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Covers voice conversion, singing voice conversion, speech to speech and source separation, the four tasks LocalAI's AudioTransform can represent. AudioTransformResult carries one dst while htdemucs and mel_band_roformer produce several named stems from a single run, so inference runs ONCE, every stem is written as a sibling file <dst-stem>.<name>.<ext>, and params[stem] selects which one dst receives, defaulting to vocals and falling back to the first output. An unknown stem name is INVALID_ARGUMENT listing the real stem names rather than a silent substitution, and the selection happens before the first write so a refused request leaves no files behind. params[stem] is consumed here and is not forwarded into the engine's request options. The stem decision lives in stem_selection, which is stdlib only and therefore tested by backend/cpp/run-unit-tests.sh. It also validates the names, because they come from the model (htdemucs reads them from the GGUF's config.sources) and each becomes a component of a path this backend writes: a name carrying a path separator would escape the caller's output directory, and two stems sharing a name would silently overwrite one another. Both files are read at their native rate and channel count. Separation forces it, since demucs and roformer refuse any rate but 44.1 kHz and lose the stereo image that separates a centred vocal from a wide mix. The conversion families all resample internally (seed_vc, vevo2, miocodec, chatterbox were each checked), so passing the file through unchanged is also strictly better than band limiting it to 16 kHz first. Verified end to end against htdemucs f16 on a 44.1 kHz stereo mix: four stems plus dst, dst byte identical to the selected stem, params[stem] selecting a different one, an unknown stem refused with no files written, and mono input preserved as mono output. Also against miocodec for the single output path, where params[stem] is refused rather than ignored. Assisted-by: Claude:claude-opus-5 [Claude Code] Signed-off-by: Ettore Di Giacinto <mudler@localai.io>