Cap the hint prompt at 120 tokens: a full 223-token prompt garbles small models

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

src/lib/hint.ts

@@ -22,10 +22,14 @@ export const HINT_STORAGE_KEY = "whisper-vocabulary-hint";
 export const HINT_MAX_CHARS = 1000;
 
 /**
- * Whisper's prompt budget: half its 448 decoder positions, minus one for
- * `<|startofprev|>` — the cap openai-whisper and faster-whisper use too.
+ * Prompt budget in tokens (without `<|startofprev|>`). Whisper itself allows
+ * 223 (half its 448 decoder positions, minus one — the openai-whisper and
+ * faster-whisper cap), but measured on en_alien a prompt that long garbles
+ * small models: base.en WER 0.339 → 0.476 with a realistic 218-token list and
+ * → 1.05 with a synthetic one, while ≤ 120 tokens stays at or below the
+ * no-hint WER (D-WS2a).
  */
-export const PROMPT_TOKEN_BUDGET = 223;
+export const PROMPT_TOKEN_BUDGET = 120;
 
 /** C0 controls + DEL, except \n and \r (line separators, handled below). Tab stays whitespace. */
 const CONTROL_CHARS = /[\u0000-\u0008\u000b\u000c\u000e-\u001f\u007f]/g;