MCP API — Audio / BGM

Overview

This section covers generating background music (BGM) and mixing it into the final video via MCP. The provider is Stable Audio 2.5 (fal.ai), and the create_bgm tool handles generation and callback end-to-end.

create_bgm

Pass a prompt and duration to generate an AI background music WAV. Async like create_clip, returns a taskId.

Parameters

• prompt (required) — BGM description in English. ", seamless loop" is auto-appended for loop-friendly output.
• secondsTotal (required) — 1-190 seconds. Match the final video length for best results.
• numInferenceSteps (optional) — 4-8, default 8. Higher = better quality, slower.
• guidanceScale (optional) — 1-25, default 1. Higher = stronger prompt adherence.
• seed (optional) — reproducibility seed
• workflowId (optional) — dashboard workflow association

Prompt Examples

• "upbeat cinematic orchestral, 120 bpm, inspirational"
• "lo-fi hip hop, relaxed, jazz piano, 85 bpm"
• "epic trailer music, tension build, strings and percussion"
• "ambient electronic, dreamy pad, 90 bpm, ethereal"

Pricing

500 coins (length-independent, auto-refunded on failure)

Output

• 44.1kHz stereo WAV
• Max 190 seconds
• Commercial license OK
• Auto-uploaded to Supabase Storage (public URL stored in bgm_url)

Automatic Mixing during Compose

If a BGM node is connected to the compose node in your workflow graph, compose_video automatically mixes it. Default processing:

1. Crossfade Loop

When BGM is shorter than the video, the loop boundary is crossfaded (0.5-1s overlap) for seamless continuation.

2. End-of-Video Fade Out

BGM volume fades to 0 in the last 1-2 seconds of the video (default fade_out_ms: 1500).

3. Volume Ducking

When source clip audio exists (Veo3/Seedance/Kling auto-generated), BGM volume is automatically lowered by -12dB.

Example Workflow

1. create_workflow → workflowId
2. create_clip for a 30s video (multiple scenes)
3. create_bgm for a 30s BGM

   {
     "prompt": "upbeat cinematic orchestral, inspirational, 120 bpm",
     "secondsTotal": 30,
     "workflowId": "wf_..."
   }

4. get_job_status polling until BGM is done (check bgm_url)
5. compose_video — BGM is auto-mixed if connected in the workflow

Note

LTX-2.3 models do not generate audio, so BGM is especially important. Veo3/Seedance/Kling generate audio automatically, so BGM should layer with ducking enabled.