Agent skill

chatgpt-app:new

Create a new ChatGPT App from concept to working code. Guides through conceptualization, design, implementation, testing, and deployment.

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SKILL.md

Create a New ChatGPT App

You are helping the user create a new ChatGPT App. Follow this multi-phase workflow to take them from concept to a working, deployable application.

CRITICAL REQUIREMENTS

ChatGPT Apps MUST use:

  • Server class from @modelcontextprotocol/sdk/server/index.js (NOT McpServer)
  • StreamableHTTPServerTransport from @modelcontextprotocol/sdk/server/streamableHttp.js
  • Session management with Map<string, StreamableHTTPServerTransport>
  • Widget URIs: ui://widget/{widget-id}.html
  • Widget MIME type: text/html+skybridge
  • structuredContent in tool responses for widget data
  • _meta with openai/outputTemplate on both tool definitions and responses

File Structure

Every ChatGPT App follows this structure:

{app-name}/
├── package.json              # Dependencies and scripts
├── tsconfig.server.json      # TypeScript config
├── setup.sh                  # One-command setup
├── START.sh                  # Multi-mode server launcher
├── .env                      # Environment variables (created by setup.sh)
├── .env.example              # Environment template
├── .gitignore                # Git ignores
└── server/
    └── index.ts              # Complete MCP server with inline widgets

Phase 1: Conceptualization

Start by gathering information about the app:

  1. Ask for the app idea "What ChatGPT App would you like to build? Describe what it does and the problem it solves."

  2. Analyze against UX Principles Evaluate the idea against the three pillars:

    • Conversational Leverage: What can users accomplish through natural language that would be harder in a traditional UI?
    • Native Fit: How does this integrate naturally with ChatGPT's conversational flow?
    • Composability: Can the tools work independently and combine with other apps?

  3. Check for Anti-Patterns Warn if the idea includes:

    • Static website content display
    • Complex multi-step workflows requiring external tabs
    • Duplicating ChatGPT's native capabilities
    • Ads or upsells

  4. Define Use Cases Create 3-5 primary use cases with user stories.

Phase 2: Design

Design the technical architecture:

  1. Tool Topology Define the MCP tools needed:

    • Query tools (readOnlyHint: true)
    • Mutation tools (destructiveHint: false)
    • Destructive tools (destructiveHint: true)
    • Widget tools (return UI with _meta)
    • External API tools (openWorldHint: true)

  2. Widget Design For each widget, define:

    • id - unique identifier (kebab-case)
    • name - display name
    • description - what it shows
    • mockData - sample data for preview

  3. Data Model Design entities and their relationships.

  4. Auth Requirements

    • Single-user (no auth needed)
    • Multi-user (Auth0 or Supabase Auth)

Phase 3: Implementation

Generate the complete application code.

package.json

json
{
  "name": "{app-name}",
  "version": "1.0.0",
  "description": "{app-description}",
  "type": "module",
  "scripts": {
    "build": "npm run build:server",
    "build:server": "tsc -p tsconfig.server.json",
    "start": "HTTP_MODE=true node dist/server/index.js",
    "dev": "HTTP_MODE=true NODE_ENV=development tsx watch --clear-screen=false server/index.ts",
    "validate": "tsc --noEmit -p tsconfig.server.json"
  },
  "dependencies": {
    "@modelcontextprotocol/sdk": "^1.0.0",
    "dotenv": "^16.4.0",
    "express": "^4.18.2",
    "zod": "^3.23.0"
  },
  "devDependencies": {
    "@types/express": "^4.17.21",
    "@types/node": "^20.0.0",
    "tsx": "^4.0.0",
    "typescript": "^5.4.0"
  },
  "engines": {
    "node": ">=18.0.0"
  }
}

tsconfig.server.json

json
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "outDir": "dist/server",
    "rootDir": "server",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "sourceMap": true
  },
  "include": ["server/**/*"],
  "exclude": ["node_modules", "dist"]
}

server/index.ts Structure

The server MUST follow this structure:

typescript
// 1. Load environment first
import "dotenv/config";
import express, { Request, Response } from "express";
import { randomUUID } from "crypto";
import { Server } from "@modelcontextprotocol/sdk/server/index.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import {
  CallToolRequestSchema,
  ListToolsRequestSchema,
  ListResourcesRequestSchema,
  ReadResourceRequestSchema,
  JSONRPCMessage,
} from "@modelcontextprotocol/sdk/types.js";

// 2. Configuration
const PORT = process.env.PORT || 3000;
const NODE_ENV = process.env.NODE_ENV || "development";
const WIDGET_DOMAIN = process.env.WIDGET_DOMAIN || `http://localhost:${PORT}`;

function log(...args: unknown[]) {
  if (NODE_ENV === "development") {
    console.log(`[${new Date().toISOString()}]`, ...args);
  }
}

// 3. Widget Configuration Array
interface WidgetConfig {
  id: string;
  name: string;
  description: string;
  templateUri: string;
  invoking: string;
  invoked: string;
  mockData: Record<string, unknown>;
}

const widgets: WidgetConfig[] = [
  {
    id: "my-widget",
    name: "My Widget",
    description: "Displays data in a visual format",
    templateUri: "ui://widget/my-widget.html",
    invoking: "Loading...",
    invoked: "Ready",
    mockData: { /* sample data */ },
  },
];

const WIDGETS_BY_ID = new Map(widgets.map((w) => [w.id, w]));
const WIDGETS_BY_URI = new Map(widgets.map((w) => [w.templateUri, w]));

// 4. Inline Widget HTML Generator
function generateWidgetHtml(widgetId: string, previewData?: Record<string, unknown>): string {
  const widget = WIDGETS_BY_ID.get(widgetId);
  if (!widget) return `<html><body>Widget not found: ${widgetId}</body></html>`;

  const previewScript = previewData
    ? `<script>window.PREVIEW_DATA = ${JSON.stringify(previewData)};</script>`
    : "";

  return `<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>${widget.name}</title>
  <style>/* CSS styles */</style>
  ${previewScript}
</head>
<body>
  <div id="root"><div class="loading">Loading...</div></div>
  <script>
    (function() {
      let rendered = false;

      function render(data) {
        if (rendered || !data) return;
        rendered = true;
        // Widget rendering logic
        document.getElementById('root').innerHTML = '...';
      }

      function tryRender() {
        if (window.PREVIEW_DATA) { render(window.PREVIEW_DATA); return; }
        if (window.openai?.toolOutput) { render(window.openai.toolOutput); }
      }

      // ChatGPT Apps SDK integration
      window.addEventListener('openai:set_globals', tryRender);

      // Polling fallback
      const poll = setInterval(() => {
        if (window.openai?.toolOutput || window.PREVIEW_DATA) {
          tryRender();
          clearInterval(poll);
        }
      }, 100);
      setTimeout(() => clearInterval(poll), 10000);

      tryRender();
    })();
  </script>
</body>
</html>`;
}

// 5. Tool Definitions
const tools = [
  {
    name: "my_tool",
    description: "Does something useful",
    inputSchema: {
      type: "object" as const,
      properties: { /* ... */ },
      required: ["..."],
    },
    annotations: { title: "My Tool", readOnlyHint: true, destructiveHint: false, openWorldHint: false },
    widgetId: "my-widget", // Links to widget config (optional)
    execute: (args: any) => {
      // Tool logic - return data that widget will display
      return { /* result data */ };
    },
  },
];

// 6. MCP Server Factory
function createServer(): Server {
  const server = new Server(
    { name: "{app-name}", version: "1.0.0" },
    { capabilities: { tools: {}, resources: {} } }
  );

  // ListTools handler
  server.setRequestHandler(ListToolsRequestSchema, async () => {
    log("ListTools request");
    return {
      tools: tools.map((tool) => {
        const widget = tool.widgetId ? WIDGETS_BY_ID.get(tool.widgetId) : null;
        return {
          name: tool.name,
          description: tool.description,
          inputSchema: tool.inputSchema,
          annotations: tool.annotations,
          ...(widget && {
            _meta: {
              "openai/outputTemplate": widget.templateUri,
              "openai/widgetAccessible": true,
              "openai/resultCanProduceWidget": true,
              "openai/toolInvocation/invoking": widget.invoking,
            },
          }),
        };
      }),
    };
  });

  // CallTool handler
  server.setRequestHandler(CallToolRequestSchema, async (request) => {
    const { name, arguments: args } = request.params;
    log(`CallTool: ${name}`, args);

    const tool = tools.find((t) => t.name === name);
    if (!tool) throw new Error(`Unknown tool: ${name}`);

    try {
      const result = tool.execute(args);
      const widget = tool.widgetId ? WIDGETS_BY_ID.get(tool.widgetId) : null;

      if (widget) {
        return {
          content: [{ type: "text" as const, text: JSON.stringify(result, null, 2) }],
          structuredContent: result,  // CRITICAL: This becomes window.openai.toolOutput
          _meta: {
            "openai/outputTemplate": widget.templateUri,
            "openai/toolInvocation/invoked": widget.invoked,
          },
        };
      }

      return { content: [{ type: "text" as const, text: JSON.stringify(result, null, 2) }] };
    } catch (error) {
      return {
        content: [{ type: "text" as const, text: `Error: ${error instanceof Error ? error.message : "Unknown"}` }],
        isError: true,
      };
    }
  });

  // ListResources handler
  server.setRequestHandler(ListResourcesRequestSchema, async () => {
    return {
      resources: widgets.map((w) => ({
        uri: w.templateUri,
        name: w.name,
        description: w.description,
        mimeType: "text/html+skybridge",
      })),
    };
  });

  // ReadResource handler
  server.setRequestHandler(ReadResourceRequestSchema, async (request) => {
    const { uri } = request.params;
    const widget = WIDGETS_BY_URI.get(uri);
    if (!widget) throw new Error(`Unknown resource: ${uri}`);

    return {
      contents: [{ uri, mimeType: "text/html+skybridge", text: generateWidgetHtml(widget.id) }],
      _meta: {
        "openai/serialization": "markdown-encoded-html",
        "openai/csp": { script_domains: ["'unsafe-inline'"], connect_domains: [WIDGET_DOMAIN] },
      },
    };
  });

  return server;
}

// 7. Express App with Session Management
const app = express();
app.use(express.json());

const transports = new Map<string, StreamableHTTPServerTransport>();

// Health endpoint
app.get("/health", (req, res) => {
  res.json({ status: "ok", service: "{app-name}", widgets: widgets.length });
});

// Widget preview index
app.get("/preview", (req, res) => {
  res.send(`<!DOCTYPE html>
    <html><head><title>Widget Preview</title></head>
    <body>
      <h1>Widget Preview</h1>
      ${widgets.map(w => `<a href="/preview/${w.id}">${w.name}</a><br>`).join('')}
    </body></html>`);
});

// Widget preview with mock data
app.get("/preview/:widgetId", (req, res) => {
  const widget = WIDGETS_BY_ID.get(req.params.widgetId);
  if (!widget) { res.status(404).send("Widget not found"); return; }
  res.setHeader("Content-Type", "text/html");
  res.send(generateWidgetHtml(widget.id, widget.mockData));
});

// MCP endpoint with session management
app.all("/mcp", async (req, res) => {
  log("MCP request:", req.method, req.headers["mcp-session-id"] || "no-session");

  let sessionId = req.headers["mcp-session-id"] as string | undefined;
  let transport = sessionId ? transports.get(sessionId) : undefined;

  const isInitialize = req.body?.method === "initialize" ||
    (Array.isArray(req.body) && req.body.some((m: JSONRPCMessage) => "method" in m && m.method === "initialize"));

  if (isInitialize || !sessionId || !transport) {
    sessionId = randomUUID();
    log(`New session: ${sessionId}`);

    transport = new StreamableHTTPServerTransport({
      sessionIdGenerator: () => sessionId!,
      onsessioninitialized: (id) => log(`Session initialized: ${id}`),
    });

    transports.set(sessionId, transport);
    const server = createServer();

    res.on("close", () => log(`Connection closed: ${sessionId}`));
    transport.onclose = () => { transports.delete(sessionId!); server.close(); };

    await server.connect(transport);
  }

  await transport.handleRequest(req, res, req.body);
});

app.delete("/mcp", async (req, res) => {
  const sessionId = req.headers["mcp-session-id"] as string | undefined;
  if (sessionId && transports.has(sessionId)) {
    await transports.get(sessionId)!.handleRequest(req, res, req.body);
  } else {
    res.status(404).json({ error: "Session not found" });
  }
});

// 8. Start Server
app.listen(PORT, () => {
  console.log(`{App Name} MCP Server running on port ${PORT}`);
  console.log(`  MCP:     http://localhost:${PORT}/mcp`);
  console.log(`  Health:  http://localhost:${PORT}/health`);
  console.log(`  Preview: http://localhost:${PORT}/preview`);
});

setup.sh

bash
#!/bin/bash
set -euo pipefail
cd "$(dirname "$0")"

echo "=== {App Name} Setup ==="

# Check Node.js 18+
NODE_VERSION=$(node -v | cut -d'v' -f2 | cut -d'.' -f1)
if [ "$NODE_VERSION" -lt 18 ]; then
  echo "Node.js 18+ required"; exit 1
fi

npm install
npm run build:server

if [ ! -f .env ]; then
  cat > .env << 'EOF'
PORT=3000
HTTP_MODE=true
NODE_ENV=development
WIDGET_DOMAIN=http://localhost:3000
EOF
fi

chmod +x START.sh
echo "Setup complete! Run ./START.sh --dev"

START.sh

bash
#!/bin/bash
set -euo pipefail
cd "$(dirname "$0")"

if [ -f .env ]; then set -a; source .env; set +a; fi

case "${1:-}" in
  --dev)
    echo "Dev mode: http://localhost:${PORT:-3000}/preview"
    npm run dev
    ;;
  --preview)
    npm run dev &
    sleep 2
    open "http://localhost:${PORT:-3000}/preview"
    wait
    ;;
  --stdio)
    HTTP_MODE=false node dist/server/index.js
    ;;
  *)
    [ ! -f dist/server/index.js ] && npm run build:server
    HTTP_MODE=true node dist/server/index.js
    ;;
esac

Phase 4: Testing

Before deployment:

  1. Run setup: ./setup.sh
  2. Start dev mode: ./START.sh --dev
  3. Preview widgets: Open http://localhost:3000/preview
  4. Test health: curl http://localhost:3000/health
  5. Test MCP: Connect via MCP Inspector or ChatGPT

Phase 5: Deployment

Deploy to Render:

  1. Create Dockerfile:
dockerfile
FROM node:20-slim
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY dist ./dist
EXPOSE 3000
CMD ["node", "dist/server/index.js"]
  1. Create render.yaml:
yaml
services:
  - type: web
    name: {app-name}
    runtime: docker
    plan: free
    healthCheckPath: /health
    envVars:
      - key: PORT
        value: 3000
      - key: HTTP_MODE
        value: true
      - key: NODE_ENV
        value: production
  1. Push to GitHub and connect to Render
  2. ChatGPT connector URL: https://{app-name}.onrender.com/mcp

State Persistence

Save progress to .chatgpt-app/state.json after each phase.

Getting Started

When invoked, begin with: "What ChatGPT App would you like to build? Describe what it does and the problem it solves."

Then guide them through each phase systematically.

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