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Two defects that made source separation unusable through LocalAI's own API, even though the backend served it correctly over gRPC. /audio/transform normalized every upload to 16 kHz mono s16 through utils.AudioToWav, with no way past it. htdemucs and mel_band_roformer refuse any rate but their checkpoint's own and separate a centred vocal from a wide mix using the stereo image, so every separation request through the HTTP API died with "HTDemucs prepare() sample rate mismatch: expected 44100, got 16000" while the same call over gRPC worked. The fold is not wrong, it is backend-specific: LocalVQE's echo cancellation genuinely wants 16 kHz mono and needs the reference in the same shape. So it becomes a declaration, BackendCapability.AudioTransformInputMono16k, set for localvqe and for nothing else. A backend that declares nothing gets its upload unchanged, which means no backend has to opt in to work. utils.AudioToWavPreservingShape is the non-folding conversion: a 16-bit PCM WAV passes through byte for byte at any rate and channel count, anything else is transcoded to WAV with its rate and channel layout kept. The other defect is that the run-once stem design bought nothing. A separation backend writes every stem beside dst from one inference, but AudioTransformResult carried only dst, so the other three were files no caller could find and a caller wanting all four had to run four separations. AudioTransformResult grows a repeated AudioTransformStem, the backend fills it, core/backend validates that each path really is inside the generated-content directory it handed over, and the endpoint publishes them as an X-Audio-Stems JSON header beside the existing X-Audio-Input-Url. JSON because a stem name is the model's own string and could contain any separator a hand-rolled format would use. Verified end to end through the HTTP endpoint with htdemucs f16 on a 44.1 kHz stereo file: 200 with a 44.1 kHz stereo body, all four stems named and fetchable through /generated-audio/, body byte identical to the selected stem, and params[stem]=drums returning a different one. The same upload sent to a model whose backend is localvqe still reaches the backend as 16 kHz mono, confirmed both by the engine's own rate refusal and by the persisted input file. Assisted-by: Claude:claude-opus-5 [Claude Code] Signed-off-by: Ettore Di Giacinto <mudler@localai.io>