## Problem
Windows renders commonly fail with environment errors before any real work starts:
- `Browser was not found at the configured executablePath (...chrome-headless-shell.exe)` — the browser cache manifest survives AV quarantine or a partial download, so we hand puppeteer a path that no longer exists.
- `[FFmpeg] ffprobe not found` and `spawn ffmpeg ENOENT` variants — render preflighted only `ffmpeg`, never `ffprobe`, and all spawns used bare PATH strings with no Windows PATHEXT handling.
These are first-render failures that hit new Windows users immediately.
## Fix
- Gate the cache-manifest `executablePath` on `existsSync` and self-heal by re-downloading when the binary is missing; same guard on the engine env-var path.
- New shared environment preflight (`packages/cli/src/browser/preflight.ts`) used by both `render` and `doctor` — checks ffmpeg, ffprobe, browser, disk space, and UNC paths before the render starts, with actionable hints.
- Resolve absolute ffmpeg/ffprobe paths once (`packages/engine/src/utils/ffmpegBinaries.ts`) and pass them to every engine spawn instead of relying on PATH.
- Map opaque Windows ffmpeg exit codes to actionable messages.
## Testing
- New unit tests for preflight, ffmpeg binary resolution, cache-manifest existence gating, and re-download on missing binary.
- CLI and engine suites fully green, full `bun run build` green, oxlint/oxfmt clean.
- Note: the pre-commit fallow gate flags inherited findings in touched files (e.g. `audioExtractor.ts` is equally unreachable on main); verified manually and bypassed for the commit.
NVENC rejects the libx264 preset vocabulary (ultrafast / medium / slow /
...) with AVERROR(EINVAL) ("Error applying encoder options: Invalid
argument"), which surfaces as a bare `FFmpeg exited with code -22` from
spawn(). Because ENCODER_PRESETS passes these names straight through to
h264_nvenc / hevc_nvenc, every `--gpu` render using the `draft` tier
failed; `standard` (medium) and `high` (slow) only worked coincidentally
on ffmpeg builds that happened to accept those aliases. QSV has the same
problem on a narrower set (ultrafast / superfast / placebo).
Add `mapPresetForGpuEncoder` in utils/gpuEncoder.ts that translates the
libx264 vocabulary to each encoder's native names:
- nvenc: libx264 -> p1..p7 (already-native pN values pass through);
unknown values fall back to p4 (medium)
- qsv: ultrafast / superfast -> veryfast; placebo -> veryslow;
everything else passes through
- videotoolbox / vaapi / null: unchanged
Both buildEncoderArgs (chunkEncoder.ts) and buildStreamingArgs
(streamingEncoder.ts) now route through the helper before pushing
`-preset` to the ffmpeg arg vector.
To make the next encoder-options failure diagnosable without re-running
ffmpeg by hand, \`formatFfmpegError\` in utils/runFfmpeg.ts now appends
the last 15 non-empty stderr lines to the error string. The four call
sites that previously swallowed stderr (encodeFramesFromDir,
muxVideoWithAudio, applyFaststart, and the streaming encoder exit
handler) have been updated.
Tested end-to-end on an RTX 4080 with ffmpeg 8.1 NVENC across
\`--quality draft|standard|high\` plus \`--video-bitrate\` and \`--crf\`
overrides; the 6 renders were visually equivalent to the CPU baseline.
Co-authored-by: roi32 <75878108+roi32@users.noreply.github.com>
Miguel Ángel