diff --git a/src/GmRelay.Bot/Infrastructure/Telegram/TelegramPlatformMessenger.cs b/src/GmRelay.Bot/Infrastructure/Telegram/TelegramPlatformMessenger.cs
index 067d0d5..3171238 100644
--- a/src/GmRelay.Bot/Infrastructure/Telegram/TelegramPlatformMessenger.cs
+++ b/src/GmRelay.Bot/Infrastructure/Telegram/TelegramPlatformMessenger.cs
@@ -59,16 +59,24 @@ public sealed class TelegramPlatformMessenger(
     public async Task UpdateScheduleAsync(PlatformScheduleMessage message, CancellationToken ct)
     {
         EnsureTelegram(message.Group.Platform);
-        if (message.ExistingMessage is null)
+        var existingMessage = message.ExistingMessage;
+        if (existingMessage is null)
         {
             throw new ArgumentException("Existing schedule message reference is required.", nameof(message));
         }
 
+        EnsureTelegram(existingMessage.Platform);
+        if (!string.Equals(message.Group.ExternalGroupId, existingMessage.ExternalGroupId, StringComparison.Ordinal) ||
+            !string.Equals(message.Group.ExternalThreadId, existingMessage.ExternalThreadId, StringComparison.Ordinal))
+        {
+            throw new ArgumentException("Existing schedule message reference must match the schedule group.", nameof(message));
+        }
+
         var renderResult = TelegramSessionBatchRenderer.Render(message.View);
         await BatchMessageEditor.EditBatchMessageAsync(
             bot,
-            chatId: ParseLong(message.Group.ExternalGroupId),
-            messageId: ParseInt(message.ExistingMessage.ExternalMessageId),
+            chatId: ParseLong(existingMessage.ExternalGroupId),
+            messageId: ParseInt(existingMessage.ExternalMessageId),
             text: renderResult.Text,
             replyMarkup: renderResult.Markup,
             ct);
diff --git a/tests/GmRelay.Bot.Tests/Infrastructure/Telegram/TelegramPlatformMessengerTests.cs b/tests/GmRelay.Bot.Tests/Infrastructure/Telegram/TelegramPlatformMessengerTests.cs
new file mode 100644
index 0000000..7652435
--- /dev/null
+++ b/tests/GmRelay.Bot.Tests/Infrastructure/Telegram/TelegramPlatformMessengerTests.cs
@@ -0,0 +1,44 @@
+using GmRelay.Bot.Infrastructure.Telegram;
+using GmRelay.Shared.Platform;
+using GmRelay.Shared.Rendering;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace GmRelay.Bot.Tests.Infrastructure.Telegram;
+
+public sealed class TelegramPlatformMessengerTests
+{
+    [Fact]
+    public async Task UpdateScheduleAsync_ShouldRejectNonTelegramExistingMessageReference()
+    {
+        var messenger = CreateMessenger();
+        var message = new PlatformScheduleMessage(
+            new PlatformGroup(PlatformKind.Telegram, "100", "Telegram group"),
+            CreateView(),
+            new PlatformMessageRef(PlatformKind.Discord, "100", null, "200"));
+
+        await Assert.ThrowsAsync<NotSupportedException>(
+            () => messenger.UpdateScheduleAsync(message, CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task UpdateScheduleAsync_ShouldRejectMismatchedGroupAndExistingMessageReference()
+    {
+        var messenger = CreateMessenger();
+        var message = new PlatformScheduleMessage(
+            new PlatformGroup(PlatformKind.Telegram, "100", "Telegram group", ExternalThreadId: "7"),
+            CreateView(),
+            new PlatformMessageRef(PlatformKind.Telegram, "101", "7", "200"));
+
+        var exception = await Assert.ThrowsAsync<ArgumentException>(
+            () => messenger.UpdateScheduleAsync(message, CancellationToken.None));
+
+        Assert.Equal("message", exception.ParamName);
+        Assert.Contains("Existing schedule message reference must match the schedule group.", exception.Message);
+    }
+
+    private static TelegramPlatformMessenger CreateMessenger() =>
+        new(null!, NullLogger<TelegramPlatformMessenger>.Instance);
+
+    private static SessionBatchViewModel CreateView() =>
+        new("Test batch", []);
+}