FFmpeg Compose

Endpoint

POST https://api.roasos.com/v1/assembly/compose

Send your API key as a bearer token. The key needs the scope api:write. Every paid request also needs an Idempotency-Key header.

The envelope fields quote_only, quote_id, webhook_url and wait_timeout_seconds behave the same on each model endpoint. Refer to Jobs and Quotes and credits.

Good at

Cut a finished ad from parts you already own: a video track for the shots, an audio track for the voice-over, an image track for a logo or an end card. Every time is in milliseconds. Keyframes inside one track must not overlap, because a track plays one source at a time; tracks overlap each other freely, which is what makes a timeline.

A timeline carries at most 8 tracks, 64 keyframes per track, 200 keyframes in total, and 12 distinct media URLs. The keyframe durations and the timeline end must each stay within 180,000 ms — three minutes. A longer timeline is refused with assembly_timeline_too_long. The whole body must be 64 KiB or smaller.

The media of this operation travels inside the timeline, in tracks[].keyframes[].url, and not in a top-level media field. EVERY one of those URLs is checked, not the first one. Each must be an immutable object on https://cdn.roasos.com that your workspace owns, in the role its track type implies: a video track takes video, an audio track takes audio, an image track takes an image. Any other URL is refused, and the refusal names the track id and the keyframe index.

The price is one credit, flat, whatever the timeline holds.

Pricing

The unit is one operation. Pricing key: compose.

The credits come from the roasOS pricing table. No other field changes the price. To get the exact price of your request, send it with quote_only: true first.

Modes

• compose

Provider endpoints: fal-ai/ffmpeg-api/compose.

Request fields

The schema is strict. A field that is not in this table gets 400 unsupported_option. The field names are the roasOS role names. One role has one name in every family.

Media

The media of this endpoint travels inside the timeline, at tracks[].keyframes[].url, and not in a top-level media field. Every URL there is checked, in the role its track type implies: a video track takes video, an audio track takes audio, an image track takes an image.

Every other role is refused: avatar_id, elements, product_id, product_images, references, reference_images, image_url, end_image_url, scene_id, scene_image, video_urls, video_url, audio_urls, audio_url, voice_id. A refused role gets 400 unsupported_option.

Prompt

None. This operation takes no prompt.

Write the timeline, not the intent. Give every track a short stable id so an error can name it. Start every clip at an exact millisecond; rounding a float here is the difference between a cut and a frame of black.

Send quote_only: true first. The quote runs the whole schema and every media check, so you learn about a refusal before you spend an idempotency key.

Not exposed

roasOS pins no provider field for this model.

Result

The result kind is video. Always present: url.

{
  "data": {
    "id": "gen_…",
    "kind": "video",
    "model": "compose",
    "status": "completed",
    "result": {
      "url": "https://cdn.roasos.com/workspaces/ws_01/result"
    }
  }
}

The files are immutable objects on https://cdn.roasos.com. A result URL is a valid media input for your next request.

Example

The three steps are always the same: quote, run, poll.

# 1. Quote. Nothing is spent.
curl -X POST https://api.roasos.com/v1/assembly/compose \
  -H "Authorization: Bearer roasos_sk_live_..." \
  -H "Idempotency-Key: 0f1c9e2a-compose-0001" \
  -H "Content-Type: application/json" \
  -d '{"quote_only":true,"tracks":[{"id":"example","type":"video","keyframes":[{"timestamp":0,"duration":1,"url":"https://cdn.roasos.com/workspaces/ws_01/uploads/video-1.mp4"}]}]}'
# 200 { "data": { "kind": "quote", "quote_id": "q_7Hk...", "credits": 1, "expires_at": "...",
#        "confirmation": { "model": "compose", "example": "per_operation" } } }

# 2. Run. Same body, same Idempotency-Key, quote_id in place of quote_only.
curl -X POST https://api.roasos.com/v1/assembly/compose \
  -H "Authorization: Bearer roasos_sk_live_..." \
  -H "Idempotency-Key: 0f1c9e2a-compose-0001" \
  -H "Content-Type: application/json" \
  -d '{"quote_id":"q_7Hk...","tracks":[{"id":"example","type":"video","keyframes":[{"timestamp":0,"duration":1,"url":"https://cdn.roasos.com/workspaces/ws_01/uploads/video-1.mp4"}]}]}'
# 201 { "data": { "id": "gen_...", "status": "queued", "credits_reserved": 1 } }

# 3. Poll, then read the result.
curl https://api.roasos.com/v1/jobs/gen_.../status -H "Authorization: Bearer roasos_sk_live_..."
curl https://api.roasos.com/v1/jobs/gen_.../result -H "Authorization: Bearer roasos_sk_live_..."

Today

The endpoint is POST /v1/assembly/compose.

It is live.

See also

• llms.txt for this model — the same facts as one plain-text file for an agent.
• GET https://api.roasos.com/v1/catalog/compose — the machine-readable form: enums, defaults, limits and prices. Refer to the API reference.
• Jobs — how to poll a job and read its result.
• Provider endpoints: fal-ai/ffmpeg-api/compose.