diff --git a/internal/mcpserver/server.go b/internal/mcpserver/server.go
index 279c902..7349a45 100644
--- a/internal/mcpserver/server.go
+++ b/internal/mcpserver/server.go
@@ -150,6 +150,9 @@ func (s Server) Tools() []Tool {
 		{
 			Name:        "list_mailboxes",
 			Description: "List visible IMAP mailboxes for the configured account.",
+			InputSchema: map[string]any{
+				"type": "object",
+			},
 		},
 		{
 			Name:        "list_messages",