diff --git a/cmd/ninja/cmd/plugin_test.go b/cmd/ninja/cmd/plugin_test.go
index 48c073b..0aa5c41 100644
--- a/cmd/ninja/cmd/plugin_test.go
+++ b/cmd/ninja/cmd/plugin_test.go
@@ -38,6 +38,76 @@ func TestCheckRepoHasHEAD_WithCommitReturnsNil(t *testing.T) {
 	}
 }
 
+func TestAutoCommitPluginMod_CommitsWhenDirty(t *testing.T) {
+	dir := t.TempDir()
+	runGit(t, dir, "init", "-q")
+	if err := os.WriteFile(filepath.Join(dir, "README.md"), []byte("hi"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	runGit(t, dir, "add", "README.md")
+	runGit(t, dir, "commit", "-qm", "init")
+
+	if err := os.WriteFile(filepath.Join(dir, "plugin.mod"),
+		[]byte("[plugin]\nname = \"x\"\nscope = \"@s\"\nversion = \"0.1.0\"\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	t.Chdir(dir)
+	if err := autoCommitPluginMod(); err != nil {
+		t.Fatalf("autoCommitPluginMod: %v", err)
+	}
+
+	subject := gitLogSubject(t, dir)
+	if subject != "Add plugin.mod" {
+		t.Errorf("expected latest commit subject 'Add plugin.mod', got %q", subject)
+	}
+}
+
+func TestAutoCommitPluginMod_NoopWhenClean(t *testing.T) {
+	dir := t.TempDir()
+	runGit(t, dir, "init", "-q")
+	if err := os.WriteFile(filepath.Join(dir, "plugin.mod"),
+		[]byte("[plugin]\nname = \"x\"\nscope = \"@s\"\nversion = \"0.1.0\"\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	runGit(t, dir, "add", "plugin.mod")
+	runGit(t, dir, "commit", "-qm", "seed")
+
+	beforeSHA := gitHeadSHA(t, dir)
+
+	t.Chdir(dir)
+	if err := autoCommitPluginMod(); err != nil {
+		t.Fatalf("autoCommitPluginMod: %v", err)
+	}
+
+	afterSHA := gitHeadSHA(t, dir)
+	if afterSHA != beforeSHA {
+		t.Errorf("expected no new commit, HEAD moved %s -> %s", beforeSHA, afterSHA)
+	}
+}
+
+func gitLogSubject(t *testing.T, dir string) string {
+	t.Helper()
+	cmd := exec.Command("git", "log", "-1", "--pretty=%s")
+	cmd.Dir = dir
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("git log: %v\n%s", err, out)
+	}
+	return strings.TrimSpace(string(out))
+}
+
+func gitHeadSHA(t *testing.T, dir string) string {
+	t.Helper()
+	cmd := exec.Command("git", "rev-parse", "HEAD")
+	cmd.Dir = dir
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("git rev-parse: %v\n%s", err, out)
+	}
+	return strings.TrimSpace(string(out))
+}
+
 func runGit(t *testing.T, dir string, args ...string) {
 	t.Helper()
 	cmd := exec.Command("git", args...)