diff --git a/build/toolbase/doc.go b/build/toolbase/doc.go
index 8bf9e25..065d96f 100644
--- a/build/toolbase/doc.go
+++ b/build/toolbase/doc.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 // toolbase implements common functionality for tools operating on a Monogon
 // workspace, notably CI and developer tools.
 package toolbase