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>

server-py/sidecar.py

@@ -52,9 +52,9 @@ SAMPLE_RATE = 16000
 
 # "Names and terms" vocabulary hint (D-WS2a). Twins of normalizeHint /
 # fitHintToBudget in src/lib/hint.ts — change both together; parity is tested
-# in tests/hint-parity.test.ts. Whisper's prompt budget is half of its 448
-# decoder positions minus one (<|startofprev|>).
-PROMPT_TOKEN_BUDGET = 223
+# in tests/hint-parity.test.ts. Whisper would allow 223 tokens; 120 is the
+# measured cap at which small models are not garbled (D-WS2a).
+PROMPT_TOKEN_BUDGET = 120
 _CONTROL_CHARS = re.compile("[\x00-\x08\x0b\x0c\x0e-\x1f\x7f]")