docs(audio-cpp): document params[text] and the pinned transform tasks

The text channel and the two task pins are both invisible from the endpoint
contract alone: nothing in the AudioTransform form tells a reader that a
speech-to-speech model needs the line it is resynthesising, and nothing says
that asking for singing voice conversion without task:svc silently gets plain
voice conversion instead. Both are the kind of thing a user only discovers
from a refusal or, worse, from output that looks right and is not.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Assisted-by: Claude:claude-opus-5 [Claude Code]

docs/content/features/audio-cpp.md

@@ -180,6 +180,36 @@ the body, is listed in the `X-Audio-Stems` response header. See
 `params[stem]` against a model that produces a single unnamed output is refused rather than
 ignored.
 
+## Text-conditioned transforms
+
+`s2s` is a text-and-prosody route, not an audio-only one: `vevo2` resynthesises a line in
+another voice and reads the line itself, refusing the run without it. `AudioTransform` has
+no text field, so the text arrives as `params[text]` (or `params[target_text]`, the name
+vevo2's own option table uses) and the backend lifts it into the engine's text input.
+`params[language]` rides along when the line is not English.
+
+```bash
+curl http://localhost:8080/audio/transformations \
+  -F model=audio-cpp-vevo2-speech-to-speech \
+  -F audio=@source.wav -F reference=@target-speaker.wav \
+  -F 'params[text]=The quick brown fox jumps over the lazy dog.' -o converted.wav
+```
+
+A model that takes no text ignores the keys, so separation and plain voice conversion are
+unaffected.
+
+## Pinned tasks
+
+Two routes are unreachable by auto-routing and need `task:` in the model config, because
+nothing in a request distinguishes them from a task the same family also advertises:
+
+- `task:svc` for singing voice conversion. `seed_vc` and `vevo2` advertise both `svc` and
+  ordinary `vc`, and no request signal means "this input is singing", so `vc` always wins.
+- `task:s2s` for speech to speech, for the same reason against the same `vc`.
+
+The gallery entries that ship these routes carry the pin already. Removing it gives plain
+voice conversion from the same weights.
+
 ## Family notes
 
 - **Supertonic**: use the `orig` GGUF package, whose weights are f32. The f16 package was

docs/content/features/audio-transform.md

@@ -56,6 +56,7 @@ form-data, returns audio bytes.
 | `sample_rate` | int | no | Desired output sample rate in Hz. Omit it for the backend's own rate; otherwise it must be between 8000 and 192000, and a value outside that range is refused with a 400 |
 | `params[<key>]` | string | no | Repeated; forwarded to backend |
 | `params[stem]` | string | no | Multi-output transforms only; picks which named output the body carries (see [stems](#multi-output-transforms-source-separation-stems)) |
+| `params[text]` | string | no | Text-conditioned transforms only; the line being resynthesised (see [text-conditioned transforms](#text-conditioned-transforms-speech-to-speech)) |
 
 First install an audio-transform model from the gallery (the examples below use `localvqe-v1.3-4.8m`):
 
@@ -143,6 +144,28 @@ The stems live in the generated-content directory beside the main output and are
 served from `/generated-audio/`. Like every other generated artifact, they are
 not swept automatically.
 
+### Text-conditioned transforms (speech to speech)
+
+Most transforms are audio in, audio out. A few are not: a speech-to-speech
+model resynthesises a line in another voice and needs to know what the line
+says. `/audio/transform` has no text field, so the text travels as
+`params[text]`.
+
+```bash
+curl -sS -X POST http://localhost:8080/audio/transformations \
+  -F model=audio-cpp-vevo2-speech-to-speech \
+  -F audio=@source.wav \
+  -F reference=@target-speaker.wav \
+  -F 'params[text]=The quick brown fox jumps over the lazy dog.' \
+  -o converted.wav
+```
+
+For these models the text is required, not a hint: the run is refused without
+it. `params[target_text]` is accepted as the same field, and
+`params[language]` sets the language when the line is not English. A model that
+does not take text ignores all three, so an ordinary separation or
+noise-suppression request is unaffected.
+
 ## Streaming endpoint
 
 `GET /audio/transformations/stream` - bidirectional WebSocket. The first