End the hint prompt in a full stop: an unterminated list made Whisper drop punctuation and loop

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

tests/hint.test.ts

@@ -72,14 +72,22 @@ describe("fitHintToBudget (D-WS2a truncation rule)", () => {
     expect(fitHintToBudget("  \n", charEncode)).toEqual([]);
   });
 
-  test("a hint within budget is encoded with Whisper's leading space", () => {
+  test("a hint within budget is encoded as a space-led sentence ending in a full stop", () => {
+    // Measured (D-WS2a): an unterminated list prompt suppresses punctuation
+    // and triggers repetition loops on smaller models; a full stop does not.
     expect(fitHintToBudget("Maxwell\nVoices of the Void", charEncode)).toEqual(
-      charEncode(" Maxwell, Voices of the Void"),
+      charEncode(" Maxwell, Voices of the Void."),
     );
   });
 
-  test("over budget: keeps whole leading words and drops a dangling comma", () => {
-    expect(charDecode(fitHintToBudget("alpha, beta gamma", charEncode, 10))).toBe(" alpha");
+  test("a hint already ending in . ? or ! gets no extra full stop", () => {
+    expect(charDecode(fitHintToBudget("Is it Maxwell?", charEncode))).toBe(" Is it Maxwell?");
+    expect(charDecode(fitHintToBudget("Dr. Bao.", charEncode))).toBe(" Dr. Bao.");
+  });
+
+  test("over budget: keeps whole leading words, drops a dangling comma, ends in a full stop", () => {
+    expect(charDecode(fitHintToBudget("alpha, beta gamma", charEncode, 10))).toBe(" alpha.");
+    expect(charDecode(fitHintToBudget("alpha, beta gamma", charEncode, 13))).toBe(" alpha, beta.");
   });
 
   test("a single word longer than the budget is cut at the budget", () => {
@@ -96,10 +104,11 @@ describe("fitHintToBudget (D-WS2a truncation rule)", () => {
     expect(ids.length).toBeLessThanOrEqual(223);
     expect(ids.length).toBeGreaterThan(200);
     const kept = charDecode(ids);
-    expect(` ${hint}`.startsWith(kept)).toBe(true);
-    // Ends on a whole term: the next character of the hint is a separator.
-    expect(kept.endsWith(",")).toBe(false);
-    expect([",", " "]).toContain(` ${hint}`.charAt(kept.length));
+    // Ends on a whole term plus the full stop: the hint continues with a separator there.
+    expect(kept.endsWith(".")).toBe(true);
+    const body = kept.slice(0, -1);
+    expect(` ${hint}`.startsWith(body)).toBe(true);
+    expect([",", " "]).toContain(` ${hint}`.charAt(body.length));
   });
 
   test("is deterministic", () => {

tests/prompt.test.ts

@@ -110,10 +110,10 @@ describe("buildPromptPrefix", () => {
     expect(buildPromptPrefix(tokenizer, " \n ")).toEqual([]);
   });
 
-  test("a hint → <|startofprev|> then the space-led hint tokens", () => {
+  test("a hint → <|startofprev|> then the space-led, full-stopped hint tokens", () => {
     expect(buildPromptPrefix(tokenizer, "Max\nVoid")).toEqual([
       PREV,
-      ...[..." Max, Void"].map((c) => c.charCodeAt(0)),
+      ...[..." Max, Void."].map((c) => c.charCodeAt(0)),
     ]);
   });