diff --git a/cmd/plug/gen.go b/cmd/plug/gen.go
index 7b4a351..b9b0fca 100644
--- a/cmd/plug/gen.go
+++ b/cmd/plug/gen.go
@@ -2,17 +2,22 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"go/format"
 	"go/types"
 	"io"
+	"io/fs"
+	"log"
 	"os"
 	"path/filepath"
 	"reflect"
+	"runtime"
 	"strings"
 
-	"github.com/lufia/plug/plugcore"
 	"golang.org/x/tools/go/ast/astutil"
+
+	"github.com/lufia/plug/plugcore"
 )
 
 type Stub struct { // Plug?
@@ -23,11 +28,25 @@ type Stub struct { // Plug?
 func Rewrite(stub *Stub) (string, error) {
 	filePath := stub.f.path
 	name := filepath.Base(filePath)
-	dir := filepath.Join("plug", stub.f.pkg.path)
+	cacheDir, err := os.UserCacheDir()
+	if err != nil {
+		return "", fmt.Errorf("failed to get cachedir: %w", err)
+	}
+	dir := filepath.Join(cacheDir, "plug", runtime.Version(), stub.f.pkg.PathVersion())
 	if err := os.MkdirAll(dir, 0755); err != nil && !os.IsExist(err) {
 		return "", fmt.Errorf("failed to create %s: %w", dir, err)
 	}
+	if verbose {
+		log.Printf("cachedir: %s\n", dir)
+	}
 	file := filepath.Join(dir, name)
+	_, err = os.Stat(file)
+	if err == nil {
+		return file, nil
+	}
+	if !errors.Is(err, fs.ErrNotExist) {
+		return "", fmt.Errorf("failed to stat %s: %w", file, err)
+	}
 	w, err := os.Create(file)
 	if err != nil {
 		return "", fmt.Errorf("failed to create %s: %w", file, err)
@@ -43,13 +62,9 @@ func Rewrite(stub *Stub) (string, error) {
 	return file, nil
 }
 
-func pkgPath(v any) string {
-	return reflect.TypeOf(v).PkgPath()
-}
-
 func rewriteFile(w io.Writer, stub *Stub) error {
 	fset := stub.f.pkg.c.Fset
-	path := pkgPath(plugcore.Object{})
+	path := reflect.TypeOf(plugcore.Object{}).PkgPath()
 	astutil.AddImport(fset, stub.f.f, path)
 
 	var buf bytes.Buffer
@@ -57,7 +72,7 @@ func rewriteFile(w io.Writer, stub *Stub) error {
 		rewriteFunc(&buf, fn)
 	}
 	if verbose {
-		fmt.Printf("====\n%s\n====\n", buf.Bytes())
+		log.Printf("====\n%s\n====\n", buf.Bytes())
 	}
 	s, err := format.Source(buf.Bytes())
 	if err != nil {
diff --git a/cmd/plug/main.go b/cmd/plug/main.go
index 03c70d0..faa23ce 100644
--- a/cmd/plug/main.go
+++ b/cmd/plug/main.go
@@ -33,7 +33,7 @@ func main() {
 	flag.BoolVar(&verbose, "v", false, "enable verbose log")
 	flag.Parse()
 
-	pkgPath, err := loadPackagePath(".")
+	pkgPath, modVers, err := loadPackagePath(".")
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -41,7 +41,7 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	stubs := Group(syms)
+	stubs := Group(syms, modVers)
 
 	var o Overlay
 	for filePath, stub := range stubs {
@@ -56,11 +56,11 @@ func main() {
 	}
 }
 
-func loadPackagePath(dir string) (string, error) {
+func loadPackagePath(dir string) (string, map[string]string, error) {
 	// loader.Import does not handle "." notation that means current package.
 	dir, err := filepath.Abs(dir)
 	if err != nil {
-		return "", err
+		return "", nil, err
 	}
 	s := dir
 	file := filepath.Join(s, "go.mod")
@@ -70,36 +70,47 @@ func loadPackagePath(dir string) (string, error) {
 			break
 		}
 		if !os.IsNotExist(err) {
-			return "", err
+			return "", nil, err
 		}
 		up := filepath.Dir(s)
 		if up == s {
-			return "", fmt.Errorf("go.mod is not exist")
+			return "", nil, fmt.Errorf("go.mod is not exist")
 		}
 		s = up
 		file = filepath.Join(s, "go.mod")
 	}
 	data, err := os.ReadFile(file)
 	if err != nil {
-		return "", err
+		return "", nil, err
 	}
-	modPath := modfile.ModulePath(data)
+
+	f, err := modfile.Parse(file, data, nil)
+	if err != nil {
+		return "", nil, err
+	}
+	modPath := f.Module.Mod.Path
 	if modPath == "" {
-		return "", fmt.Errorf("%s: invalid go.mod syntax", file)
+		return "", nil, fmt.Errorf("%s: invalid go.mod syntax", file)
 	}
 	slug, err := filepath.Rel(s, dir)
 	if err != nil {
-		return "", err
+		return "", nil, err
+	}
+	pkgPath := path.Join(modPath, filepath.ToSlash(slug))
+
+	modVers := make(map[string]string)
+	for _, r := range f.Require {
+		modVers[r.Mod.Path] = r.Mod.Version
 	}
-	return path.Join(modPath, filepath.ToSlash(slug)), nil
+	return pkgPath, modVers, nil
 }
 
 // Group returns a map of Stub indexed by filePath.
-func Group(syms []*Sym) map[string]*Stub {
+func Group(syms []*Sym, modVers map[string]string) map[string]*Stub {
 	stubs := make(map[string]*Stub)
 	for _, sym := range syms {
 		pkgPath := sym.PkgPath()
-		pkg, err := LoadPackage(pkgPath)
+		pkg, err := LoadPackage(pkgPath, modVers[pkgPath])
 		if err != nil {
 			log.Fatalf("failed to load package %s: %v\n", pkgPath, err)
 		}
diff --git a/cmd/plug/pkg.go b/cmd/plug/pkg.go
index f7f45f2..a171ec7 100644
--- a/cmd/plug/pkg.go
+++ b/cmd/plug/pkg.go
@@ -11,8 +11,17 @@ import (
 
 type Pkg struct {
 	*loader.PackageInfo
-	c    *loader.Config
-	path string
+	c       *loader.Config
+	path    string
+	version string // If it is empty, maybe it is the stdlib
+}
+
+func (pkg *Pkg) PathVersion() string {
+	s := pkg.path
+	if v := pkg.version; v != "" {
+		s += "@" + v
+	}
+	return s
 }
 
 type File struct {
@@ -30,7 +39,7 @@ type Func struct {
 
 var pkgCache = make(map[string]*Pkg)
 
-func LoadPackage(pkgPath string) (*Pkg, error) {
+func LoadPackage(pkgPath, modVersion string) (*Pkg, error) {
 	if pkg, ok := pkgCache[pkgPath]; ok {
 		return pkg, nil
 	}
@@ -42,7 +51,7 @@ func LoadPackage(pkgPath string) (*Pkg, error) {
 	if err != nil {
 		return nil, err
 	}
-	pkg := &Pkg{p.Package(pkgPath), &c, pkgPath}
+	pkg := &Pkg{p.Package(pkgPath), &c, pkgPath, modVersion}
 	pkgCache[pkgPath] = pkg
 	return pkg, nil
 }