Sdk Patterns

SDK patterns — App, plugins, routing, middleware

The @microsoft/teams.apps package is built around one class (App) and a plugin pipeline. Mastering four patterns — boot, plugin registration, routing, and middleware — covers ~80% of day-to-day work.

1. Boot the App

import { App, PUBLIC } from '@microsoft/teams.apps';
import { DevtoolsPlugin } from '@microsoft/teams.dev';

const app = new App({
  plugins: [new DevtoolsPlugin()],
  cloud: PUBLIC,
});

(async () => {
  await app.start(+(process.env.PORT ?? 3978));
})();

• app.start(port?) returns once the HTTP server is listening.
• The HTTP listener handles /api/messages (bot activities) and any routes plugins register.
• The default cloud is PUBLIC. For US Gov / GCC High / China, pass US_GOV, US_GOV_DOD, or CHINA — see deployment/references/sovereign-clouds.md.

2. Register plugins

Plugins are passed to the constructor:

new App({
  plugins: [
    new DevtoolsPlugin(),                          // local inspector
    new McpPlugin({ port: 4000 }),                 // expose tools as MCP server
    new A2AClientPlugin({ agents: [{ ... }] }),    // call remote A2A agents
    new ExpressAdapter({ app: express() }),        // run on an existing Express server
  ],
});

Plugin order matters: earlier plugins wrap later ones. Place adapter-level plugins (e.g. ExpressAdapter) last so they observe the final, fully-decorated pipeline.

3. Activity routing

The app.on(name, handler) API attaches a handler to a route. The route name is dotted:

Use a concrete <id> to scope a handler. Use the bare verb (dialog.open) to catch every id.

4. Middleware via next()

Each handler receives next. Call it to pass control to the next matching handler:

app.on('message', async (ctx) => {
  if (ctx.activity.text?.startsWith('/admin')) {
    if (!isAdmin(ctx.activity.from)) {
      await ctx.send('Not authorised.');
      return;
    }
  }
  await ctx.next();
});

app.on('message', async ({ send, activity }) => {
  await send(`You said: ${activity.text}`);
});

The first handler short-circuits non-admin /admin requests; the second runs only when next() is called.

5. Project layout

src/
├── index.ts          # App boot, plugin list, route registration
├── handlers/
│   ├── message.ts    # app.on('message', …)
│   ├── dialog.ts     # app.on('dialog.*', …)
│   └── extension.ts  # app.on('message.ext.*', …)
├── cards/            # Adaptive Card builders
├── prompts/          # AI ChatPrompt definitions
└── lib/              # Domain code

Keep index.ts short — registration only. Put handler bodies in handlers/* modules and import them. This keeps the route map readable.

6. Integrating with an existing server

If the app is not greenfield, plug into the existing Express/Fastify server via ExpressAdapter:

import express from 'express';
import { App } from '@microsoft/teams.apps';
import { ExpressAdapter } from '@microsoft/teams.apps/express';

const server = express();
const app = new App({ plugins: [new ExpressAdapter({ app: server })] });

server.get('/health', (_, res) => res.send('ok'));
app.on('message', async ({ send }) => { await send('hi'); });

server.listen(3978);

See troubleshoot/references/integrate-existing-server.md for the full migration checklist.

7. Common pitfalls

• Forgetting await app.start() — without the await, the HTTP server never binds.
• Plugins after start() — plugins must be passed at construction; calling app.use(plugin) post-start is not supported.
• Logging via console.log — use ctx.log so DevTools captures the entry alongside the activity.
• Long-running handlers — Teams expects a response within ~15 seconds. Use stream.emit() (1:1) or send a typing indicator + proactive follow-up for longer work.