fix(telegram): preserve astral chunk boundaries

extensions/telegram/src/format.test.ts

@@ -435,6 +435,10 @@ describe("markdownToTelegramHtml", () => {
       expect(containsLoneSurrogate(chunk)).toBe(false);
     }
   });
+
+  it("keeps an astral char whole when a positive limit starts on its pair", () => {
+    expect(splitTelegramHtmlChunks("A😀B", 1)).toEqual(["A", "😀", "B"]);
+  });
 });
 
 function containsLoneSurrogate(text: string): boolean {

extensions/telegram/src/format.ts

@@ -1071,14 +1071,17 @@ function findTelegramHtmlEntityEnd(text: string, start: number): number {
 }
 
 // Never return a split index that lands between a UTF-16 surrogate pair, or
-// both chunks would carry a lone surrogate that re-encodes to U+FFFD. Mirrors
-// the guard in bot/native-quote.ts `truncateUtf16Safe`.
+// both chunks would carry a lone surrogate that re-encodes to U+FFFD. If the
+// pair starts the segment, keep it whole so chunking still advances.
 function clampToSurrogateBoundary(text: string, index: number): number {
   const high = text.charCodeAt(index - 1);
   const low = text.charCodeAt(index);
   const splitsPair =
     index > 0 && high >= 0xd800 && high <= 0xdbff && low >= 0xdc00 && low <= 0xdfff;
-  return splitsPair ? index - 1 : index;
+  if (!splitsPair) {
+    return index;
+  }
+  return index > 1 ? index - 1 : index + 1;
 }
 
 function findTelegramHtmlSafeSplitIndex(text: string, maxLength: number): number {

extensions/telegram/src/telegram-outbound.test.ts

@@ -43,6 +43,17 @@ describe("telegramPlugin outbound", () => {
     expect(telegramOutbound.chunker?.(text, 4000)).toEqual([text]);
   });
 
+  it("keeps astral characters whole at positive configured chunk limits", () => {
+    clearTelegramRuntime();
+
+    expect(telegramOutbound.chunker?.("A😀B", 1)).toEqual(["A", "😀", "B"]);
+    expect(telegramOutbound.chunker?.("A😀B", 1, { formatting: { parseMode: "HTML" } })).toEqual([
+      "A",
+      "😀",
+      "B",
+    ]);
+  });
+
   it("preserves markdown tables for the configured delivery renderer", () => {
     clearTelegramRuntime();
     const text = ["| Name | Value |", "|------|-------|", "| A | 1 |"].join("\n");

src/auto-reply/chunk.test.ts

@@ -604,6 +604,10 @@ describe("chunkMarkdownTextWithMode", () => {
     expect(chunks.every((chunk) => !/[\uD800-\uDBFF]$/u.test(chunk))).toBe(true);
     expect(chunks.every((chunk) => !/^[\uDC00-\uDFFF]/u.test(chunk))).toBe(true);
   });
+
+  it("keeps an astral character whole when a positive hard limit starts on its pair", () => {
+    expect(chunkMarkdownTextWithMode("A😀B", 1, "length")).toEqual(["A", "😀", "B"]);
+  });
 });
 
 describe("resolveChunkMode", () => {

src/shared/text-chunking.ts

@@ -16,7 +16,7 @@ export function avoidTrailingHighSurrogateBreak(text: string, start: number, end
     return end;
   }
   const adjusted = end - 1;
-  return adjusted > start ? adjusted : end;
+  return adjusted > start ? adjusted : end + 1;
 }
 
 export function chunkTextByBreakResolver(