diff --git a/packages/opencode/src/agent/prompt/title.txt b/packages/opencode/src/agent/prompt/title.txt
index e7fc7b500..62960b2c4 100644
--- a/packages/opencode/src/agent/prompt/title.txt
+++ b/packages/opencode/src/agent/prompt/title.txt
@@ -12,6 +12,7 @@ Your output must be:
 </task>
 
 <rules>
+- you MUST use the same language as the user message you are summarizing
 - Title must be grammatically correct and read naturally - no word salad
 - Never include tool names in the title (e.g. "read tool", "bash tool", "edit tool")
 - Focus on the main topic or question the user needs to retrieve