diff --git a/gateway/run.py b/gateway/run.py
index 08c6a35cda5..f643eadf4a7 100644
--- a/gateway/run.py
+++ b/gateway/run.py
@@ -12422,11 +12422,12 @@ class GatewayRunner:
             if not tts_text:
                 return
 
-            # Use .mp3 extension so edge-tts conversion to opus works correctly.
-            # The TTS tool may convert to .ogg — use file_path from result.
+            # Telegram's adapter only sends native voice bubbles for OGG/Opus.
+            # Other platforms keep the existing MP3 default.
+            audio_ext = "ogg" if event.source.platform == Platform.TELEGRAM else "mp3"
             audio_path = os.path.join(
                 tempfile.gettempdir(), "hermes_voice",
-                f"tts_reply_{_uuid.uuid4().hex[:12]}.mp3",
+                f"tts_reply_{_uuid.uuid4().hex[:12]}.{audio_ext}",
             )
             os.makedirs(os.path.dirname(audio_path), exist_ok=True)
 
diff --git a/tests/gateway/test_auto_voice_reply_format.py b/tests/gateway/test_auto_voice_reply_format.py
new file mode 100644
index 00000000000..eeb39ab60e7
--- /dev/null
+++ b/tests/gateway/test_auto_voice_reply_format.py
@@ -0,0 +1,100 @@
+"""Tests for gateway auto-TTS voice reply audio format selection."""
+
+import json
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from gateway.config import Platform
+from gateway.platforms.base import MessageEvent
+from gateway.run import GatewayRunner
+from gateway.session import SessionSource
+
+
+class TestAutoVoiceReplyFormat:
+    @pytest.mark.asyncio
+    async def test_telegram_auto_voice_reply_requests_ogg_for_native_voice_bubble(self):
+        """Telegram auto-TTS should request OGG/Opus so send_voice sends a voice bubble."""
+        runner = _make_runner()
+        adapter = _make_adapter(Platform.TELEGRAM)
+        runner.adapters[Platform.TELEGRAM] = adapter
+        event = _make_event(Platform.TELEGRAM)
+        requested_paths = []
+
+        def fake_tts(*, text, output_path):
+            requested_paths.append(output_path)
+            assert output_path.endswith(".ogg")
+            Path(output_path).parent.mkdir(parents=True, exist_ok=True)
+            Path(output_path).write_bytes(b"fake ogg opus")
+            return json.dumps({
+                "success": True,
+                "file_path": output_path,
+                "provider": "gemini",
+                "voice_compatible": True,
+            })
+
+        with patch("tools.tts_tool.text_to_speech_tool", side_effect=fake_tts):
+            await runner._send_voice_reply(event, "hello from auto tts")
+
+        assert requested_paths
+        assert requested_paths[0].endswith(".ogg")
+        adapter.send_voice.assert_awaited_once()
+        assert adapter.send_voice.await_args.kwargs["audio_path"].endswith(".ogg")
+
+    @pytest.mark.asyncio
+    async def test_non_telegram_auto_voice_reply_keeps_mp3_default(self):
+        """Non-Telegram platforms should keep the current MP3 default."""
+        runner = _make_runner()
+        adapter = _make_adapter(Platform.SLACK)
+        runner.adapters[Platform.SLACK] = adapter
+        event = _make_event(Platform.SLACK)
+        requested_paths = []
+
+        def fake_tts(*, text, output_path):
+            requested_paths.append(output_path)
+            assert output_path.endswith(".mp3")
+            Path(output_path).parent.mkdir(parents=True, exist_ok=True)
+            Path(output_path).write_bytes(b"fake mp3")
+            return json.dumps({
+                "success": True,
+                "file_path": output_path,
+                "provider": "gemini",
+                "voice_compatible": False,
+            })
+
+        with patch("tools.tts_tool.text_to_speech_tool", side_effect=fake_tts):
+            await runner._send_voice_reply(event, "hello from auto tts")
+
+        assert requested_paths
+        assert requested_paths[0].endswith(".mp3")
+        adapter.send_voice.assert_awaited_once()
+        assert adapter.send_voice.await_args.kwargs["audio_path"].endswith(".mp3")
+
+
+def _make_runner() -> GatewayRunner:
+    with patch("gateway.run.GatewayRunner._load_voice_modes", return_value={}):
+        runner = GatewayRunner.__new__(GatewayRunner)
+        runner._voice_mode = {}
+        runner.adapters = {}
+    return runner
+
+
+def _make_adapter(platform: Platform) -> MagicMock:
+    adapter = MagicMock()
+    adapter.platform = platform
+    adapter.send_voice = AsyncMock()
+    return adapter
+
+
+def _make_event(platform: Platform) -> MessageEvent:
+    return MessageEvent(
+        text="trigger",
+        source=SessionSource(
+            platform=platform,
+            chat_id="123",
+            user_id="u1",
+            user_name="User",
+        ),
+        message_id="456",
+    )