diff --git a/test/sandbox/assert.go b/test/sandbox/assert.go
index e00bc5d..388d232 100644
--- a/test/sandbox/assert.go
+++ b/test/sandbox/assert.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 /*
 Package sandbox provides utilities for checking sandbox outcome.
 
diff --git a/test/sandbox/assert_test.go b/test/sandbox/assert_test.go
index c3fb580..efeb7cb 100644
--- a/test/sandbox/assert_test.go
+++ b/test/sandbox/assert_test.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 import (
diff --git a/test/sandbox/fs.go b/test/sandbox/fs.go
index 2aa3aa5..36d6c6d 100644
--- a/test/sandbox/fs.go
+++ b/test/sandbox/fs.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 import (
diff --git a/test/sandbox/fs_test.go b/test/sandbox/fs_test.go
index 4486872..8dd920b 100644
--- a/test/sandbox/fs_test.go
+++ b/test/sandbox/fs_test.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox_test
 
 import (
diff --git a/test/sandbox/mount.go b/test/sandbox/mount.go
index 2940572..791a68b 100644
--- a/test/sandbox/mount.go
+++ b/test/sandbox/mount.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 /*
diff --git a/test/sandbox/mount_test.go b/test/sandbox/mount_test.go
index 43970b2..7d6b0df 100644
--- a/test/sandbox/mount_test.go
+++ b/test/sandbox/mount_test.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox_test
 
 import (
diff --git a/test/sandbox/ptrace.go b/test/sandbox/ptrace.go
index b6e4130..3f1acad 100644
--- a/test/sandbox/ptrace.go
+++ b/test/sandbox/ptrace.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 import (
diff --git a/test/sandbox/seccomp.go b/test/sandbox/seccomp.go
index 8ef59c9..7df781d 100644
--- a/test/sandbox/seccomp.go
+++ b/test/sandbox/seccomp.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 import (
diff --git a/test/sandbox/tool/main.go b/test/sandbox/tool/main.go
index 7731074..f53950c 100644
--- a/test/sandbox/tool/main.go
+++ b/test/sandbox/tool/main.go
@@ -1,3 +1,5 @@
+//go:build testtool
+
 package main
 
 import (
diff --git a/test/sandbox/tool/package.nix b/test/sandbox/tool/package.nix
index 43e21ca..92829b2 100644
--- a/test/sandbox/tool/package.nix
+++ b/test/sandbox/tool/package.nix
@@ -17,6 +17,8 @@ buildGoModule rec {
   };
   vendorHash = null;
 
+  tags = [ "testtool" ];
+
   buildInputs = [ util-linux ];
   nativeBuildInputs = [ pkg-config ];