fix(telegram): split stop-created preview finalization path

Refactor lane preview finalization into explicit branches so stop-created
previews never duplicate sends when edit fails.

Add Telegram dispatch regressions for:
- stop-created preview edit failure (no duplicate send)
- existing preview edit failure (fallback send preserved)
- missing message id after stop-created flush (fallback send)

Thanks @obviyus for the original preview-prime direction in #27449.

Co-authored-by: Ayaan Zaidi <hi@obviy.us>

src/telegram/bot-message-dispatch.test.ts

@@ -416,6 +416,83 @@ describe("dispatchTelegramMessage draft streaming", () => {
     expect(answerDraftStream.stop).toHaveBeenCalled();
   });
 
+  it("does not duplicate final delivery when stop-created preview edit fails", async () => {
+    let messageId: number | undefined;
+    const draftStream = {
+      update: vi.fn(),
+      flush: vi.fn().mockResolvedValue(undefined),
+      messageId: vi.fn().mockImplementation(() => messageId),
+      clear: vi.fn().mockResolvedValue(undefined),
+      stop: vi.fn().mockImplementation(async () => {
+        messageId = 777;
+      }),
+      forceNewMessage: vi.fn(),
+    };
+    createTelegramDraftStream.mockReturnValue(draftStream);
+    dispatchReplyWithBufferedBlockDispatcher.mockImplementation(async ({ dispatcherOptions }) => {
+      await dispatcherOptions.deliver({ text: "Short final" }, { kind: "final" });
+      return { queuedFinal: true };
+    });
+    deliverReplies.mockResolvedValue({ delivered: true });
+    editMessageTelegram.mockRejectedValue(new Error("500: edit failed after stop flush"));
+
+    await dispatchWithContext({ context: createContext() });
+
+    expect(editMessageTelegram).toHaveBeenCalledWith(123, 777, "Short final", expect.any(Object));
+    expect(deliverReplies).not.toHaveBeenCalled();
+    expect(draftStream.stop).toHaveBeenCalled();
+  });
+
+  it("falls back to normal delivery when existing preview edit fails", async () => {
+    const draftStream = createDraftStream(999);
+    createTelegramDraftStream.mockReturnValue(draftStream);
+    dispatchReplyWithBufferedBlockDispatcher.mockImplementation(
+      async ({ dispatcherOptions, replyOptions }) => {
+        await replyOptions?.onPartialReply?.({ text: "Hel" });
+        await dispatcherOptions.deliver({ text: "Hello final" }, { kind: "final" });
+        return { queuedFinal: true };
+      },
+    );
+    deliverReplies.mockResolvedValue({ delivered: true });
+    editMessageTelegram.mockRejectedValue(new Error("500: preview edit failed"));
+
+    await dispatchWithContext({ context: createContext() });
+
+    expect(editMessageTelegram).toHaveBeenCalledWith(123, 999, "Hello final", expect.any(Object));
+    expect(deliverReplies).toHaveBeenCalledWith(
+      expect.objectContaining({
+        replies: [expect.objectContaining({ text: "Hello final" })],
+      }),
+    );
+  });
+
+  it("falls back to normal delivery when stop-created preview has no message id", async () => {
+    const draftStream = {
+      update: vi.fn(),
+      flush: vi.fn().mockResolvedValue(undefined),
+      messageId: vi.fn().mockReturnValue(undefined),
+      clear: vi.fn().mockResolvedValue(undefined),
+      stop: vi.fn().mockResolvedValue(undefined),
+      forceNewMessage: vi.fn(),
+    };
+    createTelegramDraftStream.mockReturnValue(draftStream);
+    dispatchReplyWithBufferedBlockDispatcher.mockImplementation(async ({ dispatcherOptions }) => {
+      await dispatcherOptions.deliver({ text: "Short final" }, { kind: "final" });
+      return { queuedFinal: true };
+    });
+    deliverReplies.mockResolvedValue({ delivered: true });
+
+    await dispatchWithContext({ context: createContext() });
+
+    expect(editMessageTelegram).not.toHaveBeenCalled();
+    expect(deliverReplies).toHaveBeenCalledWith(
+      expect.objectContaining({
+        replies: [expect.objectContaining({ text: "Short final" })],
+      }),
+    );
+    expect(draftStream.stop).toHaveBeenCalled();
+  });
+
   it("does not overwrite finalized preview when additional final payloads are sent", async () => {
     const draftStream = createDraftStream(999);
     createTelegramDraftStream.mockReturnValue(draftStream);

src/telegram/lane-delivery.ts

@@ -104,6 +104,55 @@ type ConsumeArchivedAnswerPreviewParams = {
 export function createLaneTextDeliverer(params: CreateLaneTextDelivererParams) {
   const getLanePreviewText = (lane: DraftLaneState) => lane.lastPartialText;
 
+  const shouldSkipRegressivePreviewUpdate = (args: {
+    currentPreviewText: string | undefined;
+    text: string;
+    skipRegressive: "always" | "existingOnly";
+    hadPreviewMessage: boolean;
+  }): boolean =>
+    Boolean(args.currentPreviewText) &&
+    args.currentPreviewText.startsWith(args.text) &&
+    args.text.length < args.currentPreviewText.length &&
+    (args.skipRegressive === "always" || args.hadPreviewMessage);
+
+  const tryEditPreviewMessage = async (args: {
+    laneName: LaneName;
+    messageId: number;
+    text: string;
+    context: "final" | "update";
+    previewButtons?: TelegramInlineButtons;
+    updateLaneSnapshot: boolean;
+    lane: DraftLaneState;
+    treatEditFailureAsDelivered: boolean;
+  }): Promise<boolean> => {
+    try {
+      await params.editPreview({
+        laneName: args.laneName,
+        messageId: args.messageId,
+        text: args.text,
+        previewButtons: args.previewButtons,
+        context: args.context,
+      });
+      if (args.updateLaneSnapshot) {
+        args.lane.lastPartialText = args.text;
+      }
+      params.markDelivered();
+      return true;
+    } catch (err) {
+      if (args.treatEditFailureAsDelivered) {
+        params.log(
+          `telegram: ${args.laneName} preview ${args.context} edit failed after stop-created flush; treating as delivered (${String(err)})`,
+        );
+        params.markDelivered();
+        return true;
+      }
+      params.log(
+        `telegram: ${args.laneName} preview ${args.context} edit failed; falling back to standard send (${String(err)})`,
+      );
+      return false;
+    }
+  };
+
   const tryUpdatePreviewForLane = async ({
     lane,
     laneName,
@@ -122,12 +171,39 @@ export function createLaneTextDeliverer(params: CreateLaneTextDelivererParams) {
     const lanePreviewMessageId = lane.stream.messageId();
     const hadPreviewMessage =
       typeof previewMessageIdOverride === "number" || typeof lanePreviewMessageId === "number";
-    if (stopBeforeEdit) {
-      if (!hadPreviewMessage && context === "final") {
-        // If debounce prevented the first preview, replace stale pending partial text
-        // before final stop() flush sends the first visible preview.
-        lane.stream.update(text);
+    const stopCreatesFirstPreview = stopBeforeEdit && !hadPreviewMessage && context === "final";
+    if (stopCreatesFirstPreview) {
+      // Final stop() can create the first visible preview message.
+      // Prime pending text so the stop flush sends the final text snapshot.
+      lane.stream.update(text);
+      await params.stopDraftLane(lane);
+      const previewMessageId = lane.stream.messageId();
+      if (typeof previewMessageId !== "number") {
+        return false;
       }
+      const currentPreviewText = previewTextSnapshot ?? getLanePreviewText(lane);
+      const shouldSkipRegressive = shouldSkipRegressivePreviewUpdate({
+        currentPreviewText,
+        text,
+        skipRegressive,
+        hadPreviewMessage,
+      });
+      if (shouldSkipRegressive) {
+        params.markDelivered();
+        return true;
+      }
+      return tryEditPreviewMessage({
+        laneName,
+        messageId: previewMessageId,
+        text,
+        context,
+        previewButtons,
+        updateLaneSnapshot,
+        lane,
+        treatEditFailureAsDelivered: true,
+      });
+    }
+    if (stopBeforeEdit) {
       await params.stopDraftLane(lane);
     }
     const previewMessageId =
@@ -138,34 +214,26 @@ export function createLaneTextDeliverer(params: CreateLaneTextDelivererParams) {
       return false;
     }
     const currentPreviewText = previewTextSnapshot ?? getLanePreviewText(lane);
-    const shouldSkipRegressive =
-      Boolean(currentPreviewText) &&
-      currentPreviewText.startsWith(text) &&
-      text.length < currentPreviewText.length &&
-      (skipRegressive === "always" || hadPreviewMessage);
+    const shouldSkipRegressive = shouldSkipRegressivePreviewUpdate({
+      currentPreviewText,
+      text,
+      skipRegressive,
+      hadPreviewMessage,
+    });
     if (shouldSkipRegressive) {
       params.markDelivered();
       return true;
     }
-    try {
-      await params.editPreview({
-        laneName,
-        messageId: previewMessageId,
-        text,
-        previewButtons,
-        context,
-      });
-      if (updateLaneSnapshot) {
-        lane.lastPartialText = text;
-      }
-      params.markDelivered();
-      return true;
-    } catch (err) {
-      params.log(
-        `telegram: ${laneName} preview ${context} edit failed; falling back to standard send (${String(err)})`,
-      );
-      return false;
-    }
+    return tryEditPreviewMessage({
+      laneName,
+      messageId: previewMessageId,
+      text,
+      context,
+      previewButtons,
+      updateLaneSnapshot,
+      lane,
+      treatEditFailureAsDelivered: false,
+    });
   };
 
   const consumeArchivedAnswerPreviewForFinal = async ({