffs, file contents, screenshots — and answer for you: their decision reaches your agent as though it were yours. That is a fair trade on a home network and a bad one on cafe or hotel wifi. Your phone also has to be on the same network; this does nothing over cellular.

Development

pnpm install
pnpm build      # shared -> ui -> server (vendors three.js, copies the shell into server/web)
pnpm test       # the server suite runs against the real built binary over stdio
pnpm typecheck

pnpm build before running anything: the server serves the built shell, and the installer registers the built dist/mcp.js by absolute path.

Examples in packages/server/examples/ are dogfood — each asks something real about this project, so building the tool and using it are the same activity.

The design decisions and their evidence live in the header comments of the files they affect, particularly packages/server/src/core/{waiters,session,library}.ts, packages/server/src/web/preview.ts and packages/ui/public/agentui/preview.js.

License

MIT