How to Use the black-forest-labs/flux-3/start-end-to-video API: image to video integration guide (2026)

black-forest-labs/flux-3/start-end-to-video is an image to video model by Black Forest Labs: FLUX 3 Start-End-to-Video generates 5-20 second transition videos between required start and end images, using prompt guidance to control action, camera movement, visual style, motion continuity, and optional synchronized audio at 720P / 1080P output. Ready-to-use REST inference API, best performance, no coldstarts, affordable pricing. On NamiFusion you can try it in the playground (exact quote shown before you submit), then call it through one unified REST API — about $0.850 per typical run.

TL;DR

· Task type: image to video, provided by Black Forest Labs

· Pricing: about $0.850 per typical run ($1 = 100 credits), pay-as-you-go

· Getting started: playground (exact quote before submit) → create an API key → one POST request

· Tunable parameters: 7 (see the table below)

· No platform watermark; commercial use allowed (per the Terms of Service)

What can you build with black-forest-labs/flux-3/start-end-to-video?

Short-form video and ad clips: go from a prompt or a single image to publishable motion content.

Product demos and concept previews: validate storyboards and visual direction cheaply before any shoot.

A social content factory: batch vertical clips and ride trending topics fast.

Remixing existing assets: image-to-video brings static work to life and extends its lifespan.

How do you call this API?

1. Sign up on NamiFusion and create a key on the API Keys page (signup bonus credits can only be used on selected models).
2. Submit a task with the cURL example below, passing your key as a Bearer token.
3. Poll the task status with the returned task_uuid and read the output URLs when completed.

# 1) Submit — returns { "task_uuid": "..." }
curl -X POST "https://www.namifusion.com/api/v1/marketplace/run/black-forest-labs/flux-3/start-end-to-video" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
  "input": {
    "prompt": "A cinematic transition from a quiet sunrise over a misty mountain lake to a vibrant golden-hour scene at the same location. The camera slowly glides forward across the water as fog lifts, light intensifies, birds begin to circle overhead, and the environment becomes warmer and more alive. Natural motion, smooth continuity between the start and end frames, realistic water ripples, subtle lens movement, and atmospheric detail.",
    "start_image": [
      "https://d1s3annbwribom.cloudfront.net/marketplace/examples/2026/10/08/4c7f8c3d8f45.jpeg"
    ],
    "end_image": [
      "https://d1s3annbwribom.cloudfront.net/marketplace/examples/2026/10/08/40dcae8c87c3.jpeg"
    ],
    "aspect_ratio": "16:9",
    "resolution": "720p",
    "duration": 5,
    "generate_audio": true
  }
}'

# 2) Poll until status is "completed", then read the output URLs
curl "https://www.namifusion.com/api/v1/marketplace/run/tasks/TASK_UUID" \
  -H "Authorization: Bearer YOUR_API_KEY"

What parameters does it take?

prompt (Prompt): required

start_image (Start Image): required

end_image (End Image): required

aspect_ratio (Aspect Ratio): optional

resolution (Resolution): optional, default "720p"

duration (Duration): optional, default 5

generate_audio (Generate Audio): optional, default true

How do you write prompts for black-forest-labs/flux-3/start-end-to-video?

1. Subject first: lead with the actor and action, then add environment, lighting and style ("an astronaut sprinting through a neon street in the rain, cinematic, shallow depth of field").
2. Concrete nouns beat stacked adjectives: "85mm portrait lens, golden-hour backlight" works far better than "very beautiful, ultra high quality".
3. Change one variable per iteration: pin the seed to reproduce a result, tweak a single phrase at a time so you know what actually moved the output.
4. State negatives positively: instead of "no blur", ask for "sharp focus, crisp details".

How much does it cost?

Pay-per-use in credits. About $0.850 per typical run ($1 = 100 credits); the exact price varies with parameters like resolution or duration, and the playground shows the exact quote before you submit.