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Install

The launcher resolves a per-platform binary through npm optional dependencies: Linux x64/arm64, macOS x64/arm64, Windows x64. Nothing is downloaded at install time, and npm ci --ignore-scripts still works. Linux builds are static, so Alpine is supported.

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The ffmpeg requirement

splice does not bundle ffmpeg. It needs a build with libx264 and libass: libx264 encodes the picture, and libass draws captions and titles. A system build can be missing libass. Homebrew on macOS has shipped such builds; on GitHub’s macOS runners, ffmpeg 9.0.2 has no ass or subtitles filter. The symptom is specific: everything renders until a caption or title is added, then ffmpeg fails with No such filter: 'subtitles'. Check a build directly:
Both must print a line. If either is empty:
That downloads a pinned static build, verifies the sha256 of the uncompressed binary against pins compiled into splice, and installs it into a per-user cache. Both ffmpeg and ffprobe must verify before either is moved into place, so a mismatched download installs nothing.
This is explicit, not a postinstall: nothing is fetched behind your back. It is about 56 MB to download and 160 MB on disk, and it needs network once.

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Which binary wins

doctor reports ffmpeg_source, and the order is fixed:
  1. SPLICE_FFMPEG — a directory holding both tools, or a path whose sibling ffprobe is preferred.
  2. A copy beside the splice binary.
  3. The splice cache, where ffmpeg install puts one.
  4. PATH.
To go back to the system build, delete the cache directory — ~/.cache/splice/ffmpeg on Linux, ~/Library/Caches/splice/ffmpeg on macOS, %LOCALAPPDATA%\splice\ffmpeg on Windows — or point SPLICE_FFMPEG at the binary you want.

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Keeping it current

splice update asks the npm registry for latest and installs it with npm install -g splice-cli@latest; it needs npm on PATH, and it only moves forward, so a local build ahead of the registry is left alone.

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The agent skill

The npm package ships a splice skill and installs it into whichever skill directories already exist (~/.claude/skills, ~/.agents/skills, ~/.pi/agent/skills). It never creates one, and a failed copy never fails the install. If you installed with --ignore-scripts, run splice-skill; --dir puts it somewhere else, and SPLICE_SKILL=none skips it.