Motion-design video analysis

Turns a short (under ~1 min) motion-design clip into two things: an analysis (how it was made and how it feels) and a rebuild spec (numbers someone can build from without reopening the video). Validated on a 28.6 s product-update clip (Linear's "Loops updates") and reused as the judge on a replica of it.

The model is a fast first reader. You are the ground truth. Both models tested on the Linear clip confidently described a "flat 2D view that becomes 3D at 7.5s"; the frames show a 3D scene from t=0. Plan for that class of error.

Prerequisites

• ffmpeg on PATH. Worker images have a full static build at /usr/local/bin/ffmpeg (has fps, tile, scale, crop, hstack; no drawtext, no ffprobe: read ffmpeg -i stderr, or run the community template's setup.sh to add ffprobe). The Remotion-bundled ffmpeg under node_modules/@remotion/compositor-* is stripped (no fps, tile or hstack); don't use it for analysis.
• Pixel maths: python3 -m venv /tmp/venv && /tmp/venv/bin/pip install numpy pillow (system pip is blocked by PEP 668; this venv install was run and works).
• An OpenRouter credential the script can use. In an Agent Swarm, bind it as an egress credential so the key never enters the script (the header Authorization: Bearer [REDACTED:OPENROUTER_API_KEY] resolves at egress in ephemeral script-run only, see Execution notes). Outside a swarm, any way of calling OpenRouter works.
• Getting bytes off agent-fs: agent-fs download <path> -o <file>. Viewer URLs serve an SPA shell, not the file; use agent-fs signed-url <path> --json when a script needs to fetch it. Upload binaries with agent-fs write <path> --file <local> (binary-safe).

Step 1 — probe the file

ffmpeg -hide_banner -i in.mp4 2>&1 | grep -E "Duration|Stream"