fix(discord): expose thread title prompt formatter

Alix-007 · Alix-007 · commit f062226e814b · 2026-07-07T13:27:30.000+08:00
diff --git a/extensions/discord/src/monitor/thread-title.generate.test.ts b/extensions/discord/src/monitor/thread-title.generate.test.ts
@@ -11,6 +11,7 @@ const prepareSimpleCompletionModelForAgentMock =
 const extractAssistantTextMock = vi.fn<typeof agentRuntimeModule.extractAssistantText>();
 
 let generateThreadTitle: typeof import("./thread-title.js").generateThreadTitle;
+let buildThreadTitleCompletionUserMessage: typeof import("./thread-title.js").buildThreadTitleCompletionUserMessage;
 
 function firstCompletionArgs(): Parameters<
   typeof agentRuntimeModule.completeWithPreparedSimpleCompletionModel
@@ -41,7 +42,9 @@ function hasLoneSurrogate(value: string): boolean {
 }
 
 beforeAll(async () => {
-  ({ generateThreadTitle } = await import("./thread-title.js"));
+  ({ buildThreadTitleCompletionUserMessage, generateThreadTitle } = await import(
+    "./thread-title.js"
+  ));
 });
 
 beforeEach(() => {
@@ -236,6 +239,19 @@ describe("generateThreadTitle", () => {
     expect(content).toContain(`${"d".repeat(319)}...`);
   });
 
+  it("builds prompt text without lone surrogates at truncation boundaries", () => {
+    const content = buildThreadTitleCompletionUserMessage({
+      sourceText: `${"m".repeat(599)}😀tail`,
+      channelName: `${"n".repeat(119)}😀tail`,
+      channelDescription: `${"d".repeat(319)}😀tail`,
+    });
+
+    expect(hasLoneSurrogate(content)).toBe(false);
+    expect(content).toContain(`${"m".repeat(599)}...`);
+    expect(content).toContain(`${"n".repeat(119)}...`);
+    expect(content).toContain(`${"d".repeat(319)}...`);
+  });
+
   it("clamps completion budget to the selected model output cap", async () => {
     prepareSimpleCompletionModelForAgentMock.mockResolvedValueOnce({
       selection: {
diff --git a/extensions/discord/src/monitor/thread-title.ts b/extensions/discord/src/monitor/thread-title.ts
@@ -52,9 +52,8 @@ export async function generateThreadTitle(params: {
   }
 
   try {
-    const promptText = truncateThreadTitleSourceText(sourceText);
-    const userMessage = buildThreadTitleUserMessage({
-      sourceText: promptText,
+    const userMessage = buildThreadTitleCompletionUserMessage({
+      sourceText,
       channelName: params.channelName,
       channelDescription: params.channelDescription,
     });
@@ -105,11 +104,12 @@ async function completeThreadTitle(params: {
   });
 }
 
-function buildThreadTitleUserMessage(params: {
+export function buildThreadTitleCompletionUserMessage(params: {
   sourceText: string;
   channelName?: string;
   channelDescription?: string;
 }): string {
+  const sourceText = truncateThreadTitleSourceText(params.sourceText);
   const channelName = normalizeTitleContextField(
     params.channelName,
     MAX_THREAD_TITLE_CHANNEL_NAME_CHARS,
@@ -125,7 +125,7 @@ function buildThreadTitleUserMessage(params: {
   if (channelDescription) {
     messageLines.push(`Channel description: ${channelDescription}`);
   }
-  messageLines.push(`Message:\n${params.sourceText}`);
+  messageLines.push(`Message:\n${sourceText}`);
   return messageLines.join("\n\n");
 }
 
