fix(telegram): include DM topic thread id in replies (#18586)

src/telegram/bot/delivery.test.ts

@@ -167,7 +167,7 @@ describe("deliverReplies", () => {
     );
   });
 
-  it("does not include message_thread_id for DMs (threads don't exist in private chats)", async () => {
+  it("includes message_thread_id for DM topics", async () => {
     const runtime = createRuntime();
     const sendMessage = vi.fn().mockResolvedValue({
       message_id: 4,
@@ -179,14 +179,14 @@ describe("deliverReplies", () => {
       replies: [{ text: "Hello" }],
       runtime,
       bot,
-      thread: { id: 1, scope: "dm" },
+      thread: { id: 42, scope: "dm" },
     });
 
     expect(sendMessage).toHaveBeenCalledWith(
       "123",
       expect.any(String),
-      expect.not.objectContaining({
-        message_thread_id: 1,
+      expect.objectContaining({
+        message_thread_id: 42,
       }),
     );
   });

src/telegram/bot/helpers.test.ts

@@ -43,15 +43,21 @@ describe("buildTelegramThreadParams", () => {
     });
   });
 
-  it("skips thread id for dm threads (DMs don't have threads)", () => {
-    expect(buildTelegramThreadParams({ id: 1, scope: "dm" })).toBeUndefined();
-    expect(buildTelegramThreadParams({ id: 2, scope: "dm" })).toBeUndefined();
+  it("includes thread id for dm topics", () => {
+    expect(buildTelegramThreadParams({ id: 1, scope: "dm" })).toEqual({
+      message_thread_id: 1,
+    });
+    expect(buildTelegramThreadParams({ id: 2, scope: "dm" })).toEqual({
+      message_thread_id: 2,
+    });
   });
 
-  it("normalizes and skips thread id for dm threads even with edge values", () => {
+  it("normalizes dm thread ids and skips non-positive values", () => {
     expect(buildTelegramThreadParams({ id: 0, scope: "dm" })).toBeUndefined();
     expect(buildTelegramThreadParams({ id: -1, scope: "dm" })).toBeUndefined();
-    expect(buildTelegramThreadParams({ id: 1.9, scope: "dm" })).toBeUndefined();
+    expect(buildTelegramThreadParams({ id: 1.9, scope: "dm" })).toEqual({
+      message_thread_id: 1,
+    });
   });
 
   it("handles thread id 0 for non-dm scopes", () => {

src/telegram/bot/helpers.ts

@@ -114,7 +114,7 @@ export function resolveTelegramThreadSpec(params: {
  * Build thread params for Telegram API calls (messages, media).
  *
  * IMPORTANT: Thread IDs behave differently based on chat type:
- * - DMs (private chats): Never send thread_id (threads don't exist)
+ * - DMs (private chats): Include message_thread_id when present (DM topics)
  * - Forum topics: Skip thread_id=1 (General topic), include others
  * - Regular groups: Thread IDs are ignored by Telegram
  *
@@ -130,9 +130,8 @@ export function buildTelegramThreadParams(thread?: TelegramThreadSpec | null) {
   }
   const normalized = Math.trunc(thread.id);
 
-  // Never send thread_id for DMs (threads don't exist in private chats)
   if (thread.scope === "dm") {
-    return undefined;
+    return normalized > 0 ? { message_thread_id: normalized } : undefined;
   }
 
   // Telegram rejects message_thread_id=1 for General forum topic

src/telegram/draft-stream.test.ts

@@ -86,12 +86,14 @@ describe("createTelegramDraftStream", () => {
     await vi.waitFor(() => expect(api.sendMessage).toHaveBeenCalledWith(123, "Hello", undefined));
   });
 
-  it("omits message_thread_id for dm threads and clears preview on cleanup", async () => {
+  it("includes message_thread_id for dm threads and clears preview on cleanup", async () => {
     const api = createMockDraftApi();
-    const stream = createThreadedDraftStream(api, { id: 1, scope: "dm" });
+    const stream = createThreadedDraftStream(api, { id: 42, scope: "dm" });
 
     stream.update("Hello");
-    await vi.waitFor(() => expect(api.sendMessage).toHaveBeenCalledWith(123, "Hello", undefined));
+    await vi.waitFor(() =>
+      expect(api.sendMessage).toHaveBeenCalledWith(123, "Hello", { message_thread_id: 42 }),
+    );
     await stream.clear();
 
     expect(api.deleteMessage).toHaveBeenCalledWith(123, 17);