fix(telegram): skip empty HTML chunks from thematic breaks (#3011)

Thematic breaks (---, ***, ___) and empty block-level markdown elements
produce empty HTML after markdown→Telegram conversion.  When these empty
chunks are sent to the Telegram Bot API, it rejects with '400: message
text is empty', which aborts the delivery loop and silently drops all
subsequent message chunks.

Fix:
1. markdownToTelegramChunks() now filters out chunks with empty HTML
   before returning, preventing empty chunks from reaching the delivery
   pipeline at all.
2. sendTelegramText() adds a defensive guard that skips empty HTML,
   protecting against any other code path that might produce one.

Closes #3011

CHANGELOG.md

@@ -6,6 +6,7 @@ Docs: https://docs.molt.bot
 Status: beta.
 
 ### Changes
+- Telegram: skip empty HTML chunks from thematic breaks to prevent delivery loop abort. (#3011) Thanks @AlexAnys.
 - Rebrand: rename the npm package/CLI to `moltbot`, add a `moltbot` compatibility shim, and move extensions to the `@moltbot/*` scope.
 - Commands: group /help and /commands output with Telegram paging. (#2504) Thanks @hougangdev.
 - macOS: limit project-local `node_modules/.bin` PATH preference to debug builds (reduce PATH hijacking risk).

src/telegram/bot/delivery.test.ts

@@ -305,4 +305,32 @@ describe("deliverReplies", () => {
     expect(sendVoice).toHaveBeenCalledTimes(1);
     expect(sendMessage).not.toHaveBeenCalled();
   });
+
+  it("skips empty chunks from thematic breaks without crashing (#3011)", async () => {
+    // A horizontal rule (---) in markdown produces empty HTML after conversion.
+    // This must not cause a Telegram "message text is empty" 400 error or
+    // abort delivery of subsequent chunks.
+    const runtime = { error: vi.fn(), log: vi.fn() };
+    const sendMessage = vi.fn().mockResolvedValue({ message_id: 1, chat: { id: "c" } });
+    const bot = { api: { sendMessage } } as unknown as Bot;
+
+    await deliverReplies({
+      replies: [{ text: "Before\n\n---\n\nAfter" }],
+      chatId: "c",
+      token: "tok",
+      runtime,
+      bot,
+      replyToMode: "off",
+      textLimit: 4000,
+    });
+
+    // All sendMessage calls should have non-empty text
+    for (const call of sendMessage.mock.calls) {
+      const text = call[1] as string;
+      expect(text.trim().length).toBeGreaterThan(0);
+    }
+    // "After" should not be silently dropped
+    const allTexts = sendMessage.mock.calls.map((c: unknown[]) => c[1] as string).join(" ");
+    expect(allTexts).toContain("After");
+  });
 });

src/telegram/bot/delivery.ts

@@ -476,6 +476,15 @@ async function sendTelegramText(
   const linkPreviewOptions = linkPreviewEnabled ? undefined : { is_disabled: true };
   const textMode = opts?.textMode ?? "markdown";
   const htmlText = textMode === "html" ? text : markdownToTelegramHtml(text);
+  // Skip empty HTML chunks — thematic breaks (---/***/___)  and empty
+  // block-level elements produce zero visible output after markdown→HTML
+  // conversion.  Sending an empty string to the Telegram API triggers a
+  // "400: message text is empty" error which aborts the entire delivery
+  // loop, silently dropping all subsequent chunks.  See #3011.
+  if (!htmlText || !htmlText.trim()) {
+    logVerbose("telegram sendTelegramText: skipping empty HTML chunk");
+    return undefined;
+  }
   try {
     const res = await withTelegramApiErrorLogging({
       operation: "sendMessage",

src/telegram/format.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 
-import { markdownToTelegramHtml } from "./format.js";
+import { markdownToTelegramChunks, markdownToTelegramHtml } from "./format.js";
 
 describe("markdownToTelegramHtml", () => {
   it("renders basic inline formatting", () => {
@@ -47,4 +47,39 @@ describe("markdownToTelegramHtml", () => {
     const res = markdownToTelegramHtml("```js\nconst x = 1;\n```");
     expect(res).toBe("<pre><code>const x = 1;\n</code></pre>");
   });
+
+  it("returns empty string for thematic break (---)", () => {
+    const res = markdownToTelegramHtml("---");
+    expect(res.trim()).toBe("");
+  });
+});
+
+describe("markdownToTelegramChunks", () => {
+  it("filters out empty chunks from thematic breaks", () => {
+    // A thematic break (---) produces empty HTML; it must be filtered
+    // out so Telegram doesn't receive an empty message.  See #3011.
+    const chunks = markdownToTelegramChunks("First paragraph\n\n---\n\nSecond paragraph", 4000);
+    // Every returned chunk must have non-empty html
+    for (const chunk of chunks) {
+      expect(chunk.html.trim().length).toBeGreaterThan(0);
+    }
+    // Both paragraphs should still be present
+    const combined = chunks.map((c) => c.html).join("");
+    expect(combined).toContain("First paragraph");
+    expect(combined).toContain("Second paragraph");
+  });
+
+  it("filters out empty chunks from *** and ___", () => {
+    const chunksAsterisk = markdownToTelegramChunks("Above\n\n***\n\nBelow", 4000);
+    const chunksUnderscore = markdownToTelegramChunks("Above\n\n___\n\nBelow", 4000);
+    for (const chunk of [...chunksAsterisk, ...chunksUnderscore]) {
+      expect(chunk.html.trim().length).toBeGreaterThan(0);
+    }
+  });
+
+  it("preserves normal chunks without filtering", () => {
+    const chunks = markdownToTelegramChunks("hello **world**", 4000);
+    expect(chunks.length).toBeGreaterThan(0);
+    expect(chunks[0].html).toContain("hello");
+  });
 });

src/telegram/format.ts

@@ -81,10 +81,18 @@ export function markdownToTelegramChunks(
     tableMode: options.tableMode,
   });
   const chunks = chunkMarkdownIR(ir, limit);
-  return chunks.map((chunk) => ({
-    html: renderTelegramHtml(chunk),
-    text: chunk.text,
-  }));
+  return (
+    chunks
+      .map((chunk) => ({
+        html: renderTelegramHtml(chunk),
+        text: chunk.text,
+      }))
+      // Filter out empty chunks produced by thematic breaks (---/***/___),
+      // empty blockquotes, or headings without text.  These render to empty
+      // HTML and would cause Telegram API "message text is empty" errors.
+      // See #3011.
+      .filter((chunk) => chunk.html.trim().length > 0)
+  );
 }
 
 export function markdownToTelegramHtmlChunks(markdown: string, limit: number): string[] {