diff --git a/extensions/telegram/src/bot-native-commands.test.ts b/extensions/telegram/src/bot-native-commands.test.ts
index 62218557969..5a5b64a8260 100644
--- a/extensions/telegram/src/bot-native-commands.test.ts
+++ b/extensions/telegram/src/bot-native-commands.test.ts
@@ -12,6 +12,7 @@ import {
   resetNativeCommandMenuMocks,
   waitForRegisteredCommands,
 } from "./bot-native-commands.menu-test-support.js";
+import { resetTelegramForumFlagCacheForTest } from "./bot/helpers.js";
 import { TELEGRAM_COMMAND_NAME_PATTERN } from "./command-config.js";
 import { pluginCommandMocks, resetPluginCommandMocks } from "./test-support/plugin-command.js";
 
@@ -101,6 +102,7 @@ describe("registerTelegramNativeCommands", () => {
   });
 
   beforeEach(() => {
+    resetTelegramForumFlagCacheForTest();
     resetNativeCommandMenuMocks();
     resetPluginCommandMocks();
   });
diff --git a/extensions/telegram/src/bot.create-telegram-bot.test.ts b/extensions/telegram/src/bot.create-telegram-bot.test.ts
index 5ae87c83074..fceb0da8546 100644
--- a/extensions/telegram/src/bot.create-telegram-bot.test.ts
+++ b/extensions/telegram/src/bot.create-telegram-bot.test.ts
@@ -46,6 +46,7 @@ const {
   getTelegramSequentialKey,
   setTelegramBotRuntimeForTest,
 } = await import("./bot.js");
+const { resetTelegramForumFlagCacheForTest } = await import("./bot/helpers.js");
 let createTelegramBot: (
   opts: Parameters<typeof import("./bot.js").createTelegramBot>[0],
 ) => ReturnType<typeof import("./bot.js").createTelegramBot>;
@@ -131,6 +132,7 @@ describe("createTelegramBot", () => {
     process.env.TZ = ORIGINAL_TZ;
   });
   beforeEach(() => {
+    resetTelegramForumFlagCacheForTest();
     setTelegramBotRuntimeForTest(
       telegramBotRuntimeForTest as unknown as Parameters<typeof setTelegramBotRuntimeForTest>[0],
     );
diff --git a/extensions/telegram/src/bot/helpers.test.ts b/extensions/telegram/src/bot/helpers.test.ts
index 12d0550fcb7..d950f0977fc 100644
--- a/extensions/telegram/src/bot/helpers.test.ts
+++ b/extensions/telegram/src/bot/helpers.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it, vi } from "vitest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
 import {
   buildTelegramRoutingTarget,
   buildTelegramThreadParams,
@@ -12,6 +12,7 @@ import {
   resolveTelegramDirectPeerId,
   resolveTelegramForumFlag,
   resolveTelegramForumThreadId,
+  resetTelegramForumFlagCacheForTest,
 } from "./helpers.js";
 
 describe("resolveTelegramForumThreadId", () => {
@@ -34,6 +35,10 @@ describe("resolveTelegramForumThreadId", () => {
 });
 
 describe("resolveTelegramForumFlag", () => {
+  beforeEach(() => {
+    resetTelegramForumFlagCacheForTest();
+  });
+
   it("keeps explicit forum metadata when Telegram already provides it", async () => {
     const getChat = vi.fn(async () => ({ is_forum: false }));
     await expect(
diff --git a/extensions/telegram/src/bot/helpers.ts b/extensions/telegram/src/bot/helpers.ts
index 89ddbff4518..0a26488c7ea 100644
--- a/extensions/telegram/src/bot/helpers.ts
+++ b/extensions/telegram/src/bot/helpers.ts
@@ -44,6 +44,10 @@ const TELEGRAM_FORUM_FLAG_CACHE_MAX_CHATS = 1024;
 const TELEGRAM_FORUM_FLAG_CACHE_TTL_MS = 10 * 60_000;
 const telegramForumFlagByChatId = new Map<string, { expiresAtMs: number; isForum: boolean }>();
 
+export function resetTelegramForumFlagCacheForTest(): void {
+  telegramForumFlagByChatId.clear();
+}
+
 function cacheTelegramForumFlag(chatId: string | number, isForum: boolean, nowMs = Date.now()) {
   const cacheKey = String(chatId);
   if (