fix(session-cost-usage): keep emoji / surrogate pairs intact during first user message truncation

wings1029 · claude · wings1029 · commit e17feaa8e5ea · 2026-07-07T20:35:02.000+08:00
The discoverAllSessions helper in session-cost-usage.ts truncates the
first user message with .slice(0, 100), which counts UTF-16 code units.
When an emoji straddles the cut boundary, .slice() produces a lone
surrogate that renders as � in session titles and usage API output.

Replace .slice(0, 100) with the already-imported truncateUtf16Safe()
to avoid splitting surrogate pairs at the boundary.

Co-Authored-By: Claude &lt;noreply@anthropic.com&gt;
diff --git a/src/infra/session-cost-usage.test.ts b/src/infra/session-cost-usage.test.ts
@@ -2721,6 +2721,47 @@ describe("session cost usage", () => {
     });
   });
 
+  it("does not split surrogate pairs when truncating the first user message during discovery", async () => {
+    const root = await makeSessionCostRoot("discover-utf16-first-msg");
+    const sessionsDir = path.join(root, "agents", "main", "sessions");
+    await fs.mkdir(sessionsDir, { recursive: true });
+
+    // "a" (1 code unit) + 50 🦞 (100 code units) = 101 code units > 100
+    // .slice(0, 100) would cut between the surrogates of the last emoji
+    const content = "a" + "🦞".repeat(50);
+    const sessionFile = path.join(sessionsDir, "sess-emoji.jsonl");
+    await fs.writeFile(
+      sessionFile,
+      JSON.stringify({
+        type: "message",
+        timestamp: "2026-02-21T17:47:00.000Z",
+        message: { role: "user", content },
+      }),
+      "utf-8",
+    );
+
+    await withStateDir(root, async () => {
+      const sessions = await discoverAllSessions();
+      expect(sessions).toHaveLength(1);
+      const msg = sessions[0]?.firstUserMessage;
+      expect(msg).toBeDefined();
+
+      // Must not include a dangling high surrogate
+      for (let i = 0; i < msg!.length; i++) {
+        const cp = msg!.charCodeAt(i);
+        if (cp >= 0xd800 && cp <= 0xdbff) {
+          // High surrogate must be followed by a low surrogate
+          expect(msg!.charCodeAt(i + 1)).toBeGreaterThanOrEqual(0xdc00);
+          expect(msg!.charCodeAt(i + 1)).toBeLessThanOrEqual(0xdfff);
+        }
+      }
+
+      // After safe truncation, the last code unit should not be a lone surrogate
+      const lastCode = msg!.charCodeAt(msg!.length - 1);
+      expect(lastCode < 0xd800 || lastCode > 0xdbff).toBe(true);
+    });
+  });
+
   it("falls back to archived reset transcripts for per-session detail queries", async () => {
     const root = await makeSessionCostRoot("session-archive-fallback");
     const sessionsDir = path.join(root, "agents", "main", "sessions");
diff --git a/src/infra/session-cost-usage.ts b/src/infra/session-cost-usage.ts
@@ -2160,7 +2160,7 @@ export async function discoverAllSessions(params?: {
             if (message?.role === "user") {
               const content = message.content;
               if (typeof content === "string") {
-                firstUserMessage = content.slice(0, 100);
+                firstUserMessage = truncateUtf16Safe(content, 100);
               } else if (Array.isArray(content)) {
                 for (const block of content) {
                   if (
@@ -2170,7 +2170,7 @@ export async function discoverAllSessions(params?: {
                   ) {
                     const text = (block as Record<string, unknown>).text;
                     if (typeof text === "string") {
-                      firstUserMessage = text.slice(0, 100);
+                      firstUserMessage = truncateUtf16Safe(text, 100);
                     }
                     break;
                   }
