Approval Gate — an MCP App

A human approval gate for AI agents, built as an MCP App. Before an agent does something consequential — send an email, issue a refund, publish a post, delete data — it calls a tool that renders an interactive card inline in the chat. A human clicks Approve, Edit, or Reject, and only then does the agent proceed.

It runs in any MCP Apps host: Claude (web/desktop), ChatGPT, VS Code, Goose, and others.

Why this and not a dashboard? The highest-leverage use of MCP Apps in real workflows isn't a fancy chart — it's the confirmation gate that stops an agent acting on a misread "yes". The artifact changes per client; the pattern doesn't.

How it works

MCP Apps extend MCP with one idea: a tool can declare a UI resource. When the tool is called, the host fetches that resource and renders it in a sandboxed iframe, then relays messages between the UI and your server.

This project wires up that pattern for an approval gate using three tools and one UI resource:

                        ┌─────────────────────────────────────────────┐
   agent calls          │  request_approval  (declares the UI resource)│
   request_approval ───▶│      → creates a pending ApprovalRequest      │
                        │      → host renders ui://approval-gate card   │
                        └───────────────────────┬─────────────────────┘
                                                 │  card shows the artifact
                                                 ▼
                                   human clicks Approve / Edit / Reject
                                                 │
                        ┌───────────────────────▼─────────────────────┐
   the card calls       │  submit_approval_decision                    │
   (via the host) ─────▶│      → records the decision (once only)      │
                        │      → returns the outcome to the agent       │
                        └───────────────────────┬─────────────────────┘
                                                 │  "APPROVED" / "…WITH EDITS" / "REJECTED"
                                                 ▼
                          agent proceeds with the action — or doesn't

   list_approvals  →  read-only audit trail of every gate and its decision

The agent's contract is simple: call request_approval, then wait. It must not perform the real action until submit_approval_decision reports approved or edited. On edited, it uses the reviewer's corrected values; on rejected, it stops.

Related MCP server: vantagate-mcp-server

Project structure

mcp-approval-gate/
├── server.ts                # MCP server: registers the 3 tools + the UI resource
├── main.ts                  # Entry point: stdio (--stdio) or Streamable HTTP
├── mcp-app.html             # View shell (Vite bundles the React app into it)
├── src/
│   ├── types.ts             # Shared, type-only model (ApprovalRequest, …)
│   ├── schemas.ts           # Zod input schemas for the tools
│   ├── store.ts             # In-memory store + audit trail (swap for a DB in prod)
│   └── ui/
│       ├── main.tsx         # React entry
│       ├── useApprovalApp.ts# Hook around the ext-apps `App` class (connect/receive/submit/theme)
│       ├── ApprovalCard.tsx # The card: view / edit / reject / outcome states
│       └── styles.css       # Theme-adaptive styling (matches host light/dark)
├── vite.config.ts           # Bundles the View into a single self-contained HTML file
├── tsconfig.json            # Typecheck for the View
├── tsconfig.server.json     # Typecheck for the server
└── test-smoke.mjs           # End-to-end runtime test over stdio

Prerequisites

• Node.js 20.19+ or 22+

• An MCP Apps-capable host to see the UI (Claude Desktop, ChatGPT, VS Code, Goose, …), or the basic-host from the ext-apps repo for local testing.

Setup

npm install
npm run build      # typechecks, then bundles the View into dist/mcp-app.html

The server serves dist/mcp-app.html as the UI resource, so you must build before running.

Run

Streamable HTTP (default, on http://localhost:3001/mcp):

npm start

stdio (for Claude Desktop and other local hosts):

npm run start:stdio

During development, npm run dev rebuilds the View on change and restarts the server.

Use it in Claude Desktop

Build first (npm run build), then add this to your claude_desktop_config.json (use an absolute path):

{
  "mcpServers": {
    "approval-gate": {
      "command": "npx",
      "args": ["-y", "tsx", "/ABSOLUTE/PATH/TO/mcp-approval-gate/main.ts", "--stdio"]
    }
  }
}

Restart Claude Desktop, then try a prompt like:

Draft a reply to Jane approving her $40 refund, and ask me to approve it before sending.

The agent calls request_approval, the card appears inline, and your click decides what happens next.

Try it locally with basic-host

The ext-apps repo ships a reference host you can run against this server over HTTP:

# terminal 1 — this project
npm start

# terminal 2 — the reference host
git clone https://github.com/modelcontextprotocol/ext-apps.git
cd ext-apps && npm install && cd examples/basic-host && npm start
# open http://localhost:8080, point it at http://localhost:3001/mcp,
# call request_approval, and interact with the card

Test

test-smoke.mjs boots the server over stdio and drives the full flow — initialize, list tools, call request_approval, read the UI resource, submit an edited decision, verify the one-decision-only guard, and check the audit trail:

npm run build && node test-smoke.mjs

Adapting it

• Change the artifact, keep the gate. request_approval takes a generic fields: [{ label, value, multiline? }] list, so the same card reviews an email, a refund, a social post, or a config change. Only the calling agent's prompt changes.

• Make it durable. The store in src/store.ts is an in-memory Map — fine for a demo or single instance, lost on restart and not shared across replicas. Back it with Postgres or Redis for production; the function signatures (createRequest, recordDecision, listRequests) are the seam to replace.

• Add structured output. Each tool currently returns text. Define an outputSchema and return structuredContent if your host consumes structured tool results.

Tech

TypeScript · React 19 · Vite (single-file bundle) · @modelcontextprotocol/ext-apps · @modelcontextprotocol/sdk. Built against the MCP Apps spec version 2026-01-26.

License

MIT — see LICENSE.