diff --git a/src/slack/monitor/message-handler/prepare.ts b/src/slack/monitor/message-handler/prepare.ts
index 8a2a9e111..d8f587afb 100644
--- a/src/slack/monitor/message-handler/prepare.ts
+++ b/src/slack/monitor/message-handler/prepare.ts
@@ -437,7 +437,10 @@ export async function prepareSlackMessage(params: {
     });
   }
 
-  const slackTo = isDirectMessage ? `user:${message.user}` : `channel:${message.channel}`;
+  // Always use channel ID for replies - user:X triggers conversations.open which
+  // creates a bot↔user DM (App Home) instead of posting to the original channel.
+  // This broke @mentions in DMs between two other users. See #2412.
+  const slackTo = `channel:${message.channel}`;
 
   const channelDescription = [channelInfo?.topic, channelInfo?.purpose]
     .map((entry) => entry?.trim())