diff --git a/internal/sidecarassets/embed_stub_test.go b/internal/sidecarassets/embed_stub_test.go
new file mode 100644
index 0000000..8d27408
--- /dev/null
+++ b/internal/sidecarassets/embed_stub_test.go
@@ -0,0 +1,27 @@
+//go:build !withsidecar
+
+package sidecarassets
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestStubBuild_IsPlaceholder(t *testing.T) {
+	if !IsPlaceholder() {
+		t.Error("default build (no -tags withsidecar) must report a placeholder")
+	}
+	if EmbeddedSize() != 0 {
+		t.Errorf("placeholder build must embed no JAR, got %d bytes", EmbeddedSize())
+	}
+}
+
+func TestStubBuild_ExtractErrors(t *testing.T) {
+	_, err := Extract(t.TempDir())
+	if err == nil {
+		t.Fatal("Extract must fail when no JAR is embedded")
+	}
+	if !strings.Contains(err.Error(), "withsidecar") {
+		t.Errorf("error should tell the user to rebuild with -tags withsidecar, got %v", err)
+	}
+}