---
name: scenario-video-assembly
description: "Use when generated clips must become a finished video on Scenario via MCP: cutting a shot list together, laying a timeline, concatenating with transitions, overlaying a logo or title card, adding a music bed or voiceover, burning in or exporting captions and subtitles, trimming and splitting, or producing vertical and square variants of one master. Keywords: edit, assemble, stitch, concat, compositor, timeline, layers, overlay, captions, SRT, ad, UGC, social cut."
license: MIT
---

# Scenario Video Assembly

## Overview

Scenario has no editing tools on the MCP surface. Every compositor, concatenator, trimmer and captioner is a deterministic model run through `model_run`. The compositor, concatenator and trimmer ids are fixed and named below; Scenario ships two captioners, so that one is discovered with `recommend`. Connection and the core loop: see the `scenario` skill; the clips themselves: see `scenario-video`. If a sibling skill named here is missing from your available skills, ask the user to install it (`npx skills add scenario-labs/skills --skill <name>`); unattended, proceed from tool schemas and flag the gap.

## Quick reference: pick the backend first

| Need                                       | Model                          |
| ------------------------------------------ | ------------------------------ |
| Clips end to end, optional transitions     | `model_scenario-video-concat`  |
| Anything overlapping: overlay, bed, titles | `model_scenario-compose-video` |
| Still layout (thumbnails, sheets, key art) | `model_scenario-compose-image` |

These ids are constants: each is Scenario's single deterministic tool for its operation, so discovery would only re-derive it.

Concat is sequential: `videos` takes 2 to 50 files (never 1), `preserveAudio` (default true) carries each clip's own track into the cut, and the optional `transitions` array's "length must be number of videos - 1". Each entry is an object `{type, duration}`, never a bare string: `type` is one of 18 names read off the schema (`fade`, the default, is the plain crossfade; `dissolve` is a different effect; `none` a hard cut) and `duration` runs 0.1 to 5 seconds, default 0.5. Omit the array for hard cuts throughout, the right call whenever the brief does not ask one shot to flow into the next.

`preserveAudio` is not a guarantee of sync: a concatenated cut has come back with its audio drifting against the picture, with and without transitions (a confirmed defect at authoring time). A concat of clips carrying dialogue or effects is verified, never trusted: `asset_get` the output and compare `properties.duration` with the inputs' sum (each transition shortens the cut by up to its `duration`), then download it and check that the audio and video streams run the same length (a local probe of the download is inspection, not editing) and that a visible cue (a cut, a mouth opening, an impact) lands on its sound. When speech must stay locked to the picture, assemble in Video Studio instead: each clip layer carries its own track at its `startTime`, so a drift introduced by joining has nowhere to arise, at the price of hard cuts or per-layer fades in place of the transition set. Concat stays the tool for silent clips and for cuts whose bed is laid afterwards.

Video Studio is an absolute timeline: `layers` holds 1 to 50 image, video or audio sources placed by `startTime` and stacked by `zIndex` (higher in front), with per-layer `fadeIn`/`fadeOut` rather than a between-clip array, so a cross-fade between two clips belongs in concat. At least one layer must be a video, so a music bed over a still is rejected.

## The Video Studio contract

Per layer, three timing ideas have confusable names:

- `startTime` and `endTime` place the layer on the timeline.
- `trimStart` and `trimEnd` are amounts shaved off the head and tail of the source, not in and out points: to play the first 20 seconds of a 90-second track, set `trimEnd: 70`.
- `duration` overrides the layer's length. The top-level `duration` is a different field, setting the whole composition, and applies only when `durationMode` is `"custom"`.

Everything is in seconds at `step: 0.1`, so do not promise frame-accurate cuts. `x` and `y` are strings taking pixels, percentages or words, and `anchor` says which point of the layer they address: `top-left`, `top-center`, `top-right`, `center-left`, `center`, `center-right`, `bottom-left`, `bottom-center`, `bottom-right` (the middle one is bare `center`), defaulting to `"top-left"`. A bottom-right logo is `x: "right"`, `y: "bottom"`, `anchor: "bottom-right"`, flush to the edge unless you inset it with a percentage.

`canvasMode` and `durationMode` both default to `"auto"`, computed from the layers: a custom frame is `canvasMode: "custom"` plus numeric `canvasWidth` and `canvasHeight` (there are no top-level `width`/`height`, and unlike the layers' string `width`/`height` these are numbers), and a custom length is `durationMode: "custom"` plus the top-level `duration`. `fit` only acts on a layer that also sets the layer's own `width` and `height`, strings like `x` and `y`: to reframe a clip, set both to the canvas size as bare pixel numbers in strings (`width: "1920"`, `height: "1080"`, the form of the schema's own `"0"` default; a percent string also works) and pass `"cover"` (crops) or `"contain"` (letterboxes), since the default `"fill"` stretches. Give an overlay layer an explicit `width` and `height` even at its native size: leaving them empty is documented as preserving the original but does not reliably, and a 560x80 strip came back at 1792x256 on a 1920x1080 canvas. The output field is `videoOutputFormat` here, `imageOutputFormat` in Image Studio, and `outputFormat` in concat and the trim utilities.

A layer's `type` (image, video, audio, or mask) is optional and otherwise inferred from `source`. There is no text layer, so titles, lower thirds and end cards arrive as rendered PNGs composited as image layers; `scenario-text-overlay` renders them letter-perfect.

## Worked example: three clips, a music bed, and captions

1. For every source, `asset_get` and read `properties`: the real `duration` (audio carries it too), `frameRate` on video, and `width`/`height` on video and images alike. A model asked for eight seconds does not always return exactly eight.
2. Add them up to get each clip's `startTime`, then `model_schema_get` on `model_scenario-compose-video`.
3. `model_run` it with `layers` holding the three clips at their computed `startTime`s (`zIndex: 0`), a logo PNG pinned by `x`/`y`/`anchor` at `zIndex: 1`, and the music as an audio layer trimmed to the clips' total with `trimEnd`, since auto `durationMode` computes from every layer, the bed included. `volume` (0 to 2) and `mute` are per-layer and apply to video layers too, so balance the bed from either side. Set `canvasMode: "custom"` with `canvasWidth` and `canvasHeight`; `fps` defaults to 30 and takes whole numbers from 1 to 120, so set it to the clips' measured `frameRate` when they agree (rounded), which is what step 1 read it for.
4. `jobs_wait`, then caption the finished cut rather than each clip: `model_scenario-caption-studio` (an authoring-time pick, since Scenario ships two captioners: re-discover with `recommend`) takes it as `video` and transcribes whatever audio the master actually carries, so lower the bed's `volume` rather than muting the dialogue. For Caption Studio, follow `scenario-caption-studio` for corrected-SRT input and rendered-caption verification rather than inferring an upload route from a file-typed schema: the exposed `upload_asset` kinds do not include text. It burns captions in by default, returns a sidecar when `outputSrt` is true, and positions them with `textPosition` (`top`, `middle` or `bottom`, default `bottom`, so move it off a bottom-edge overlay). Other captioners use their own live input contracts.
5. `asset_display` to review, then `asset_download` with `format` left unset (it is an image conversion target) for the file.

For a revision handoff, retain the clean master, source assets, actual submitted layer payload, measured timings, narration text and voice settings, title PNGs with their editable layout settings, and any corrected SRT and returned caption theme. Keep output ids linked to the recipe. Revise the requested layer and recaption from the clean master; do not stack new captions over a burned export. These records support reconstruction, not a promise of a native editor project or a tested replay in a new environment.

Trim, split and resize have their own tool models. Note `model_scenario-video-cut`'s `startTime`/`endTime` are source in and out points, not timeline placement.

## Common mistakes

- Reaching for an MCP tool like `video_compose`, or for local ffmpeg to do the edit itself. The editing surface is `model_run` on tool models; probing a download is fine.
- Handing concat one clip, or one transition per clip: the minimum is 2 videos, and transitions run one fewer.
- Trusting `dry_run` here as validation: it returns a cost estimate only, and assembly payloads are the most structurally complex in the catalog.
- Captioning a concatenated master before checking its sync: the captioner times its lines to the audio it hears, so drifted audio ships drifted captions on top of it.