diff --git a/x/build/build.go b/x/build/build.go
index 0b99626ed..10fae033c 100644
--- a/x/build/build.go
+++ b/x/build/build.go
@@ -21,7 +21,6 @@ import (
 	"fmt"
 	goast "go/ast"
 	"go/types"
-	"path"
 	"path/filepath"
 
 	"github.com/goplus/gop/ast"
@@ -31,6 +30,7 @@ import (
 	"github.com/goplus/gop/token"
 	"github.com/goplus/gox"
 	"github.com/goplus/gox/packages"
+	"github.com/goplus/mod/modfile"
 )
 
 type Class = cl.Class
@@ -76,7 +76,7 @@ func (p *Package) ToAst() *goast.File {
 }
 
 func ClassKind(fname string) (isProj, ok bool) {
-	ext := path.Ext(fname)
+	ext := modfile.ClassExt(fname)
 	switch ext {
 	case ".gmx":
 		return true, true
diff --git a/x/build/build_test.go b/x/build/build_test.go
index 116776ebe..b09985c55 100644
--- a/x/build/build_test.go
+++ b/x/build/build_test.go
@@ -44,6 +44,7 @@ func init() {
 	build.RegisterClassFileType(".tspx", "MyGame", []*build.Class{
 		{Ext: ".tspx", Class: "Sprite"},
 	}, "github.com/goplus/gop/cl/internal/spx")
+	build.RegisterClassFileType("_yap.gox", "App", nil, "github.com/goplus/yap")
 }
 
 func gopClTest(t *testing.T, gopcode interface{}, expected string) {
@@ -76,6 +77,7 @@ func TestKind(t *testing.T) {
 	testKind(t, "main.gmx", true, true)
 	testKind(t, "Cat.tspx", false, true)
 	testKind(t, "main.tspx", true, true)
+	testKind(t, "blog_yap.gox", true, true)
 }
 
 func TestGop(t *testing.T) {