diff --git a/gateway/platforms/base.py b/gateway/platforms/base.py
index 1025964dc43b..caa429462c22 100644
--- a/gateway/platforms/base.py
+++ b/gateway/platforms/base.py
@@ -13,6 +13,7 @@
 import random
 import re
 import socket as _socket
+import tempfile
 import subprocess
 import sys
 import time
@@ -4965,8 +4966,21 @@ async def _stop_typing_task() -> None:
                             speech_text = self.prepare_tts_text(text_content)
                             if not speech_text:
                                 raise ValueError("Empty text after markdown cleanup")
+                            # Pass an explicit output_path with the correct
+                            # extension for the current platform.  The
+                            # session contextvar (HERMES_SESSION_PLATFORM) is
+                            # not populated at this call site — it is set by
+                            # _set_session_env() inside _handle_message_with_agent
+                            # and cleared on return.  Without an explicit path
+                            # text_to_speech_tool falls back to .mp3, which
+                            # causes Telegram to render sendAudio instead of
+                            # sendVoice.
+                            _tts_ext = "ogg" if self.platform == Platform.TELEGRAM else "mp3"
+                            _ts = datetime.now().strftime("%Y%m%d_%H%M%S")
+                            _tts_out = Path(tempfile.gettempdir()) / f"auto_tts_{_ts}.{_tts_ext}"
                             tts_result_str = await asyncio.to_thread(
-                                text_to_speech_tool, text=speech_text
+                                text_to_speech_tool, text=speech_text,
+                                output_path=str(_tts_out),
                             )
                             tts_data = _json.loads(tts_result_str)
                             _tts_path = tts_data.get("file_path")
diff --git a/tests/gateway/test_base_topic_sessions.py b/tests/gateway/test_base_topic_sessions.py
index 4de540b49d1d..03d5f964166a 100644
--- a/tests/gateway/test_base_topic_sessions.py
+++ b/tests/gateway/test_base_topic_sessions.py
@@ -353,3 +353,40 @@ async def test_telegram_auto_tts_send_failure_keeps_followup_text(self, tmp_path
                 "metadata": {"thread_id": "17585", "notify": True},
             }
         ]
+
+    @pytest.mark.asyncio
+    async def test_telegram_auto_tts_passes_explicit_ogg_output_path(self, tmp_path):
+        """Auto-TTS must pass an explicit .ogg output_path on Telegram.
+
+        Regression test for #57049: the session contextvar
+        (HERMES_SESSION_PLATFORM) is not populated at the auto-TTS call
+        site in base.py, so text_to_speech_tool falls back to .mp3.
+        The fix passes an explicit output_path with the correct
+        extension based on self.platform.
+        """
+        adapter = DummyTelegramAdapter()
+        adapter._keep_typing = self._hold_typing()
+        adapter._should_auto_tts_for_chat = lambda _chat_id: True
+        adapter.play_tts = AsyncMock(return_value=SendResult(success=True, message_id="tts-1"))
+        adapter.set_message_handler(lambda _event: asyncio.sleep(0, result="Short reply"))
+
+        tts_path = tmp_path / "reply.ogg"
+        tts_path.write_text("audio", encoding="utf-8")
+        event = self._make_voice_event()
+
+        captured_kwargs = {}
+
+        def capture_tts(*args, **kwargs):
+            captured_kwargs.update(kwargs)
+            return json.dumps({"file_path": str(tts_path)})
+
+        with patch("tools.tts_tool.check_tts_requirements", return_value=True), patch(
+            "tools.tts_tool.text_to_speech_tool",
+            side_effect=capture_tts,
+        ):
+            await adapter._process_message_background(event, build_session_key(event.source))
+
+        assert "output_path" in captured_kwargs, "text_to_speech_tool should receive explicit output_path"
+        assert captured_kwargs["output_path"].endswith(".ogg"), (
+            f"Telegram auto-TTS must use .ogg extension, got: {captured_kwargs['output_path']}"
+        )