Azure Voice Live STT: How to enforce a hard per-session language lock (es-ES only)?

Question

Azure Voice Live STT: How to enforce a hard per-session language lock (es-ES only)?

Jurado, Jose Luis 0

Issue context:

We are using Azure Voice Live realtime STT in production voice sessions and need strict monolingual behavior per session.

Observed behavior:

We send session.update with input_audio_transcription.language set to es-ES.
- We also set turn_detection.type to azure_semantic_vad_multilingual.
- - Despite language=es-ES, STT still detects/transcribes other languages when speakers switch language.
- Expected behavior:
- - A hard language lock so STT only recognizes/transcribes Spanish (es-ES) for that session.
- Current payload example:
- {
- "type": "session.update",
- "session": {
- ```
"input_audio_transcription": {
```
- ```
  "model": "azure-speech",
```
- ```
  "language": "es-ES"
```
- ```
},
```
- ```
"turn_detection": {
```
- ```
  "type": "azure_semantic_vad_multilingual",
```
- ```
  "threshold": 0.5,
```
- ```
  "prefix_padding_ms": 500,
```
- ```
  "silence_duration_ms": 1400,
```
- ```
  "barge_in": true
```
- ```
}
```
- }
- }
Question:
1. Is strict hard language lock supported today for Voice Live STT per session?
2. 1. If yes, what exact parameter(s) and API version enforce it?
  2. 1. If not, what is the recommended workaround and roadmap?
  3. Any official guidance is appreciated.

AdityaJadhav-2653 0 Reputation points

2026-07-01T09:11:12.89+00:00
The issue isn't actually your transcription setting; it's your VAD (Voice Activity Detection).

When you set turn_detection.type to azure_semantic_vad_multilingual, you are essentially throwing a massive wrench into your own language lock. That specific VAD type tells Azure, "Hey, look out for speech boundaries across multiple different languages." Because it’s actively scanning for other languages to figure out when someone stops talking, it forces the downstream STT engine to switch into a multilingual mode. It completely overrides that "language": "es-ES" tag you passed it.

How to actually lock it down

To force a hard lock, you have to strip out the multilingual VAD. You need to use the standard, single-language semantic VAD instead.

If you drop the _multilingual suffix, the pipeline falls back strictly to a single language context. Here is exactly what your payload needs to look like to make that happen:

JSON

{

What to expect next

Once you swap azure_semantic_vad_multilingual to azure_semantic_vad, Azure will stop trying to be smart about other languages.

Because it's forced into a Spanish-only acoustic model, if a speaker switches to English or Hindi mid-session, the system won't transcribe it in that language anymore. Instead, it will either try to force those sounds into Spanish words (which will just look like gibberish phonetic Spanish) or it will drop the confidence score so low that it outputs nothing at all. Which, based on what you're building, sounds exactly like the guardrail you want.

Make sure you're hitting the latest API version (like 2026-04-10) when you push this config update, and that should solve the bleeding issue for you.
Jurado, Jose Luis 0 Reputation points

2026-07-01T09:44:30.9+00:00

I updated to turn_detection.type=azure_semantic_vad and api_version=2026-04-10, and my logs confirm both values are sent in session.update. However, English and Portuguese speech is still transcribed when input_audio_transcription.language=es-ES, so why is the change applied in payload but not enforcing a strict Spanish-only STT lock, and what exact setting should reject non-Spanish input?