Offer Whisper's Translate to English only on models trained for it (canTranslate)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/Architecture.md

@@ -141,7 +141,7 @@ environment). See `WorkBreakdown.md` for the itemized checklist.)
    `AudioContext.decodeAudioData`, downmixes to mono, resamples to 16 kHz,
    transfers the `Float32Array` to the worker. For multilingual models the
    `transcribe` message also carries `language` (or auto-detect) and `task`
-   (transcribe | translate).
+   (transcribe | translate — translate only for models with `canTranslate`, D-WS4a).
 5. Worker runs the pipeline (chunked, with timestamps), streaming partial results
    back via `{type:"partial"}` messages; final result as `{type:"result"}`.
 

docs/Decisions.md

@@ -1224,3 +1224,20 @@ whole-file output is equally bad, which is exactly what the absolute bar
 exists to catch.
 **Consequence:** full `bun test` ≈ 19 min on CPU (en_alien ≈ 20 min of it
 on a cold run); the fast suite excludes this file.
+
+## D-WS4a — Whisper's "Translate to English" only on models trained for it (`canTranslate`)
+**Status:** Accepted (2026-10-29) · spec 001 Goal 4a
+`WhisperModel` gains `canTranslate: boolean` (`src/lib/models.ts`): **true** for multilingual
+tiny / base / small / medium, **false** for every `.en` model (no task token) and for
+`whisper-large-v3-turbo` — OpenAI's model card says turbo was fine-tuned on transcription data only
+and is not trained for translation, so its "translate" output is unreliable. Any future turbo-based
+fine-tune defaults to false until tested. `TranscriptionOptions` renders the Task control only where
+true (language stays). **Stale task:** the `task` state is kept, but `taskForModel(modelId, task)`
+maps "translate" to "transcribe" for a model that can't translate; App applies it on model switch
+(picker and the server-engine auto-switch to turbo) and again when building every file / live run's
+options, so a leftover "translate" can never reach the worker or the server. **Not changed:** the
+server's `parseLanguageAndTask` (D-032) still accepts "translate" for any model — the browser never
+sends it for turbo now, and a direct API caller asking faster-whisper to translate with turbo gets
+what faster-whisper does; rejecting it server-side would be a behaviour change for API users with no
+UI benefit. Speech in another language than English can still end up in English (or Danish) through
+text translation of the finished transcript (D-WS4b), for every model.

docs/WorkBreakdown.md

@@ -65,6 +65,8 @@ Notes (M1):
       surface active backend in UI
 - [x] Handle model switching (dispose old pipeline, load new)
 - [x] Language / task options for multilingual models (transcribe vs translate)
+- [x] Whisper translate only on models trained for it (`canTranslate`; hidden for
+      large-v3-turbo and `.en`, stale "translate" resets to transcribe — D-WS4a)
 - [~] Verify quantized variants (q4/q8/fp16) on both backends; record working
       combinations in `models.ts`