Fix #147: truncate task prompt by byte length for Telegram's 4096-byte limit

src/bot/callbacks/scheduled-task-callback-handler.ts

@@ -154,14 +154,37 @@ function formatDateTime(dateIso: string | null, timezone: string): string {
   }
 }
 
+const TASK_DETAIL_PROMPT_BYTE_BUDGET = 3800;
+
+function truncatePromptForDetails(prompt: string): string {
+  if (Buffer.byteLength(prompt, "utf-8") <= TASK_DETAIL_PROMPT_BYTE_BUDGET) {
+    return prompt;
+  }
+
+  const budget = TASK_DETAIL_PROMPT_BYTE_BUDGET - 3;
+  let lo = 0;
+  let hi = prompt.length;
+
+  while (lo < hi) {
+    const mid = (lo + hi + 1) >>> 1;
+    if (Buffer.byteLength(prompt.slice(0, mid), "utf-8") <= budget) {
+      lo = mid;
+    } else {
+      hi = mid - 1;
+    }
+  }
+
+  return `${prompt.slice(0, lo)}...`;
+}
+
 function formatTaskDetails(task: ScheduledTask): string {
   const variant = task.model.variant ? ` (${task.model.variant})` : "";
   const model = `${task.model.providerID}/${task.model.modelID}${variant}`;
   const cronLine =
     task.kind === "cron" ? `${t("tasklist.details.cron", { cron: task.cron })}\n` : "";
 
   return t("tasklist.details", {
-    prompt: task.prompt,
+    prompt: truncatePromptForDetails(task.prompt),
     project: `${task.projectWorktree}\n${t("status.line.model", { model })}`,
     schedule: task.scheduleSummary,
     cronLine,

tests/bot/commands/tasklist.test.ts

@@ -257,4 +257,83 @@ describe("bot/commands/tasklist", () => {
       show_alert: true,
     });
   });
+
+  it("truncates long ASCII prompt by byte length to fit Telegram limit", async () => {
+    interactionManager.start({
+      kind: "custom",
+      expectedInput: "callback",
+      metadata: {
+        flow: "tasklist",
+        stage: "list",
+        messageId: 700,
+      },
+    });
+
+    const longPrompt = "A".repeat(4000);
+    mocked.getScheduledTaskMock.mockReturnValue(
+      createTask("task-long", {
+        prompt: longPrompt,
+      }),
+    );
+
+    const ctx = createCallbackContext("tasklist:open:task-long", 700);
+    await handleTaskListCallback(ctx);
+
+    const [text] = (ctx.editMessageText as ReturnType<typeof vi.fn>).mock.calls[0] as [string];
+    expect(text).toContain("...");
+    expect(Buffer.byteLength(text, "utf-8")).toBeLessThanOrEqual(4096);
+  });
+
+  it("keeps short prompts intact without truncation", async () => {
+    interactionManager.start({
+      kind: "custom",
+      expectedInput: "callback",
+      metadata: {
+        flow: "tasklist",
+        stage: "list",
+        messageId: 800,
+      },
+    });
+
+    const shortPrompt = "Check weather";
+    mocked.getScheduledTaskMock.mockReturnValue(
+      createTask("task-short", {
+        prompt: shortPrompt,
+      }),
+    );
+
+    const ctx = createCallbackContext("tasklist:open:task-short", 800);
+    await handleTaskListCallback(ctx);
+
+    const [text] = (ctx.editMessageText as ReturnType<typeof vi.fn>).mock.calls[0] as [string];
+    expect(text).toContain(shortPrompt);
+    expect(text).not.toContain("...");
+  });
+
+  it("truncates non-ASCII prompts by byte length (Arabic/Cyrillic)", async () => {
+    interactionManager.start({
+      kind: "custom",
+      expectedInput: "callback",
+      metadata: {
+        flow: "tasklist",
+        stage: "list",
+        messageId: 900,
+      },
+    });
+
+    // Arabic chars are ~2 bytes each in UTF-8 — 2500 chars = ~5000 bytes
+    const arabicPrompt = "مرحباً".repeat(500);
+    mocked.getScheduledTaskMock.mockReturnValue(
+      createTask("task-ar", {
+        prompt: arabicPrompt,
+      }),
+    );
+
+    const ctx = createCallbackContext("tasklist:open:task-ar", 900);
+    await handleTaskListCallback(ctx);
+
+    const [text] = (ctx.editMessageText as ReturnType<typeof vi.fn>).mock.calls[0] as [string];
+    expect(text).toContain("...");
+    expect(Buffer.byteLength(text, "utf-8")).toBeLessThanOrEqual(4096);
+  });
 });