diff --git a/ds4_agent.c b/ds4_agent.c
index 1c5209154..917bd2909 100644
--- a/ds4_agent.c
+++ b/ds4_agent.c
@@ -6272,6 +6272,8 @@ static char *agent_tool_edit(agent_worker *w, const agent_tool_call *call) {
     const char *new_text = agent_tool_arg_value(call, "new");
     if (!old || !old[0]) return xstrdup("Tool error: edit requires non-empty old text\n");
     if (!new_text) return xstrdup("Tool error: edit requires new text\n");
+    if (strstr(new_text, "[upto]"))
+        return xstrdup("Tool error: new text must not contain the [upto] anchor marker\n");
 
     char err[256];
     char *data = NULL;