> ## Documentation Index
> Fetch the complete documentation index at: https://docs.rime.ai/llms.txt
> Use this file to discover all available pages before exploring further.
# Build a voice agent (Express)
> Build a voice agent with Rime TTS on an Express server: one route proxies synthesis so your API key stays server-side while the browser records speech and plays back Rime audio.
This guide builds a working voice agent on Express: the browser captures speech, your Express server decides what to say, and Rime speaks the reply.
There is no Rime npm package to install — and you don't need one. Rime is a single HTTPS API; the route below is plain `fetch`, and Express is the only dependency.
```
Browser (mic + playback) → POST /api/tts (Express) → POST https://users.rime.ai/v1/rime-tts
```
## Prerequisites
* A Rime API key from the [API Tokens page](https://app.rime.ai/tokens), exported as `RIME_API_KEY`
* Node.js 20.11+ and `npm install express`
## 1. Server: Express app with a TTS route
Create `server.mjs`:
```javascript server.mjs theme={null}
import path from "node:path";
import express from "express";
const app = express();
app.use(express.json());
app.post("/api/tts", async (req, res) => {
const { text, speaker = "astra", modelId = "coda" } = req.body ?? {};
if (!text) return res.status(400).json({ error: "text is required" });
const rimeRes = await fetch("https://users.rime.ai/v1/rime-tts", {
method: "POST",
headers: {
Authorization: `Bearer ${process.env.RIME_API_KEY}`,
"Content-Type": "application/json",
Accept: "audio/mpeg", // MP3; see the API reference for Opus, WAV, PCM, μ-law
},
body: JSON.stringify({ text, speaker, modelId }),
});
if (!rimeRes.ok) {
return res
.status(rimeRes.status)
.json({ error: `Rime API error ${rimeRes.status}`, detail: await rimeRes.text() });
}
res.setHeader("Content-Type", "audio/mpeg");
res.send(Buffer.from(await rimeRes.arrayBuffer()));
});
app.get("/", (_req, res) => {
res.sendFile(path.join(import.meta.dirname, "index.html"));
});
app.listen(3000, () => console.log("Rime voice agent on http://localhost:3000"));
```
Verify it works before touching the frontend:
```bash theme={null}
RIME_API_KEY=your_key node server.mjs &
curl -X POST http://localhost:3000/api/tts \
-H 'Content-Type: application/json' \
-d '{"text": "Hello from my Express voice agent."}' \
--output test.mp3
```
`test.mp3` should be playable audio. A 401 means `RIME_API_KEY` isn't visible to the server process.
## 2. Client: mic in, Rime audio out
Create `index.html` next to `server.mjs`. It uses the browser's built-in `SpeechRecognition` for input (Chrome/Edge/Safari), a stub `respond()` function as the agent brain, and your `/api/tts` route for the voice:
```html index.html theme={null}
Rime voice agent
Rime voice agent
```
Open `http://localhost:3000`, click **Speak**, say something, and the agent answers in Rime's `astra` voice.
## Want streaming?
For incremental synthesis — audio starts playing while later sentences are still generating — attach a WebSocket bridge to the same HTTP server: browser WS ↔ Express server ↔ `wss://users-ws.rime.ai/ws3` with header auth. Rime's WebSocket endpoints authenticate with an `Authorization` header, which browser WebSockets cannot send, so the bridge always lives server-side. The complete bridge implementation (using the `ws` package with `new WebSocketServer({ server })`) is in the [Next.js guide](/docs/voice-agent-nextjs#3-optional-stream-over-websockets) — it works unchanged with Express's underlying HTTP server via `const server = app.listen(3000)`. The `/ws3` message schema is in the [Coda WebSocket reference](/api-reference/coda/websockets-json).
## Next steps
Endpoints, word-level timestamps, context IDs, and interruption handling.
Swap `astra` for any Coda voice across six languages.
Choose between Opus, MP3, WAV, PCM, and μ-law for your latency budget.
The same agent with zero dependencies — just node:http and built-in fetch.