fix(telegram): fallback to plain text when threaded markdown renders empty

Minimal fix path for Telegram empty-text failures in threaded replies.

- fallback to plain text when formatted htmlText is empty
- retry plain text on parse/empty-text API errors
- add focused regression test for threaded mode case

Related: #25091
Supersedes alternative fix path in #17629 if maintainers prefer minimal scope.

src/telegram/bot/delivery.test.ts

@@ -244,6 +244,40 @@ describe("deliverReplies", () => {
     );
   });
 
+  it("falls back to plain text when markdown renders to empty HTML in threaded mode", async () => {
+    const runtime = { error: vi.fn(), log: vi.fn() };
+    const sendMessage = vi.fn(async (_chatId: string, text: string) => {
+      if (text === "") {
+        throw new Error("400: Bad Request: message text is empty");
+      }
+      return {
+        message_id: 6,
+        chat: { id: "123" },
+      };
+    });
+    const bot = { api: { sendMessage } } as unknown as Bot;
+
+    await deliverReplies({
+      replies: [{ text: ">" }],
+      chatId: "123",
+      token: "tok",
+      runtime,
+      bot,
+      replyToMode: "off",
+      textLimit: 4000,
+      thread: { id: 42, scope: "forum" },
+    });
+
+    expect(sendMessage).toHaveBeenCalledTimes(1);
+    expect(sendMessage).toHaveBeenCalledWith(
+      "123",
+      ">",
+      expect.objectContaining({
+        message_thread_id: 42,
+      }),
+    );
+  });
+
   it("uses reply_to_message_id when quote text is provided", async () => {
     const runtime = createRuntime();
     const sendMessage = vi.fn().mockResolvedValue({

src/telegram/bot/delivery.ts

@@ -41,6 +41,7 @@ const TELEGRAM_MEDIA_SSRF_POLICY = {
   allowedHostnames: ["api.telegram.org"],
   allowRfc2544BenchmarkRange: true,
 };
+const EMPTY_TEXT_ERR_RE = /message text is empty/i;
 
 export async function deliverReplies(params: {
   replies: ReplyPayload[];
@@ -553,6 +554,30 @@ async function sendTelegramText(
   const linkPreviewOptions = linkPreviewEnabled ? undefined : { is_disabled: true };
   const textMode = opts?.textMode ?? "markdown";
   const htmlText = textMode === "html" ? text : markdownToTelegramHtml(text);
+  const fallbackText = opts?.plainText ?? text;
+  const hasFallbackText = fallbackText.trim().length > 0;
+  const sendPlainFallback = async () => {
+    if (!hasFallbackText) {
+      return undefined;
+    }
+    const res = await withTelegramApiErrorLogging({
+      operation: "sendMessage",
+      runtime,
+      fn: () =>
+        bot.api.sendMessage(chatId, fallbackText, {
+          ...(linkPreviewOptions ? { link_preview_options: linkPreviewOptions } : {}),
+          ...(opts?.replyMarkup ? { reply_markup: opts.replyMarkup } : {}),
+          ...baseParams,
+        }),
+    });
+    return res.message_id;
+  };
+
+  // Markdown can occasionally render to empty HTML (for example syntax-only chunks).
+  // Telegram rejects those sends, so fall back to plain text early.
+  if (!htmlText.trim()) {
+    return await sendPlainFallback();
+  }
   try {
     const res = await withTelegramApiErrorLogging({
       operation: "sendMessage",
@@ -570,21 +595,9 @@ async function sendTelegramText(
     return res.message_id;
   } catch (err) {
     const errText = formatErrorMessage(err);
-    if (PARSE_ERR_RE.test(errText)) {
-      runtime.log?.(`telegram HTML parse failed; retrying without formatting: ${errText}`);
-      const fallbackText = opts?.plainText ?? text;
-      const res = await withTelegramApiErrorLogging({
-        operation: "sendMessage",
-        runtime,
-        fn: () =>
-          bot.api.sendMessage(chatId, fallbackText, {
-            ...(linkPreviewOptions ? { link_preview_options: linkPreviewOptions } : {}),
-            ...(opts?.replyMarkup ? { reply_markup: opts.replyMarkup } : {}),
-            ...baseParams,
-          }),
-      });
-      runtime.log?.(`telegram sendMessage ok chat=${chatId} message=${res.message_id} (plain)`);
-      return res.message_id;
+    if (PARSE_ERR_RE.test(errText) || EMPTY_TEXT_ERR_RE.test(errText)) {
+      runtime.log?.(`telegram formatted send failed; retrying without formatting: ${errText}`);
+      return await sendPlainFallback();
     }
     throw err;
   }