fusion-runtime 0.1.2

A self-hosted voice agent runtime. Speech-to-text, the LLM and text-to-speech run together on one machine and stream into each other, so a reply starts playing while it's still being generated.

On an RTX 3090 with a 7B model: about 490 ms of processing once a turn ends, or 991 ms stopwatched from your last syllable — the difference is a silence wait you can configure. 127 tokens/sec, interruptions honoured mid-sentence.

Quickstart

Requires Python 3.11–3.13.

pip install fusion-runtime

An agent is one file. This is the whole thing:

# agent.py
from fusion_runtime import Agent, LLM, STT, TTS, Turns

agent = Agent(
    name="shopkart-orders",
    prompt="You are the order line for ShopKart. Keep answers to one short sentence.",
    greeting="Hi, this is ShopKart. How can I help with your order?",   # said as soon as a caller connects
    stt=STT("whisper-tiny.en"),          # or "whisper-small" for better accuracy
    llm=LLM("qwen2.5-0.5b-q4", max_tokens=256),
    tts=TTS("kokoro-v1.0", voice="af_heart"),
    turns=Turns(wait_ms=500, interrupt_after_ms=300),
)

frun models pull agent.py     # exactly the models it names, nothing else
frun up agent.py              # add --reload to restart on every edit

Then talk to it from a second terminal:

pip install "fusion-runtime[talk]"
frun talk

That is the whole loop — one file, two commands, a conversation. Talk over the agent to interrupt it.

frun up with no file runs a default agent if you just want to hear it work, and frun doctor checks libraries, GPU, models and audio and says how to fix what it finds.

In a browser instead

The runtime serves a browser client at http://localhost:8000 — the same one you would embed in your own page.

With no keys configured, open it and click Talk. With keys configured (FUSION_ACCEPTED_KEYS), a page can't hold a secret, so it needs a short-lived session token:

frun token        # prints a URL with a token in it — open that

Tokens are single-use and expire in about a minute. The page is handed its next one over the socket it already has, so a conversation keeps going without asking again. If you open the bare URL on a server with keys, the connection closes and the page says the token wasn't accepted.

Naming models

A model is a catalog id (frun models list), a file path, hf:owner/repo for anything on Hugging Face, or a URL for an OpenAI-compatible endpoint. A model on vLLM, SGLang or llama-server is named with that server in front — vllm:hf:Qwen/Qwen2.5-7B-Instruct-AWQ — and found at the server's usual address (url= for another). frun up starts vLLM and SGLang itself.

Settings are checked against the runtime that runs the model, so a misspelt one fails at startup with the name it probably meant instead of being ignored. For llama.cpp that includes flash_attn, kv_cache_type="q8_0" (half the context memory), use_mlock, main_gpu and llama_kwargs for anything else; for an endpoint, extra_body for server-specific sampling.

Secrets never go in the agent file — it names the variable holding a key (api_key_env="GROQ_API_KEY"), so agent.py is safe to commit.

Tools

A tool is a function. The model reads its name, docstring and type hints, calls it when it needs to, and answers with what it returned:

from fusion_runtime import Agent, LLM, tool

@tool
async def order_status(order_id: str) -> dict:
    """Look up where an order is and when it will arrive.

    Args:
        order_id: The order number, as the caller reads it out.
    """
    return await orders.lookup(order_id)

agent = Agent(prompt="...", llm=LLM("vllm:hf:Qwen/Qwen2.5-7B-Instruct-AWQ"), tools=[order_status])

What the model says before calling ("Let me check.") is spoken while the tool runs. Each call has a timeout (@tool(timeout_s=...), 10 s by default); ordinary functions run on a worker thread; a tool that fails tells the model what went wrong rather than ending the call; and talking over the wait cancels it. After max_tool_rounds calls in one turn (4) the model has to answer.

Tools need an LLM server that can call them — vLLM or SGLang (frun up starts them with tool calling on), llama-server (--jinja) or a hosted API. The in-process llama.cpp runtime can't, and frun up says so at startup. A runnable version is examples/tools_agent.py.

On your own site

<script src="https://your-server/fusion-runtime.js"></script>
<button id="talk"></button>
<script>FusionRuntime.attach({ button: "#talk" });</script>

The runtime serves the browser client it uses itself, so the page you demo with is the one your site embeds. With no url it connects back to wherever the script came from.

Browsers only allow a microphone on https://, so a deployment needs TLS and wss://. A page never holds an API key: your backend mints it a short-lived token.

Authentication

frun key new
FUSION_ACCEPTED_KEYS=web:frun_kR7m...

Without keys the server answers on localhost only, and frun up --host 0.0.0.0 refuses to start. frun talk, a backend or curl send the key in an Authorization header; a browser page gets a short-lived, single-use token from POST /v1/sessions instead, because a page can hold neither a secret nor a header.

Concurrency caps, message and audio limits, idle timeouts, origin allowlists and proxy trust all have working defaults — see the docs.

Performance

Measured, not estimated. The production profile as it ships — RTX 3090, Qwen 7B q4 + Whisper small + Kokoro on the one card — through the browser client, 21 September 2026:

	Median	Range
Processing — turn ends, audio comes back	~490 ms	288–657
Stopwatch from your last syllable	991 ms	858–1061
↳ of which: silence wait before the turn is judged over	~500 ms	turns.wait_ms
Speech-to-text	119 ms	58–329
LLM first token	27 ms	20–70
First token → first audio (a sentence gets written, then spoken)	430 ms	320–509
Text-to-speech real-time factor	0.09	speech is synthesized ~11× faster than real time
LLM tokens/sec	127	106–130

Two numbers, because there are two honest answers. A stopwatch started at your last syllable reads 991 ms. About 500 ms of that is the runtime waiting through silence to decide you've finished — which elapses while you're still finishing, so people don't experience it as waiting. What a caller feels is closer to the 490 ms of processing. Quote whichever you like, but say which one: a voice stack claiming a number under 500 ms is almost always measuring from "we decided the caller stopped", not "the caller stopped".

The stages don't sum, and that's not sleight of hand. Transcription of what you already said runs during the silence wait. And "first token → first audio" is mostly the language model writing a sentence — text-to-speech can't start on half a clause — so it is not a measure of how fast Kokoro is. Kokoro's own speed is the real-time factor: 0.09, or about 126 ms of compute for 1.4 seconds of speech.

Every figure is the runtime's own per-turn telemetry (frun talk --verbose, or the browser console), so you can reproduce them rather than trusting ours. Barge-in fired on every attempt.

Several callers at once

Measured on the same 3090, real WebSocket sessions, three turns each (scripts/concurrency_check.py). Response is the server's own figure: the turn ends, audio comes back.

Callers	In-process llama.cpp	vLLM	SGLang
1	~460 ms	398 ms	410 ms
4	~740 ms	698 ms	848 ms
8	~4600 ms	1086 ms	1054 ms
12	~7500 ms	1083 ms	1264 ms
16	—	2064 ms	1598 ms

With the model in-process, four callers is the ceiling. At twelve, the language model's first token takes 4790 ms of a 5312 ms response, while speech-to-text stays at 76 ms and text-to-speech at 469 ms: one llama.cpp context decodes one reply at a time.

With vLLM or SGLang, twelve callers answer in about a second. Both batch every caller's reply into each step on the GPU, and the language model's first token stayed between 36 and 72 ms from one caller to sixteen. The limit moves to speech, which still runs one caller at a time: Kokoro's first audio grows from ~350 ms alone to 1.5–2 s at sixteen callers, and Whisper grows with how long people talk (a 3-second question: ~200 ms alone, 1.6–2.2 s at sixteen). Batching speech is the next step, not the language model.

Tools hold up under load. Every caller asked about the same order on every turn: vLLM looked it up in 122 of 123 turns, SGLang in 109. On SGLang every session looked it up the first time; the turns without a lookup were the question asked a second, third or fourth time, answered from the lookup already in the conversation. That is correct, but if your data can change during a call, tell the agent to look things up again.

Measured 26 September 2026: Qwen2.5-7B-Instruct-AWQ, --gpu-memory-utilization 0.6, --max-model-len 4096, Whisper small and Kokoro on the same card, both servers with default settings otherwise. The in-process column is Qwen 7B q4 GGUF on llama.cpp, 21 September.

To run the model on vLLM or SGLang, name it in the agent and frun up starts the server for you:

llm = LLM("vllm:hf:Qwen/Qwen2.5-7B-Instruct-AWQ")     # frun up starts vLLM on :8002
llm = LLM("sglang:hf:Qwen/Qwen2.5-7B-Instruct-AWQ")   # ...or SGLang on :30000
llm = LLM("http://gpu-box:8000/v1", model_name="...")  # or any OpenAI-compatible server you run

Install the engine once, in its own environment (it brings its own torch and CUDA), and tell fusion where it is:

python3 -m venv ~/vllm-env && ~/vllm-env/bin/pip install vllm
export FUSION_LLM_ENGINE_ENV=~/vllm-env
frun up agent.py

frun up starts the server before loading speech, so it takes its share of the GPU first (60% by default, which fits a 4-bit 7B model next to Whisper and Kokoro on a 24 GB card), restarts it if it exits, and stops it with Ctrl+C. GPU memory, context length, tool parsing, extra engine flags and logs are in the docs.

Docker

	Image	Compose file
A laptop, or any machine without an NVIDIA GPU	ghcr.io/samarthurs18/fusion-runtime:cpu	docker/docker-compose.yml
An NVIDIA GPU server	ghcr.io/samarthurs18/fusion-runtime:latest	docker/docker-compose.gpu.yml (fusion + vLLM)

cd docker
docker compose run --rm fusion models pull     # once
docker compose up                              # or: -f docker-compose.gpu.yml

On a laptop, pip install fusion-runtime is simpler still. Keys, models and your own agent file are in the docs.

The frun CLI

frun up [agent.py]	Starts the server, and vLLM or SGLang if the agent names one. --host, --port, --reload, --llm-log
frun talk	Talks to it from a terminal, with a latency summary per turn
frun models list / pull	What's available, and downloading it
frun key new / keys list / token	Keys and browser tokens
frun doctor	Checks the machine and says how to fix what's wrong
frun version	The installed version. --version and -V work too

fusion-runtime works as an alias for frun.

Using it as a library

frun up agent.py covers running an agent. The pipeline can also run inside your own process — for a queue worker, a test, or a batch job over recorded calls — with no server involved. See examples/sdk_example.py, which is runnable, and the docs.

Documentation and contact

Everything else — configuration, turn detection, languages, the server API, telemetry, limits, GPU setup and deployment — is at fusion-runtime.dev/docs.

Site and docs	fusion-runtime.dev
Questions, or anything else	hello@fusion-runtime.dev
Security problems	security@fusion-runtime.dev — not a public issue, please (why)

Development

uv sync --extra dev --extra talk     # or: pip install -e ".[dev,talk]"
pytest

CI runs the suite on Python 3.11, 3.12 and 3.13. CONTRIBUTING.md has the layout, the design rules a review will hold you to, and how to add a runtime.

License

Apache-2.0. Embed it in a commercial product, rebrand it, ship it closed — keep the copyright notice and the NOTICE file in what you distribute, and don't use the project's name to imply it endorses you.

The models it downloads by default are permissive too (Whisper MIT, Silero VAD MIT, Qwen2.5 Apache-2.0, Kokoro Apache-2.0), so the whole default path is clear for commercial use. A model you point it at yourself carries its own licence — check that one before you ship it.