create-mcp-app

# Create MCP App Build interactive UIs that run inside MCP-enabled hosts like Claude Desktop. An MCP App combines an MCP tool with an HTML resource to display rich, interactive content. ## Core Concept: Tool + Resource Every MCP App requires two parts linked together: 1. **Tool** - Called by the LLM/host, returns data 2. **Resource** - Serves the bundled HTML UI that displays the data 3. **Link** - The tool's `_meta.ui.resourceUri` references the resource ``` Host calls tool → Server returns result → Host renders resource UI → UI receives result ``` ## Quick Start Decision Tree ### Framework Selection | Framework | SDK Support | Best For | |-----------|-------------|----------| | React | `useApp` hook provided | Teams familiar with React | | Vanilla JS | Manual lifecycle | Simple apps, no build complexity | | Vue/Svelte/Preact/Solid | Manual lifecycle | Framework preference | Note that if the user prefers to write MCP Servers in Golang or Rust you should ask the user if they have a specific framework in mind for that language and if not help them search for a suitable framework / library that supports MCP Apps. ### Project Context **Adding to existing MCP server:** - Import `registerAppTool`, `registerAppResource` from SDK - Add tool registration with `_meta.ui.resourceUri` - Add resource registration serving bundled HTML **Creating new MCP server:** - Set up server with transport (stdio or HTTP) - Register tools and resources - Configure build system with `vite-plugin-singlefile` ## Getting Reference Code Clone the SDK repository for working examples and API documentation: ```bash git clone --branch "v$(npm view @modelcontextprotocol/ext-apps version)" --depth 1 https://github.com/modelcontextprotocol/ext-apps.git /tmp/mcp-ext-apps ``` ### Framework Templates Learn and adapt from `/tmp/mcp-ext-apps/examples/basic-server-{framework}/`: | Template | Key Files | |----------|-----------| | `basic-server-vanillajs/` | `server.ts`, `src/mcp-app.ts`, `mcp-app.html` | | `basic-server-react/` | `server.ts`, `src/mcp-app.tsx` (uses `useApp` hook) | | `basic-server-vue/` | `server.ts`, `src/App.vue` | | `basic-server-svelte/` | `server.ts`, `src/App.svelte` | | `basic-server-preact/` | `server.ts`, `src/mcp-app.tsx` | | `basic-server-solid/` | `server.ts`, `src/mcp-app.tsx` | Each template includes: - Complete `server.ts` with `registerAppTool` and `registerAppResource` - Client-side app with all lifecycle handlers - `vite.config.ts` with `vite-plugin-singlefile` - `package.json` with all required dependencies - `.gitignore` excluding `node_modules/` and `dist/` ### API Reference (Source Files) Read JSDoc documentation directly from `/tmp/mcp-ext-apps/src/`: | File | Contents | |------|----------| | `src/app.ts` | `App` class, handlers (`ontoolinput`, `ontoolresult`, `onhostcontextchanged`, `onteardown`), lifecycle | | `src/server/index.ts` | `registerAppTool`, `registerAppResource`, tool visibility options | | `src/spec.types.ts` | All type definitions: `McpUiHostContext`, CSS variable keys, display modes | | `src/styles.ts` | `applyDocumentTheme`, `applyHostStyleVariables`, `applyHostFonts` | | `src/react/useApp.tsx` | `useApp` hook for React apps | | `src/react/useHostStyles.ts` | `useHostStyles`, `useHostStyleVariables`, `useHostFonts` hooks | ### Advanced Examples | Example | Pattern Demonstrated | |---------|---------------------| | `examples/shadertoy-server/` | **Streaming partial input** + visibility-based pause/play (best practice for large inputs) | | `examples/wiki-explorer-server/` | `callServerTool` for interactive data fetching | | `examples/system-monitor-server/` | Polling pattern with interval management | | `examples/video-resource-server/` | Binary/blob resources | | `examples/sheet-music-server/` | `ontoolinput` - processing tool args before execution completes | | `examples/threejs-server/` | `ontoolinputpartial` - streaming/progressive rendering | | `examples/map-server/` | `updateModelContext` - keeping model informed of UI state | | `examples/transcript-server/` | `updateModelContext` + `sendMessage` - background context updates + user-initiated messages | | `examples/basic-host/` | Reference host implementation using `AppBridge` | ## Critical Implementation Notes ### Adding Dependencies Use `npm install` to add dependencies rather than manually writing version numbers: ```bash npm install @modelcontextprotocol/ext-apps @modelcontextprotocol/sdk zod ``` This lets npm resolve the latest compatible versions. Never specify version numbers from memory. ### TypeScript Server Execution Use `tsx` as a devDependency for running TypeScript server files: ```bash npm install -D tsx ``` ```json "scripts": { "serve": "tsx server.ts" } ``` Note: The SDK examples use `bun` but generated projects should use `tsx` for broader compatibility. ### Handler Registration Order Register ALL handlers BEFORE calling `app.connect()`: ```typescript const app = new App({ name: "My App", version: "1.0.0" }); // Register handlers first app.ontoolinput = (params) => { /* handle input */ }; app.ontoolresult = (result) => { /* handle result */ }; app.onhostcontextchanged = (ctx) => { /* handle context */ }; app.onteardown = async () => { return {}; }; // Then connect await app.connect(); ``` ### Tool Visibility Control who can access tools via `_meta.ui.visibility`: ```typescript // Default: visible to both model and app _meta: { ui: { resourceUri, visibility: ["model", "app"] } } // UI-only (hidden from model) - for refresh buttons, form submissions _meta: { ui: { resourceUri, visibility: ["app"] } } // Model-only (app cannot call) _meta: { ui: { resourceUri, visibility: ["model"] } } ``` ### Host Styling Integration **Vanilla JS** - Use helper functions: ```typescript import { applyDocumentTheme, applyHostStyleVariables, applyHostFonts } from "@modelcontextprotocol/ext-apps"; app.onhostcontextchanged = (ctx) => { if (ctx.theme) applyDocumentTheme(ctx.theme); if (ctx.styles?.variables) applyHostStyleVariables(ctx.styles.variables); if (ctx.styles?.css?.fonts) applyHostFonts(ctx.styles.css.fonts); }; ``` **React** - Use hooks: ```typescript import { useApp, useHostStyles } from "@modelcontextprotocol/ext-apps/react"; const { app } = useApp({ appInfo, capabilities, onAppCreated }); useHostStyles(app); // Injects CSS variables to document, making var(--*) available ``` **Using variables in CSS** - After applying, use `var()`: ```css .container { background: var(--color-background-secondary); color: var(--color-text-primary); font-family: var(--font-sans); border-radius: var(--border-radius-md); } .code { font-family: var(--font-mono); font-size: var(--font-text-sm-size); line-height: var(--font-text-sm-line-height); color: var(--color-text-secondary); } .heading { font-size: var(--font-heading-lg-size); font-weight: var(--font-weight-semibold); } ``` Key variable groups: `--color-background-*`, `--color-text-*`, `--color-border-*`, `--font-sans`, `--font-mono`, `--font-text-*-size`, `--font-heading-*-size`, `--border-radius-*`. See `src/spec.types.ts` for full list. ### Safe Area Handling Always respect `safeAreaInsets`: ```typescript app.onhostcontextchanged = (ctx) => { if (ctx.safeAreaInsets) { const { top, right, bottom, left } = ctx.safeAreaInsets; document.body.style.padding = `${top}px ${right}px ${bottom}px ${left}px`; } }; ``` ### Streaming Partial Input For large tool inputs, use `ontoolinputpartial` to show progress during LLM generation. The partial JSON is healed (always valid), enabling progressive UI updates. **Spec:** [ui/notifications/tool-input-partial](https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/2026-01-26/apps.mdx#streaming-tool-input) ```typescript app.ontoolinputpartial = (params) => { const args = params.arguments; // Healed partial JSON - always valid, fields appear as generated // Use args directly for progressive rendering }; app.ontoolinput = (params) => { // Final complete input - switch from preview to full render }; ``` **Use cases:** | Pattern | Example | |---------|---------| | Code preview | Show streaming code in `<pre>`, render on complete (`examples/shadertoy-server/`) | | Progressive form | Fill form fields as they stream in | | Live chart | Add data points to chart as array grows | | Partial render | Render incomplete structured data (tables, lists, trees) | **Simple pattern (code preview):** ```typescript app.ontoolinputpartial = (params) => { codePreview.textContent = params.arguments?.code ?? ""; codePreview.style.display = "block"; canvas.style.display = "none"; }; app.ontoolinput = (params) => { codePreview.style.display = "none"; canvas.style.display = "block"; render(params.arguments); }; ``` ### Visibility-Based Resource Management Pause expensive operations (animations, WebGL, polling) when view scrolls out of viewport: ```typescript const observer = new IntersectionObserver((entries) => { entries.forEach((entry) => { if (entry.isIntersecting) { animation.play(); // or: startPolling(), shaderToy.play() } else { animation.pause(); // or: stopPolling(), shaderToy.pause() } }); }); observer.observe(document.querySelector(".main")); ``` ### Fullscreen Mode Request fullscreen via `app.requestDisplayMode()`. Check availability in host context: ```typescript let currentMode: "inline" | "fullscreen" = "inline"; app.onhostcontextchanged = (ctx) => { // Check if fullscreen available if (ctx.availableDisplayModes?.includes("fullscreen")) { fullscreenBtn.style.display = "block"; } // Track current mode if (ctx.displayMode) { currentMode = ctx.displayMode; container.classList.toggle("fullscreen", currentMode === "fullscreen"); } }; async function toggleFullscreen() { const newMode = currentMode === "fullscreen" ? "inline" : "fullscreen"; const result = await app.requestDisplayMode({ mode: newMode }); currentMode = result.mode; } ``` **CSS pattern** - Remove border radius in fullscreen: ```css .main { border-radius: var(--border-radius-lg); overflow: hidden; } .main.fullscreen { border-radius: 0; } ``` See `examples/shadertoy-server/` for complete implementation. ## Common Mistakes to Avoid 1. **Handlers after connect()** - Register ALL handlers BEFORE calling `app.connect()` 2. **Missing single-file bundling** - Must use `vite-plugin-singlefile` 3. **Forgetting resource registration** - Both tool AND resource must be registered 4. **Missing resourceUri link** - Tool must have `_meta.ui.resourceUri` 5. **Ignoring safe area insets** - Always handle `ctx.safeAreaInsets` 6. **No text fallback** - Always provide `content` array for non-UI hosts