# Helios API

> Helios as a hosted realtime API. One continuous, steerable clip on the main_video track. $0.0017 per billable second.

Source: https://windflow.online/models/helios · Realtime video generation · Windflow Labs

## What it is

Helios is a 14-billion-parameter, open-source video diffusion model from the PKU-YuanGroup research lab, built to generate long video continuously rather than as a fixed-length clip. On Windflow it runs as a realtime session: you set the opening prompt, start the stream, and keep steering the same continuous clip while frames arrive live.

## Specifications

- **Architecture:** 14B Diffusion Transformer, autoregressive chunked diffusion
- **Resolution:** Negotiated per session
- **Playout:** Negotiated per session
- **Chunk:** 33 native frames in the production profile
- **Control latency:** First chunk after roughly one chunk of generation
- **Inputs:** Text prompt (required before start) and an optional reference image for image-to-video
- **Outputs:** One continuous, steerable clip on the main_video track
- **Delivery:** WebSocket (H.264) · WebRTC
- **Session protocol:** sdk.realtime.v2
- **Price:** $0.0017 per billable second

## Pricing

$0.0017 per billable session-second ($6.12/hr of billable time). Billing starts at the session's first generation_started event and stops when the session is released or ends. Queueing, worker startup, time before generation starts and the documented reconnect grace are not billed. After generation starts, paused or unwatched time remains billable; release the owner session to stop the meter.

## Use cases

- Continuous generative video for ambient scenes, live backdrops and long takes within a session's limits.
- Prompt-steered storytelling where each prompt change continues the scene instead of cutting away.
- Image-to-video runs that start from a reference image and keep going.
- Prototyping interactive video products before committing to a heavier world model.

## Where it comes from

- [PKU-YuanGroup/Helios on GitHub](https://github.com/PKU-YuanGroup/Helios) — Apache-2.0

## FAQ

### Can I change the prompt while Helios is running?

Yes. set_prompt updates the running session while retaining latent scene memory; it does not guarantee unchanged subjects or geometry. Use scene_cut when you intend to clear that memory and start independent content.

### Can Helios start from an image?

Yes. A reference image is optional and turns the run into image-to-video. A text prompt is required before the stream starts, because it anchors the first chunk.

### Does prompt_accepted mean the new scene is already visible?

No. It acknowledges an ordered revision. prompt_applied identifies a model-conditioning boundary, and content_visible reports the first presented frame of the corresponding content epoch on the supported WebSocket path. Measure those stages separately.

### Can Helios edit a live webcam feed?

The published Helios profile generates from text and an optional reference image; it does not accept a source clip or webcam track. Evaluate SANA-Streaming or JoyAI-Video-Edit for live source-video editing.

### How much does Helios cost on Windflow?

Helios costs $0.0017 per billable session-second ($6.12/hr of billable time). Billing starts at the session's first generation_started event and stops when the session is released or ends. Queueing, worker startup, time before generation starts and the documented reconnect grace are not billed. After generation starts, paused or unwatched time remains billable; release the owner session to stop the meter. There is no subscription.

### How do I get access to Helios?

Windflow is in private beta. Request access from the sign-up page, or ask for a live demo of Helios. Approved accounts start with $5 and four concurrent sessions.

### Is Helios always running?

No. Model capacity is brought up on demand rather than kept standing. Live readiness for each model is published at api.windflow.online/v1/status, and a live demo can be arranged on request.

## Related guides

- [Choosing a realtime video model: a decision guide for 12 hosted models](https://windflow.online/guides/choosing-a-realtime-video-model)
- [How per-second pricing works for realtime video, with worked examples](https://windflow.online/guides/how-per-second-pricing-works)
- [How to add live AI video to your app and pay per second](https://windflow.online/guides/add-live-ai-video-to-your-app)
- [How to measure realtime video API latency and responsiveness](https://windflow.online/guides/measure-realtime-video-latency)
- [How to use the Helios API](https://windflow.online/guides/helios-api)
- [Realtime video API: generate, edit and control live AI video](https://windflow.online/guides/realtime-video-api)
- [Streaming a video diffusion model to the browser: WebRTC or WebSocket?](https://windflow.online/guides/stream-video-diffusion-webrtc-websocket)
- [Helios prompting guide: write and steer a live video](https://windflow.online/guides/helios-prompting-guide)
- [How to recover a realtime video stream after a dropped connection](https://windflow.online/guides/recover-a-dropped-realtime-video-stream)
- [How realtime video models work: autoregressive diffusion, chunks and the latency budget](https://windflow.online/guides/how-realtime-video-models-work)