Ffmpeg Skill

ffmpeg-skill

Scripts live in scripts/ next to this file; run them with python3 <skill-dir>/scripts/<name>.py, and delivery templates in templates/. This file is enough to do a job: the table below routes the request, and --help on the script about to run is the cheapest full flag list. A reference file costs as much as this one; open one only for a question you have: references/scripts.md (every flag of all 42 scripts), references/devices.md (iPhone HDR, GoPro, DJI, screen recordings, Zoom), references/gotchas.md (the long form of the one-line rules at the end). MCP: tools/list shows only the core 12 by default; the other 30 (per this table) stay callable by name via tools/call, described by contract --json; set FFMPEG_SKILL_MCP_FULL=1 for all 42.

Shared flags, on every script: --dry-run; --json (output path, a probe of the output, the commands run); --json-brief (status/output/verified plus a summary; prefer on writing steps); --fast (preview quality); --progress; --timeout SECONDS (kind: timeout, default 1800); --overwrite (step 7); --plan FILE (the dry run as a plan render.py FILE runs later; refuses if an input changed). Re-encoding tools also take --codec h264|hevc|av1|prores and --quality N: unset, SDR is x264, HDR is x265 Main10; prores needs -o NAME.mov, h264 refuses HDR (color.py --to-sdr first).

Writing tools run nothing under --dry-run; the measuring tools (probe, check, sync, multicam, scenes, cropdetect, report, silence, loudness, stabilize) may still run ffmpeg/ffprobe, skipping artifacts/side files (--edl, --sheet, a generated .ass); verify ignores the flag. Per tool: contract --json dry_run.

Workflow (in this order)

0. Environment, only on failure. Never start a job with doctor: a broken machine fails with kind: missing_tool or an ffmpeg error naming the filter/encoder. Run python3 <skill-dir>/scripts/_contract.py doctor (or npx ffmpeg-skill doctor) after such a failure, or when asked what the machine can do: read ok/usable, report the missing capability. contract --json's tool schema is for a planning agent, not this workflow.
1. Probe what you must plan from. Run probe.py on each input you plan from — duration, fps, resolution, codecs, channels, variable_frame_rate_suspected — and whenever the user asks about a file. No separate probe before every edit: every writing tool's --json already carries its input and a probe of the output. Plan from real numbers, never assumptions.
2. Prefer lossless. If the request can be met without re-encoding (plain cuts on keyframes, remuxing, audio-only changes), do not re-encode. cut.py and loudness.py stream-copy video by default; --accurate on cut.py only for frame-exact cuts.
3. Plan with --dry-run --json, then execute. Trust --json, not a dry run's summary line, for any number in the plan (a dimension there can be a placeholder). Use before long encodes, to report facts. --fast is preview quality, --progress prints percent/ETA on stderr. Never point -o at a file you did not create in this job unless the user asked for it to be replaced; pass --overwrite only then.
4. Chain in a sensible order. A delivery request with no other editing is one template run (render.py --template NAME INPUT), not a hand-built chain. Otherwise: colour (HDR→SDR / LUT) → cut → join → silence → fit → caption/overlay → sync → audio → loudness → export. Frame changes before captions, so text is sized for the final frame. Re-encode as few times as possible: intermediates at CRF 18, export.py last. Three or more steps: render.py with a project.json.
5. Check the deliverable. Before reporting, run check.py OUTPUT --platform X for the destination named (a template run already does). Format rows (codec, pixel format, size, true peak, colour tags, VFR) are safe to fix. Judgement rows change content: duration (cut loses material), aspect (crop loses edges), fps (drops motion), loudness (ambience must not be boosted) — fix only when the request implies the answer, else state the choice and its cost. Mention WARNs; do not chase them.
6. Verify the output. Confirm duration, resolution, fps and audio match the request, from the writing tool's --json/--json-brief probe or probe.py, and report those numbers ("final.mp4: 59.98 s, 1080x1920, 30 fps, AAC stereo"). A step is done only when the script exited 0 and the output probes as expected: a non-zero exit, a missing or empty file, or a probe that contradicts the request is a failure reported with the script's error message.
7. Keep the user's originals. Never overwrite the source; write new files next to input or where asked. An existing output path is refused (kind: input); --overwrite is the one way to say "yes, replace it".
8. Look at the picture. Whenever the picture changed (captions, overlays, graphics, crop/pad, resize, colour, transitions, a join.py scaling a clip to the first clip's frame, color.py --to-sdr) run look.py OUTPUT --tiles 3x2 (or --at T for one frame) and view the PNG (4x3 for whole-clip layout). Not finished until Look: names that PNG — a probe cannot see a caption on a face. Audio-only: Look: not needed. What to look for splits like check.py's rows in step 5:
   • Mechanical (this skill's own job to verify and report): the specified text/logo is at the specified position, subtitles appear at the specified timestamps, dimensions are even. Letterboxing from fit.py --fit pad is the correct result of that mode, never a defect to flag.
   • Judgement (report it, don't silently pass or fail): whether a subject or face is cut off, text sits over a face, colours look washed out, a transition lands. These need deciding what the subject is, the calling agent's call — say what you see in one line; they judge. With no vision capability, write Look: PATH (pixels not inspected; agent has no image view) — never claim a picture was inspected when it wasn't, without stalling for a capability that isn't there.

Before you run anything: what to ask, what to assume

Ask a short question only when the answer changes the output materially and the request does not imply it. When several things are open at once (a vague "make it for social media" leaves destination, aspect, length and captions open), don't ask one per turn: propose one bundle with your defaults and let the user change any part ("Reels: 9:16 padded, 60 s, -14 LUFS, no captions — OK?"). One question, one answer, then the run. Never ask for what probe.py can tell you.

• Destination decides aspect, length limit, loudness and codec; "for Reels" answers all four and names a template. No destination and a plain cut/caption: keep the source format and say so. If the user says "export", "post" or "deliver", ask where.
• Duration ("make it 60 s") without a method: speed up for ≤1.5× changes, trim otherwise, say which. Ask when the content is a talk (trimming loses words) and the change is large.
• Captions without a text source: --transcribe if a local whisper exists, else ask for the text or a timed file; never invent dialogue.
• Fonts and brand: if the user mentions a brand, colours or "our font", ask for or create brand.json once, reuse it.
• CJK / non-Latin text: let the tool pick the font by script (--font turns it off); --lang ja|ko for Han-only text. doctor --json .fonts.scripts says what renders here. Tofu is a failed job.
• Crop position for --fit crop: centre by default, but when the request or the source names an off-centre subject ("keep the product on the right", someone visibly off-centre in the sheet) use --crop-x/--crop-y (0=left/top, 1=right/bottom) instead of guessing centre, or ask which edge to keep.
• Anything else (transition type, caption style): pick the conventional default, say what you picked, offer the alternative in one line.

What this skill does and does not decide

This skill cuts, joins, measures, syncs, exports and checks files — it executes an edit, it does not decide one. What belongs to the human, the calling agent or another skill:

• Which cut is right, or whether a deliverable is approvable — this skill measures and reports (check.py's PASS/WARN/FAIL, cut.py's duration error); the user or production agent decides whether that ships.
• What makes a highlight interesting — scenes.py --highlights ranks by a measured proxy (audio energy, duration): candidates, not a verdict.
• Thumbnail or cover composition — a design decision, not measurement.
• Understanding what a video is about — there is no vision here beyond look.py's contact sheets, for the calling agent's eyes, not for this skill to interpret.
• Judging what looks good — "apply this LUT", "correct exposure by +0.3 stops" (color.py) is mechanical; "grade this scene to look cinematic" belongs to a colour-grading skill (color-grading-skill) that decides the parameters, then calls color.py.
• The words in a caption — cue text is burned as written. Too long for the frame means a smaller size, --max-lines, or the user's own edit; never rewrite, shorten or paraphrase it, even when asked to "make it fit" — say so, offer --max-lines 1 at a smaller size.
• Picking a subject or region you were not given — "crop to x=200,y=0" is mechanical once the box is known; "crop to keep the speaker in frame" needs deciding what the speaker is, a judgement for the calling agent (from a look.py sheet).

The line: same input + same explicit parameters always producing the same verifiable output belongs here; anything depending on taste, understanding or what looks or sounds good belongs to whoever makes that judgement.

If a request needs an FFmpeg feature none of the 42 scripts expose, say so and name the closest built-in option — never guess a raw ffmpeg/ffprobe invocation outside scripts/*.py. It bypasses every guarantee this skill makes, so never a fallback when a script's flag doesn't cover something.

Request → script

This table and doctor --json's tools list are the source of truth for what exists: name only a script you have seen in one of them (there is no doctor.py, no trim.py, no subtitle.py).

Timestamp flags (--start, --end, --at, --from, --duration, --offset, and the times in cue and chapter files) take seconds, mm:ss(.fff), hh:mm:ss(.fff) or SMPTE hh:mm:ss:ff with @fps (00:01:02:15@29.97) — paste an NLE cue sheet as-is; length flags (--min-silence, --margin, --fade) are plain seconds.