audio joins the vocabulary, and -vn is a flag rather than an option

FORMATS was mp4/webm/mp3/image2 — a web-delivery list, because that is what av-club
was first for. a right-click on a .mkv that offers "mp4 or webm" is not a conversion
tool, so: mkv, mov for video; m4a, flac, wav, ogg, opus beside mp3 for audio.

the KEY is what a caller asks for; `container` is what ffmpeg is told, because
ffmpeg's names for two of them are not what anybody calls them — `matroska` for
.mkv, `ipod` for .m4a. making a caller know that is making them learn ffmpeg trivia
to convert a file.

audio encoders too: libmp3lame, libvorbis, flac, alac, pcm_s16le beside aac and
libopus, plus q:a for the encoders that want a quality rather than a bitrate, and
map_metadata/id3v2_version so tags survive a conversion.

A REAL BUG FOUND ON THE WAY, and it had shipped. `an` was declared as an enum whose
only value was "1", so it emitted `-an 1` — and `-an` takes no argument, so ffmpeg
read the `1` as an output filename and died with "Error opening output files:
Invalid argument", naming neither the option nor the problem. `an` had never worked.
I reproduced it exactly by adding `vn` the same way.

so there is a `flag` kind now: given `true` it emits the option alone, and anything
else is refused with a message that says so. `vn: "1"` is a 400 rather than a
mystery.

verified through the running service: mkv remux, mov, and extraction to all six
audio formats — mp3, flac, wav, ogg, opus, m4a — all 200.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

sidecar/ffmpeg/run.ts

@@ -38,8 +38,12 @@ export type FfmpegRequest = {
 
 const BITRATE = /^[0-9]+(\.[0-9]+)?[kKmM]?$/;
 
-function value(spec: FfmpegParam, raw: unknown, path: string): string {
+/** `null` means "emit the flag with no value" — see the `flag` kind in vocabulary.ts. */
+function value(spec: FfmpegParam, raw: unknown, path: string): string | null {
   switch (spec.kind) {
+    case "flag":
+      if (raw !== true) throw invalid(`${path} must be true, or left out`, path);
+      return null;
     case "int":
       if (typeof raw !== "number" || !Number.isInteger(raw)) throw invalid(`${path} must be an integer`, path);
       if (raw < spec.min || raw > spec.max) throw invalid(`${path} must be between ${spec.min} and ${spec.max}`, path);
@@ -71,7 +75,11 @@ function options(table: Record<string, FfmpegParam>, raw: unknown, path: string)
     if (!spec) {
       throw invalid(`Unknown option ${JSON.stringify(key)}. Accepted here: ${Object.keys(table).join(", ")}`, `${path}.${key}`);
     }
-    args.push(`-${key}`, value(spec, entry, `${path}.${key}`));
+    const resolved = value(spec, entry, `${path}.${key}`);
+    // A flag is the option and nothing else. Pushing an empty string here would hand ffmpeg an
+    // argument it reads as a filename, which is the bug this replaced.
+    if (resolved === null) args.push(`-${key}`);
+    else args.push(`-${key}`, resolved);
   }
   return args;
 }
@@ -199,7 +207,9 @@ export async function runFfmpeg(body: FfmpegRequest): Promise<FfmpegResult> {
       ...presetOptions,
       // After the preset, so an explicit option wins.
       ...outputArgs,
-      "-f", format,
+      // ffmpeg's own container name, which is not always what the format is called — `matroska` for
+      // mkv, `ipod` for m4a. The caller says the familiar one.
+      "-f", FORMATS[format]!.container ?? format,
       output,
     ];