---
type: email
from: guillermoAlv <notifications@github.com>
subject: [openai/openai-agents-python] Voice Incompatibility Issue with Twilio

 Media Streams: fable, onyx, and nova voices (Issue #1941)
received: 2026-03-06 08:09:31
date_header: Mon, 20 Oct 2025 03:57:26 -0700
priority: normal
status: awaiting_approval
uid: 33059
---

## Email Content

**From:** guillermoAlv <notifications@github.com>
**Subject:** [openai/openai-agents-python] Voice Incompatibility Issue with Twilio

 Media Streams: fable, onyx, and nova voices (Issue #1941)
**Date:** Mon, 20 Oct 2025 03:57:26 -0700

### Body (preview)

guillermoAlv created an issue (openai/openai-agents-python#1941)



### Please read this first



- **Have you read the docs?** Yes, [Agents SDK docs](https://openai.github.io/openai-agents-python/)

- **Have you searched for related issues?** Yes, no similar issues found regarding this voices



### Describe the bug

When using the OpenAI Realtime API with Twilio Media Streams, three specific voices (fable, onyx, and nova) do not produce intelligible speech audio. After a significant delay, they return audio that sounds like distorted noise rather than human speech. All other voices (alloy, echo, shimmer, ash, ballad, coral, sage, verse) work perfectly.





### Debug information

- Agents SDK version: v0.3.3

- OpenAI SDK version: >=1.55.1

- Python version: Python 3.13

- Audio format: g711_ulaw (both input and output)

- Integration: Twilio Media Streams

- Sample rate: 8 kHz



### Repro steps



Use the [twilio example](https://github.com/openai/openai-agents-python/tree/main/examples/realtime/twilio) with the following configuration:

```

session = await runner.run(

    model_config={

        "api_key": api_key,

        "initial_model_settings": {

            "input_audio_format": "g711_ulaw",

            "output_audio_format": "g711_ulaw",

            "voice": "fable",  # or "onyx" or "nova"

            "turn_detection": {

                "type": "semantic_vad",

                "interrupt_response": True,

                "create_response": True,

            },

        },

    }

)

```



Note: Changing the voice to shimmer, coral, alloy, or any other voice (except fable, onyx, nova) resolves the issue immediately.



### Expected behavior

All voices should produce clear, intelligible speech audio when using g711_ulaw format with Twilio Media Streams, similar to how alloy, echo, shimmer, ash, ballad, coral, sage, and verse work correctly.



Hypothesis: This might be related to how these specific voices are encoded/compressed in G.711 µ-law for

---

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