Ffmpeg

FFmpeg Skill

Hard rules

1. Never overwrite the original. Write to a new file. Derive the name if none given: video.mp4 becomes video_compressed.mp4. Never use -y on the same path as the input.

2. Never destroy quality. Minimum floors:

   Codec	Flag	Floor	Good default
   H.264 (libx264)	-crf	28 max	18-23
   H.265 (libx265)	-crf	32 max	24-28
   VP9 (libvpx-vp9)	-crf	40 max	31-36
   AAC	-b:a	128k min	192k
   MP3	-q:a	4 max	2

   If the target size requires going below these floors, say so. Offer the best size achievable at the floor instead.

3. File over 1 GB? Stop. Say: "This file is [size]. Processing will take a while and produce large intermediates. Continue?" Only proceed with confirmation.

Step 1 — Inspect first

Always run this before building any command:

ffprobe -v quiet -print_format json -show_format -show_streams "<input>"

Read the size, codec, resolution, bitrate, duration. Check the 1 GB rule here.

Step 2 — Pick the right approach

Compression: libx264 -crf 23 -preset slow. Copy audio unless asked otherwise: -c:a copy.

Format conversion: If only the container changes, use -c copy. No re-encode. No quality loss. Instant.

Trimming: -ss before -i for fast seek. -c copy when possible.

Audio extraction: -vn -c:a copy to extract without re-encoding.

Scaling: -vf scale=1280:-2 to preserve aspect ratio. Always pair with a re-encode.

Step 3 — Show the command before running

ffmpeg -i "input.mp4" -c:v libx264 -crf 23 -preset slow -c:a copy "input_compressed.mp4"
# -crf 23     : quality (lower = better)
# -preset slow : smaller file, same quality
# -c:a copy   : audio untouched

Show it. Explain non-obvious flags. Ask before running on large files or long operations.

Step 4 — Report after

• Original size vs output size.
• Any quality warnings from stderr.
• Output file path.

Output larger than input? Flag it. Suggest -c copy if applicable.

Concerns

Situation	Say
Target size needs CRF above floor	"Hitting [size] needs CRF [N]. Below acceptable quality. Best I can do at the floor is [size]."
User says "compress as much as possible"	"How small do you need it? Below CRF [N] it degrades noticeably."
Input already matches target format	"Already [format]. I can remux without re-encoding. Lossless and instant."
User asks to overwrite original	"Not overwriting the original. Saving as [derived name] instead."
File over 1 GB	Stop. Confirm before proceeding.