fix(mistral): keep the deployment voice default and drop the unreachable api base fallback

Review turned up two real problems in the TTS path.

Router.aspeech forwarded voice=None whenever the caller omitted it, which overwrote a
voice set in the deployment's litellm_params, so a configured fallback voice was
ignored on voice-less requests. It now leaves the key alone when no voice is passed.

get_complete_url also fell back to MISTRAL_API_BASE, but speech() always receives a
non-null api_base from get_llm_provider, whose mistral branch only reads
MISTRAL_AZURE_API_BASE and otherwise hardcodes the public host. That branch could
never run, and its unit test asserted a behavior the real path does not have. The
working override is api_base on the deployment, now pinned by an end-to-end test

litellm/llms/mistral/audio_speech/transformation.py

@@ -115,7 +115,7 @@ class MistralTextToSpeechConfig(BaseTextToSpeechConfig):
         api_base: str | None,
         litellm_params: Mapping[str, object],
     ) -> str:
-        configured_base: Final = (api_base or get_secret_str("MISTRAL_API_BASE") or self.TTS_BASE_URL).rstrip("/")
+        configured_base: Final = (api_base or self.TTS_BASE_URL).rstrip("/")
         versioned_base: Final = configured_base if configured_base.endswith("/v1") else f"{configured_base}/v1"
         return f"{versioned_base}/audio/speech"
 

litellm/router.py

@@ -4422,7 +4422,7 @@ class Router:
                 **{
                     **data,
                     "input": input,
-                    "voice": voice,
+                    **({"voice": voice} if voice is not None else {}),
                     "client": model_client,
                     **kwargs,
                 }

tests/test_litellm/llms/mistral/audio_speech/test_mistral_audio_speech_transformation.py

@@ -84,8 +84,7 @@ def test_transform_request_omits_voice_for_ref_audio_cloning():
     }
 
 
-def test_get_complete_url_default_base(monkeypatch: pytest.MonkeyPatch):
-    monkeypatch.delenv("MISTRAL_API_BASE", raising=False)
+def test_get_complete_url_default_base():
     config: Final = MistralTextToSpeechConfig()
     url: Final = config.get_complete_url(model="voxtral-mini-tts-2603", api_base=None, litellm_params={})
     assert url == SPEECH_URL
@@ -101,13 +100,6 @@ def test_get_complete_url_custom_base_always_versioned(api_base: str):
     assert url == "https://custom.api.example.com/v1/audio/speech"
 
 
-def test_get_complete_url_host_only_env_base_gets_v1(monkeypatch: pytest.MonkeyPatch):
-    monkeypatch.setenv("MISTRAL_API_BASE", "https://api.mistral.ai")
-    config: Final = MistralTextToSpeechConfig()
-    url: Final = config.get_complete_url(model="voxtral-mini-tts-2603", api_base=None, litellm_params={})
-    assert url == SPEECH_URL
-
-
 def test_validate_environment_sets_bearer_header():
     config: Final = MistralTextToSpeechConfig()
     headers: Final = config.validate_environment(

tests/test_litellm/test_main.py

@@ -3287,3 +3287,21 @@ def test_speech_mistral_dispatches_and_decodes_audio(respx_mock: respx.MockRoute
     }
     assert mock_route.calls.last.request.headers["authorization"] == "Bearer sk-mistral-test"
     assert response.content == audio_bytes
+
+
+def test_speech_mistral_routes_to_configured_api_base(respx_mock: respx.MockRouter, monkeypatch: pytest.MonkeyPatch):
+    monkeypatch.setenv("MISTRAL_API_KEY", "sk-mistral-test")
+    audio_bytes: Final = b"ID3-gateway-bytes"
+    gateway_route: Final = respx_mock.post("https://mistral.gateway.internal/v1/audio/speech").mock(
+        return_value=httpx.Response(200, json={"audio_data": base64.b64encode(audio_bytes).decode()})
+    )
+
+    response: Final = litellm.speech(
+        model="mistral/voxtral-mini-tts-2603",
+        input="hello from litellm",
+        voice="en_paul_neutral",
+        api_base="https://mistral.gateway.internal",
+    )
+
+    assert gateway_route.called
+    assert response.content == audio_bytes

tests/test_litellm/test_router.py

@@ -12278,6 +12278,56 @@ async def test_router_aspeech_without_voice_dispatches_ref_audio_cloning(respx_m
     assert response.content == audio_bytes
 
 
+@pytest.mark.asyncio
+async def test_router_aspeech_without_voice_keeps_deployment_default_voice(respx_mock, monkeypatch):
+    import base64
+
+    monkeypatch.setenv("MISTRAL_API_KEY", "sk-mistral-test")
+    monkeypatch.setattr(litellm, "disable_aiohttp_transport", True)
+    audio_bytes = b"RIFFfake-wav-bytes"
+    respx_mock.post("https://api.mistral.ai/v1/audio/speech").respond(
+        json={"audio_data": base64.b64encode(audio_bytes).decode()}
+    )
+    router = Router(
+        model_list=[
+            {
+                "model_name": "voxtral-tts",
+                "litellm_params": {"model": "mistral/voxtral-mini-tts-2603", "voice": "en_paul_neutral"},
+            }
+        ]
+    )
+
+    await router.aspeech(model="voxtral-tts", input="use my default")
+
+    request_body = json.loads(respx_mock.calls.last.request.content)
+    assert request_body["voice_id"] == "en_paul_neutral"
+
+
+@pytest.mark.asyncio
+async def test_router_aspeech_request_voice_overrides_deployment_default(respx_mock, monkeypatch):
+    import base64
+
+    monkeypatch.setenv("MISTRAL_API_KEY", "sk-mistral-test")
+    monkeypatch.setattr(litellm, "disable_aiohttp_transport", True)
+    audio_bytes = b"RIFFfake-wav-bytes"
+    respx_mock.post("https://api.mistral.ai/v1/audio/speech").respond(
+        json={"audio_data": base64.b64encode(audio_bytes).decode()}
+    )
+    router = Router(
+        model_list=[
+            {
+                "model_name": "voxtral-tts",
+                "litellm_params": {"model": "mistral/voxtral-mini-tts-2603", "voice": "en_paul_neutral"},
+            }
+        ]
+    )
+
+    await router.aspeech(model="voxtral-tts", input="override me", voice="gb_oliver_neutral")
+
+    request_body = json.loads(respx_mock.calls.last.request.content)
+    assert request_body["voice_id"] == "gb_oliver_neutral"
+
+
 class TestPreRoutingTierDrivesFallbacks:
     """#38832: a complexity/auto router picks a tier behind the router name, but fallback
     lookup stayed on the router name, so the tier's configured chain never ran and a