fix(voice): send language and prompt in streamed STT

src/agents/voice/models/openai_stt.py

@@ -175,6 +175,15 @@ async def _event_listener(self) -> None:
 
     async def _configure_session(self) -> None:
         assert self._websocket is not None, "Websocket not initialized"
+        transcription_config: dict[str, Any] = {"model": self._model}
+        if self._settings.language is not None:
+            if self._model in {"gpt-transcribe", "gpt-live-transcribe"}:
+                transcription_config["languages"] = [self._settings.language]
+            else:
+                transcription_config["language"] = self._settings.language
+        if self._settings.prompt is not None:
+            transcription_config["prompt"] = self._settings.prompt
+
         await self._websocket.send(
             json.dumps(
                 {
@@ -184,7 +193,7 @@ async def _configure_session(self) -> None:
                         "audio": {
                             "input": {
                                 "format": {"type": "audio/pcm", "rate": 24000},
-                                "transcription": {"model": self._model},
+                                "transcription": transcription_config,
                                 "turn_detection": self._turn_detection,
                             }
                         },

tests/voice/test_openai_stt_session_config.py

@@ -0,0 +1,97 @@
+import json
+from unittest.mock import AsyncMock
+
+import pytest
+
+from agents.voice import StreamedAudioInput, STTModelSettings
+from agents.voice.models.openai_stt import OpenAISTTTranscriptionSession
+
+
+@pytest.mark.asyncio
+async def test_streaming_stt_sends_language_and_prompt() -> None:
+    session = OpenAISTTTranscriptionSession(
+        input=StreamedAudioInput(),
+        client=AsyncMock(api_key="FAKE_KEY"),
+        model="gpt-4o-transcribe",
+        settings=STTModelSettings(language="fr", prompt="domain vocabulary"),
+        trace_include_sensitive_data=False,
+        trace_include_sensitive_audio_data=False,
+    )
+    websocket = AsyncMock()
+    session._websocket = websocket
+
+    await session._configure_session()
+
+    payload = json.loads(websocket.send.await_args.args[0])
+    assert payload["session"]["audio"]["input"]["transcription"] == {
+        "model": "gpt-4o-transcribe",
+        "language": "fr",
+        "prompt": "domain vocabulary",
+    }
+
+
+@pytest.mark.asyncio
+async def test_streaming_stt_sends_plural_languages_for_gpt_transcribe() -> None:
+    session = OpenAISTTTranscriptionSession(
+        input=StreamedAudioInput(),
+        client=AsyncMock(api_key="FAKE_KEY"),
+        model="gpt-transcribe",
+        settings=STTModelSettings(language="fr", prompt="domain vocabulary"),
+        trace_include_sensitive_data=False,
+        trace_include_sensitive_audio_data=False,
+    )
+    websocket = AsyncMock()
+    session._websocket = websocket
+
+    await session._configure_session()
+
+    payload = json.loads(websocket.send.await_args.args[0])
+    assert payload["session"]["audio"]["input"]["transcription"] == {
+        "model": "gpt-transcribe",
+        "languages": ["fr"],
+        "prompt": "domain vocabulary",
+    }
+
+
+@pytest.mark.asyncio
+async def test_streaming_stt_sends_plural_languages_for_gpt_live_transcribe() -> None:
+    session = OpenAISTTTranscriptionSession(
+        input=StreamedAudioInput(),
+        client=AsyncMock(api_key="FAKE_KEY"),
+        model="gpt-live-transcribe",
+        settings=STTModelSettings(language="fr", prompt="domain vocabulary"),
+        trace_include_sensitive_data=False,
+        trace_include_sensitive_audio_data=False,
+    )
+    websocket = AsyncMock()
+    session._websocket = websocket
+
+    await session._configure_session()
+
+    payload = json.loads(websocket.send.await_args.args[0])
+    assert payload["session"]["audio"]["input"]["transcription"] == {
+        "model": "gpt-live-transcribe",
+        "languages": ["fr"],
+        "prompt": "domain vocabulary",
+    }
+
+
+@pytest.mark.asyncio
+async def test_streaming_stt_omits_unset_language_and_prompt() -> None:
+    session = OpenAISTTTranscriptionSession(
+        input=StreamedAudioInput(),
+        client=AsyncMock(api_key="FAKE_KEY"),
+        model="gpt-4o-transcribe",
+        settings=STTModelSettings(),
+        trace_include_sensitive_data=False,
+        trace_include_sensitive_audio_data=False,
+    )
+    websocket = AsyncMock()
+    session._websocket = websocket
+
+    await session._configure_session()
+
+    payload = json.loads(websocket.send.await_args.args[0])
+    assert payload["session"]["audio"]["input"]["transcription"] == {
+        "model": "gpt-4o-transcribe"
+    }