Detect the language once for auto-detect file passes on the chunked path (D-053)

Transformers.js has no Whisper language detection and silently decoded
Danish as English whenever "Skip silence" was off or unavailable.

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

docs/Architecture.md

@@ -163,6 +163,9 @@ environment). See `WorkBreakdown.md` for the itemized checklist.)
    (`VocabularyHint`, stored in localStorage by `lib/hint.ts`), raw as typed (D-048).
 5. Worker runs the pipeline (chunked, with timestamps), streaming partial results
    back via `{type:"partial"}` messages; final result as `{type:"result"}`.
+   With auto-detect on a multilingual model, it first detects the language on
+   the first 30 s (`worker/language.ts`, D-053) and pins it — Transformers.js
+   itself would default to English.
    A hint becomes `<|startofprev|> hint.` once per pass (`worker/prompt.ts`,
    cut to a 120-token prompt budget at a whole word) and is prepended
    to the decoder input of every 30 s window by a wrapper around the model's

docs/Decisions.md

@@ -1821,3 +1821,28 @@ chunked path. Live mode is unaffected.
 **Caveat:** the WER numbers behind this use reference transcripts generated
 with Whisper (owner, 2026-10-30), so they favour Whisper-like output; re-check
 once correct transcripts are available.
+
+## D-053 — "Auto" language on the chunked file path: detect once on the first 30 s, then pin it
+**Status:** Accepted (2026-10-30)
+Transformers.js 4.3.2 still has no Whisper language detection
+(`modeling_whisper.js`: `// TODO: Implement language detection` → "No
+language specified - defaulting to English"). With "Auto-detect" selected,
+a file transcribed on the chunked path — the fallback whenever "Skip
+silence" (D-052) is off or its VAD model is unavailable — was decoded as
+English: Danish `da_festuge` came out as a rough English translation (WER
+0.94 vs 0.14 with `da`, found in D-049's measurements).
+**Decision:** `src/worker/language.ts` `detectFileLanguage` reuses the
+long-form pipeline's one-step detector (D-049: encoder pass + one decoder
+step restricted to language tokens) on the first 30 s of the file, and the
+worker pins the result for the whole chunked pass. Only file passes on
+multilingual models without an explicit language; `.en` models and an
+explicit language are unchanged.
+**Verified:** `tests/live-real-model.test.ts` "auto language on the chunked
+file path": da_festuge → `da` (WER within its bar), en_aiproblem and
+en_alien → `en` (Large v3 Turbo, onnxruntime-node).
+**Cost:** one extra encoder pass per file (the chunked pipeline can't reuse
+it). **Not covered:** live passes with "auto" still decode as English —
+detecting on every short live pass would add an encoder run per pass; it
+needs a once-per-session design (WorkBreakdown).
+**Caveat:** leading silence or music in the first 30 s can mislead
+detection; long-form detects on the first *speech* chunk instead.

docs/WorkBreakdown.md

@@ -441,7 +441,7 @@ Found in spec 001 workstream 6 (D-046), open:
       this repo has.
 
 Found while building the long-form pipeline (spec 001 Goal 3, D-049), 2026-10-29:
-- [ ] "Auto-detect" on the default (chunked) file path does not detect: with no
+- [x] "Auto-detect" on the default (chunked) file path does not detect: with no
       `language`, Transformers.js 4.2 logs "No language specified - defaulting to
       English" and decodes Danish audio as an English translation (da_festuge WER
       0.94 vs 0.14 with `da`). Long-form ("Skip silence") detects once and pins
@@ -449,6 +449,11 @@ Found while building the long-form pipeline (spec 001 Goal 3, D-049), 2026-10-29
 - [ ] Spec 001's measurement fallback mentions a hint list of the names in
       `en_alien`'s reference; `tests/meetingFixture.ts` returns audio + reference
       only. Add the list when the vocabulary hint (workstream 2) lands.
+      — Fixed for file passes: detect once on the first 30 s and pin (D-053).
+- [ ] "Auto-detect" on **live** passes (browser engine) still decodes as English:
+      each live pass would need its own detection (an extra encoder run per
+      pass). Needs a once-per-session design (detect on the first committed
+      segment, then pin) — D-053.
 
 - [ ] Parakeet v3 also transcribes 24 other European languages, Danish included, but the app
       offers it English-only (spec 001) — unmeasured on `da_festuge`. Candidate for a later spec.