Why capability preflight comes before FFmpeg execution

An FFmpeg command can be syntactically valid and still fail because the local build lacks an encoder, muxer, filter, protocol, or suitable input stream. Checking only that ffmpeg exists moves this failure into the destructive part of a job, where partial outputs and misleading automation become possible.

PyFFmpegCore inventories the selected binary, maps each typed workflow to required capabilities, probes the input, checks stream requirements, output collision, writable space, estimated disk space, and container support, then returns one versioned PreflightReport. Human --explain and machine --plan-json views serialize the same facts.

pyffmpegcore profile run web/mp4-compatible \
  --input source.mov --output web.mp4 \
  --explain

If libx264 is absent, the report names encoder:libx264 and supplies a tested fallback or platform remedy when the capability catalog has one. It does not silently choose a different output contract.

The implementation lives in pyffmpegcore/capabilities.py and pyffmpegcore/preflight.py; the cross-platform catalog contract is exercised by scripts/validate_capability_catalog.py. The maintained rules are bounded: preflight reduces known environmental failure, but it cannot prove subjective media quality or prevent the operating system from changing after the check.