fix: preserve Cartesia inference timestamp spacing

.changeset/cartesia-inference-timestamp-spacing.md

@@ -0,0 +1,5 @@
+---
+'@livekit/agents': patch
+---
+
+Preserve spaces in Cartesia inference TTS aligned transcripts.

agents/src/inference/tts.ts

@@ -845,7 +845,11 @@ export class SynthesizeStream<TModel extends TTSModels> extends BaseSynthesizeSt
               let aligned: TimedString[] = [];
               if (serverEvent.words && serverEvent.words.length > 0) {
                 aligned = serverEvent.words.map((w) =>
-                  createTimedString({ text: w.word, startTime: w.start, endTime: w.end }),
+                  createTimedString({
+                    text: provider === 'cartesia' ? `${w.word} ` : w.word,
+                    startTime: w.start,
+                    endTime: w.end,
+                  }),
                 );
               } else if (serverEvent.chars && serverEvent.chars.length > 0) {
                 aligned = serverEvent.chars.map((c) =>

agents/src/inference/tts_alignment.test.ts

@@ -0,0 +1,68 @@
+// SPDX-FileCopyrightText: 2026 LiveKit, Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+import { once } from 'node:events';
+import type { AddressInfo } from 'node:net';
+import { expect, it } from 'vitest';
+import { WebSocketServer } from 'ws';
+import { SynthesizeStream } from '../tts/tts.js';
+import { TTS } from './tts.js';
+
+it('adds separators to Cartesia inference word timestamps', async () => {
+  const server = new WebSocketServer({ host: '127.0.0.1', port: 0 });
+  await once(server, 'listening');
+  const address = server.address() as AddressInfo;
+
+  server.on('connection', (socket) => {
+    socket.on('message', (raw) => {
+      const event = JSON.parse(raw.toString()) as { type: string };
+      if (event.type !== 'session.flush') return;
+
+      socket.send(
+        JSON.stringify({
+          type: 'output_alignment',
+          session_id: 'session-1',
+          words: [
+            { word: 'hello', start: 0, end: 0.2 },
+            { word: 'world.', start: 0.2, end: 0.4 },
+          ],
+        }),
+      );
+      socket.send(
+        JSON.stringify({
+          type: 'output_audio',
+          session_id: 'session-1',
+          audio: Buffer.alloc(3200).toString('base64'),
+        }),
+      );
+      socket.send(JSON.stringify({ type: 'done', session_id: 'session-1' }));
+    });
+  });
+
+  const tts = new TTS({
+    model: 'cartesia/sonic-3',
+    modelOptions: { add_timestamps: true },
+    apiKey: 'test-key',
+    apiSecret: 'test-secret',
+    baseURL: `http://127.0.0.1:${address.port}`,
+  });
+
+  try {
+    const stream = tts.stream();
+    stream.pushText('hello world.');
+    stream.endInput();
+
+    const words: string[] = [];
+    for await (const event of stream) {
+      if (event !== SynthesizeStream.END_OF_STREAM) {
+        words.push(...(event.timedTranscripts ?? []).map((word) => word.text));
+      }
+    }
+
+    expect(words).toEqual(['hello ', 'world. ']);
+  } finally {
+    await tts.close();
+    for (const client of server.clients) client.close();
+    await new Promise<void>((resolve) => server.close(() => resolve()));
+  }
+});