Build A Real-Time Voice Assistant with Mistral and FastRTC

Tue, 15 Jul 2025 19:34:36 +0000

In this post, I will demonstrate how to build a real-time voice assistant using Mistral AI and FastRTC.

Mistral AI is one of the leading LLM providers, and they have made their LLM API easily accessible to developers.

FastRTC, on the other hand, is a real-time communication library for Python that enables you to quickly turn any Python function into real-time audio and video stream over WebRTC or WebSockets.

Building A Real-time Voice Assistant

First, let's install the required libraries by running the code below in your terminal

pip install mistalai fastrtc

Next, set an environment variable and import the libraries. Create a .env file in your project and save your Mistral API key there

MISTRAL_API_KEY = "<your-api-key>"

Import the libraries

from mistralai import Mistral
from fastrtc import (ReplyOnPause, Stream, get_stt_model, get_tts_model)
from dotenv import load_dotenv
import os

load_dotenv()

To get your Mistral API key, you will need to create an account on their website.

In the above, we have imported Mistral and the specific methods that we need from FastRTC namely ReplyOnPause(), Stream(), get_stt_model(), and get_tts_model().

ReplyOnPause(): This method takes a Python audio function. It monitors the audio, and when it detects a pause, it takes it as a cue to give a reply.

Stream(): This method streams the audio reply.

get_stt_model(): This is used to access the speech-to-text model that is used to convert audio to text.

get_tts_model(): This is used to access the text-to-speech model that is used to convert text back into audio.

Now, let's activate the Mistral client with our API key stored in the .env file

api_key = os.environ["MISTRAL_API_KEY"]
model = "mistral-large-latest"

client = Mistral(api_key=api_key)

Here we are using Mistral large model. However, you can try out other Mistral models too. Additionally, you can actually plug any LLM into FastRTC and receive real-time voice responses.

We will now build the audio function that will take a prompt and return a response

stt_model = get_stt_model()
tts_model = get_tts_model()


def echo(audio):
    prompt = stt_model.stt(audio)
    chat_response = client.chat.complete(
    model = model,
    messages = [
        {
            "role": "user",
            "content": f"{prompt}"
        },
      ]
    )

    for audio_chunk in tts_model.stream_tts_sync(chat_response.choices[0].message.content):
        yield audio_chunk

Above, we wrote a function called echo, and the function takes an audio input, then passes that to the speech-to-text method, which is converted to a user prompt and given to the LLM. The response from the LLM is then passed to a text-to-speech method and is streamed synchronously.

Finally, we will run the application

stream = Stream(ReplyOnPause(echo), modality="audio", mode="send-receive")
stream.ui.launch()

This will launch the UI below at the URL: http://127.0.0.1:7860/

Now, you can activate the microphone and say something to your assistant, who will give you a reply immediately.

Change Voice

If you do not like the default voice, you can change that by passing an instance of KokoroTTSOptions() to the text-to-speech method.

First import KokoroTTSOptions() from FastRTC by adding it to the import tuple

from fastrtc import (ReplyOnPause, Stream, get_stt_model, get_tts_model, KokoroTTSOptions)

Next, define the options

tts_model = get_tts_model(model="kokoro")

options = KokoroTTSOptions(
    voice="af_bella",
    speed=1.0,
    lang="en-us"
)

Then pass the options to the text-to-speech method in your audio function

for audio_chunk in tts_model.stream_tts_sync(chat_response.choices[0].message.content, options = options)
        yield audio_chunk

For more voice options, you can check out KokoroTTS documentation

Complete Project Code

Here is the complete code that we have used to create the real-time voice assistant

import os
from fastrtc import (ReplyOnPause, Stream, get_stt_model, get_tts_model, KokoroTTSOptions)
from dotenv import load_dotenv
from mistralai import Mistral

load_dotenv()

api_key = os.environ["MISTRAL_API_KEY"]
model = "mistral-large-latest"

client = Mistral(api_key=api_key)

options = KokoroTTSOptions(
    voice="af_bella",
    speed=1.0,
    lang="en-us"
)

stt_model = get_stt_model()
tts_model = get_tts_model(model="kokoro")

def echo(audio):
    prompt = stt_model.stt(audio)
    chat_response = client.chat.complete(
    model = model,
    messages = [

        {
            "role": "user",
            "content": f"{prompt}"
        },
      ]
    )

    for audio_chunk in tts_model.stream_tts_sync(chat_response.choices[0].message.content, options=options):
        yield audio_chunk

if__name__=="__main__":
       stream = Stream(ReplyOnPause(echo), modality="audio", mode="send-receive")
       stream.ui.launch()

Now, instead of typing a prompt, you can give voice commands to an LLM and have it speak its response, just like a natural human conversation.

I hope you found this post useful. If you did, please share it with others; it might benefit them, too. Thanks for reading!

You can follow me on the following platforms:

GitHub: Ifeanyi55

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Coder Legion - Recent posts tagged mistral

Build A Real-Time Voice Assistant with Mistral and FastRTC

Building A Real-time Voice Assistant

Change Voice

Complete Project Code