fix(telegram): recover incomplete preview finalization (#71554)

Fix Telegram partial-stream preview finalization so ambiguous final edit failures fall back to a final send when the visible preview is a strict prefix of the answer.

Includes archived-preview regression coverage and generated config metadata refresh.

Thanks @sahilsatralkar.

Co-authored-by: Sahil Satralkar <62758655+sahilsatralkar@users.noreply.github.com>

extensions/telegram/src/config-ui-hints.ts

@@ -31,7 +31,7 @@ export const telegramChannelConfigUiHints = {
   },
   streaming: {
     label: "Telegram Streaming Mode",
-    help: 'Unified Telegram stream preview mode: "off" | "partial" | "block" | "progress" (default: "partial"). "progress" maps to "partial" on Telegram. Legacy boolean/streamMode keys are auto-mapped.',
+    help: 'Unified Telegram stream preview mode: "off" | "partial" | "block" | "progress" (default: "partial"). "progress" maps to "partial" on Telegram. Legacy boolean/streamMode keys are detected; run doctor --fix to migrate.',
   },
   "streaming.mode": {
     label: "Telegram Streaming Mode",

extensions/telegram/src/lane-delivery-text-deliverer.ts

@@ -38,6 +38,12 @@ function isMissingPreviewMessageError(err: unknown): boolean {
   return MESSAGE_NOT_FOUND_RE.test(extractErrorText(err));
 }
 
+function isIncompleteFinalPreviewPrefix(previewText: string, finalText: string): boolean {
+  const preview = previewText.trimEnd();
+  const final = finalText.trimEnd();
+  return preview.length > 0 && preview.length < final.length && final.startsWith(preview);
+}
+
 export type LaneName = "answer" | "reasoning";
 
 export type DraftLaneState = {
@@ -232,6 +238,7 @@ export function createLaneTextDeliverer(params: CreateLaneTextDelivererParams) {
     lane: DraftLaneState;
     finalTextAlreadyLanded: boolean;
     retainAlternatePreviewOnMissingTarget: boolean;
+    targetPreviewText: string;
   }): Promise<PreviewEditResult> => {
     try {
       await params.editPreview({
@@ -294,7 +301,14 @@ export function createLaneTextDeliverer(params: CreateLaneTextDelivererParams) {
           );
           return "fallback";
         }
-        // Default: ambiguous error — prefer incomplete over duplicate
+        if (isIncompleteFinalPreviewPrefix(args.targetPreviewText, args.text)) {
+          params.log(
+            `telegram: ${args.laneName} preview final edit failed and existing preview is an incomplete prefix; falling back to standard send (${String(err)})`,
+          );
+          return "fallback";
+        }
+        // Default: ambiguous error — retain when fallback may duplicate a final
+        // edit that already landed or when the preview is not known-incomplete.
         params.log(
           `telegram: ${args.laneName} preview final edit failed with ambiguous error; keeping existing preview to avoid duplicate (${String(err)})`,
         );
@@ -324,6 +338,7 @@ export function createLaneTextDeliverer(params: CreateLaneTextDelivererParams) {
       messageId: number,
       finalTextAlreadyLanded: boolean,
       retainAlternatePreviewOnMissingTarget: boolean,
+      targetPreviewText: string,
     ) =>
       tryEditPreviewMessage({
         laneName,
@@ -335,6 +350,7 @@ export function createLaneTextDeliverer(params: CreateLaneTextDelivererParams) {
         lane,
         finalTextAlreadyLanded,
         retainAlternatePreviewOnMissingTarget,
+        targetPreviewText,
       });
     const finalizePreview = (
       previewMessageId: number,
@@ -357,6 +373,7 @@ export function createLaneTextDeliverer(params: CreateLaneTextDelivererParams) {
         previewMessageId,
         finalTextAlreadyLanded,
         retainAlternatePreviewOnMissingTarget,
+        currentPreviewText,
       );
     };
     if (!lane.stream) {

extensions/telegram/src/lane-delivery.test.ts

@@ -231,7 +231,8 @@ describe("createLaneTextDeliverer", () => {
 
   it("retains preview when an existing preview final edit fails with ambiguous error", async () => {
     const harness = createHarness({ answerMessageId: 999 });
-    // Plain Error with no error_code → ambiguous, prefer incomplete over duplicate
+    // Plain Error with no error_code → ambiguous. Retain unless the preview is
+    // known to be an incomplete prefix of the final text.
     harness.editPreview.mockRejectedValue(new Error("500: preview edit failed"));
 
     await expectFinalPreviewRetained({
@@ -241,6 +242,20 @@ describe("createLaneTextDeliverer", () => {
     expect(harness.editPreview).toHaveBeenCalledTimes(1);
   });
 
+  it("falls back when an ambiguous final edit failure would leave an incomplete preview", async () => {
+    const harness = createHarness({
+      answerMessageId: 999,
+      answerLastPartialText: "Hello fi",
+    });
+    harness.editPreview.mockRejectedValue(new Error("500: preview edit failed"));
+
+    await expectFinalEditFallbackToSend({
+      harness,
+      text: HELLO_FINAL,
+      expectedLogSnippet: "preview is an incomplete prefix; falling back",
+    });
+  });
+
   it("falls back when Telegram reports the current final edit target missing", async () => {
     const harness = createHarness({ answerMessageId: 999 });
     harness.editPreview.mockRejectedValue(new Error("400: Bad Request: message to edit not found"));
@@ -478,6 +493,29 @@ describe("createLaneTextDeliverer", () => {
     expect(harness.deletePreviewMessage).toHaveBeenCalledWith(5555);
   });
 
+  it("falls back when an archived preview ambiguous final edit would leave an incomplete prefix", async () => {
+    const harness = createHarness();
+    harness.archivedAnswerPreviews.push({
+      messageId: 5555,
+      textSnapshot: "Hello fi",
+      deleteIfUnused: true,
+    });
+    harness.editPreview.mockRejectedValue(new Error("500: preview edit failed"));
+
+    await expectFinalEditFallbackToSend({
+      harness,
+      text: HELLO_FINAL,
+      expectedLogSnippet: "preview is an incomplete prefix; falling back",
+    });
+    expect(harness.editPreview).toHaveBeenCalledWith(
+      expect.objectContaining({
+        messageId: 5555,
+        text: HELLO_FINAL,
+      }),
+    );
+    expect(harness.deletePreviewMessage).toHaveBeenCalledWith(5555);
+  });
+
   it("keeps the active preview when an archived final edit target is missing", async () => {
     const harness = createHarness({ answerMessageId: 999 });
     seedArchivedAnswerPreview(harness);