name: mcp-app-builder description: Build new MCP Apps (MCP servers with React UI output) using @modelcontextprotocol/ext-apps and the MCP SDK. Use when asked to scaffold or implement MCP App servers, add UI-rendering tools/resources, or migrate a standard MCP server to an MCP App with Vite single-file UI bundles.

MCP App Builder

Overview

Create MCP Apps that expose tools with visual React UIs for ChatGPT or Claude. Follow the exact dependency versions and server/UI patterns in references/mcp-app-spec.md.

Workflow

1. Clarify requirements: what data to visualize, UI pattern (card, table, chart, dashboard, form), data source, and how many tools (start with 1-2).
2. Design tools and UI mapping: define tool names, zod input schemas, output shape, and UI resource URIs (ui://.../app.html). Map each tool to one React entrypoint and one HTML file.
3. Scaffold the project: start from assets/mcp-app-template/ when possible, then customize tool names, schemas, and UI. Ensure package.json uses the exact versions, plus tsconfig.json, vite.config.ts, Tailwind + PostCSS, and per-tool build scripts.
4. Implement the server: use registerAppTool/registerAppResource, zod schemas directly, createServer() factory per request, and createMcpExpressApp with app.all("/mcp", ...).
5. Implement the UI: use useApp + useHostStyles, parse tool results, handle loading/error/empty states, and apply safe-area insets.
6. Build and test: run npm run build, then npm run serve, then verify via a tunnel if needed.

Hard Requirements

• Use the exact dependency versions listed in references/mcp-app-spec.md.
• Use registerAppTool/registerAppResource and zod schemas directly (not JSON Schema objects).
• Create a new McpServer instance per request via createServer().
• Use createMcpExpressApp and app.all("/mcp", ...).
• Bundle UI into single-file HTML via vite-plugin-singlefile.
• Use host CSS variables for theme compatibility.

References

• references/mcp-app-spec.md (authoritative spec, patterns, code templates, gotchas)

Assets

• assets/mcp-app-template/ (ready-to-copy MCP App skeleton with one tool + UI)