youtube-chapter-clipper skill

---
name: youtube-chapter-clipper
description: Generate chapter clips from YouTube videos with yt-dlp and ffmpeg. Use when asked to download YouTube videos/subtitles, generate fine-grained chapters, cut precise clips, or generate per-chapter English SRTs.
---

# YouTube Chapter Clipper

## Overview

Generate chapter clips from a YouTube video by downloading MP4 + English subtitles, segmenting content, cutting clips, and producing per-chapter English SRTs. Chapter length is user-selectable (1-2, 2-3, or 3-4 minutes).

## Workflow

### 1) Use the automation script to reduce tokens

- Prefer `scripts/smart_edit.py` for end-to-end runs (download, chaptering, clip cut, subtitle slicing).
- The script uses heuristic chaptering to avoid AI token usage.
- Create and use a local venv (no external packages required):
  - `python3 -m venv .venv`
  - `source .venv/bin/activate`
  - `python scripts/smart_edit.py --help`
  - Speed-focused default: `--mode fast` (approximate cuts, faster encode, optional downscale).
  - Use `--mode accurate` when you need precise boundaries.

### 2) Confirm inputs and environment

- Ask for the YouTube URL and whether English subtitles are available (manual preferred; auto as fallback).
- Check tools: `yt-dlp` and `ffmpeg`. If missing, install before proceeding.
- Use command templates in `references/commands.md`.

### 3) Download source video and subtitles

- Check current directory for existing source files before downloading:
  - If `<id>.mp4` and `<id>.en.vtt` already exist, skip yt-dlp download.
- Download highest 1080p MP4 and English VTT. Save in current directory with ID-based names:
  - `<id>.mp4`
  - `<id>.en.vtt` (or `<id>.en.auto.vtt` if manual subs absent)
- Also capture video metadata (id, title, duration, uploader) for reporting.
  - The script handles this when `--url` is provided.

### 4) Prepare output directory

- Create output directory using the original video title:
  - Replace spaces with underscores.
  - Remove/replace filesystem-unsafe characters.
- Place all chapter clips and subtitle files into this directory.

### 5) Generate fine-grained chapters (user-selected length)

- Ask the user to choose a chapter length preset: 1-2, 2-3, or 3-4 minutes.
- Perform AI analysis (critical step):
  - Read the full subtitle content.
  - Understand the semantic flow and topic transitions.
  - Identify natural topic switch points.
- Draft chapter boundaries based on semantic topic changes and sentence boundaries.
- Target the selected range; avoid cutting mid-sentence.
- Prefer semantic breaks (new concept, example, recap) over strict timing.
- Produce a chapter list with:
  - `title`, `start`, `end`, `reason`
  - The script uses `--chapter-preset` (or `--min-seconds/--target-seconds/--max-seconds` for custom).

### 6) Cut precise clips (speed vs accuracy)

- Use ffmpeg with accurate trimming and stable outputs. Always re-encode:
  - Place `-ss` after `-i` for accurate seeking.
  - Use `libx264` + `aac`, `-movflags +faststart`, and `-pix_fmt yuv420p` to maximize player compatibility.
  - Use a fast preset (e.g., `-preset veryfast`) to avoid long encodes and timeouts.
- Run clips serially and avoid external timeouts that kill ffmpeg mid-write.
- After each clip, validate with `ffprobe`; retry once if validation fails.
- If speed is the priority (listening practice), prefer approximate cuts:
  - Put `-ss` before `-i` to avoid decoding from the start every time.
  - Use `-preset ultrafast` and a higher CRF (e.g., 28).
  - Optionally downscale (e.g., width 1280) to reduce encode time.
- Name each clip with an ordered prefix: `<nn>_<chapter_title>.mp4` using safe filenames:
  - Use a 2-digit index starting at 01.
  - Replace spaces with underscores.
  - Remove filesystem-unsafe characters.

### 7) Extract and convert subtitles per chapter

- Extract VTT segment for each chapter by time range.
- Convert each segment to SRT:
  - `<nn>_<chapter_title>.en.srt`
  - The script deletes per-chapter VTT unless `--keep-vtt` is set.

### 8) Report outputs

- Print output directory path, chapter list, and generated files.

## Output Rules

- Source files stay in current directory (`<id>.mp4`, `<id>.en.vtt`).
- All chapter clips and subtitle files are placed in the per-video directory named after the sanitized title.
- Use consistent time formats (`HH:MM:SS.mmm`).

## References

- Command templates and copy/paste examples: `references/commands.md`
- Automation: `scripts/smart_edit.py`