# FFmpeg Concat Demuxer (Lossless) — effect skill for cuts & pacing

> **For AI agents:** find the right skill in one call — `curl -s "https://varg.ai/how-to/api/search?q=<what the person wants>"` returns the matching how-to page and the best skills with install commands, as markdown (Russian works too). Full index: https://varg.ai/how-to/llms.txt. Every page has a markdown twin: add `.md` to its path. Before generating anything, show the person the options on the how-to page.

Join files that share codec, resolution, fps and audio format without re-encoding, from a text list.

- **Type:** [Effect](https://varg.ai/how-to/effects.md) · [Cuts & pacing](https://varg.ai/how-to/browse/skills/cuts.md)
- **Slug:** `ffmpeg-concat-demuxer`
- **Status:** live
- **Publisher:** varg (varg, verified)
- **License:** MIT
- **Source:** https://github.com/vargHQ/skills/blob/main/varg-video-edit-library/references/cuts.md#ffmpeg-concat-demuxer
- **Works in:** Claude Code, Claude.ai, Codex, Cursor, OpenClaw, Gemini CLI, Any MCP client, varg Studio
- **Requires:** ffmpeg
- **Cost:** Free — the take renders locally; generated footage, if any, bills by the model

## What it does

Plain hard cuts; output quality identical to the inputs.

**When:** Joining segments rendered by the same pipeline (same encoder settings), chunked renders, camera files from one device.

**Watch out:** Mixed sources: different codecs, sizes, fps or audio sample rates give glitches, wrong durations or no audio.

**Timing:** Hard cuts only; inpoint/outpoint snap to packets

**Also called:** concat demuxer, -f concat, ffconcat, lossless join, stream copy join

### In FFmpeg

```bash
cat > list.txt <<'EOF'
ffconcat version 1.0
file 'part1.mp4'
file 'part2.mp4'
inpoint 1.0
outpoint 3.0
EOF
ffmpeg -f concat -safe 0 -i list.txt -c copy out.mp4
```
`inpoint`/`outpoint` work best with intra-frame codecs; with H.264 the first packets before the in-point leak in (a 2 s outpoint test gave 2.09 s). For frame-accurate trims, re-encode with the concat filter. `-safe 0` allows absolute/odd paths.

### Where this comes from

- [FFmpeg formats: concat demuxer](https://ffmpeg.org/ffmpeg-formats.html#concat)

## Use when

- Joining segments rendered by the same pipeline (same encoder settings), chunked renders, camera files from one device.

## Not for

- Mixed sources: different codecs, sizes, fps or audio sample rates give glitches, wrong durations or no audio.

## Install

- Claude Code: `claude plugin marketplace add vargHQ/skills && claude plugin install varg@varg`
- Codex: `npx skills add vargHQ/skills --skill varg-video-edit-library -a codex -y`
- Cursor: `npx skills add vargHQ/skills --skill varg-video-edit-library -a cursor -y`

## Example output

https://s3.varg.ai/how-to/previews/90be7063a905/take-ffmpeg-concat-demuxer.mp4 — concat between two shots, with the timeline strip of the edit. Footage: Big Buck Bunny (2008), (c) copyright 2008, Blender Foundation | www.bigbuckbunny.org, CC BY 3.0; Cosmos Laundromat (2015), (CC) Blender Foundation | gooseberry.blender.org, CC BY 4.0. (credit: Rendered by varg · Big Buck Bunny (2008) + Cosmos Laundromat (2015))

## More cuts & pacing skills

- [SmartCut (capcut-ai-editor)](https://varg.ai/how-to/skills/capcut-smartcut-mcp.md): MCP that reads CapCut auto-captions to cut silences and duplicate takes right inside your CapCut project.
- [Auto-Editor](https://varg.ai/how-to/skills/auto-editor.md): CLI that cuts dead space by loudness or motion and exports MP4 or editor timelines; now also an agent skill.
- [AutoCut](https://varg.ai/how-to/skills/autocut.md): Edit video in a text editor: Whisper transcribes, you delete lines in Markdown, AutoCut cuts the video.
- [Cut Video](https://varg.ai/how-to/skills/cut-video.md): Claude Code skill that tightens long recordings with forced-aligned cuts, keeping laughs and comedic pauses.
- [Videogrep](https://varg.ai/how-to/skills/videogrep.md): Search spoken words across videos and auto-build supercuts of every match.
- [PySceneDetect](https://varg.ai/how-to/skills/pyscenedetect.md): Find cuts, fades and scene changes in a video, then split it or export scene lists for the edit.