diff --git a/chfs-ng/internal/api/server.go b/chfs-ng/internal/api/server.go
index ed136af..b03fac0 100644
--- a/chfs-ng/internal/api/server.go
+++ b/chfs-ng/internal/api/server.go
@@ -302,10 +302,10 @@ func (s *Server) rename(w http.ResponseWriter, r *http.Request) {
 	}
 	if old != neu {
 		if _, e := s.Store.Stat(neu); e == nil {
-			if e := checkDeleteTree(s.Store, a, neu); e != nil {
-				writeJSONError(w, http.StatusForbidden, "has no privilege")
-				return
-			}
+			// CHFS rename is create-only: an existing destination is rejected
+			// rather than overwritten, even when the caller has delete access.
+			writeJSONError(w, http.StatusBadRequest, "invalid parameter")
+			return
 		} else if !os.IsNotExist(e) {
 			http.Error(w, e.Error(), 500)
 			return
@@ -453,6 +453,13 @@ func (s *Server) newtext(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	p := path.Join(dir, filename)
+	if _, e := s.Store.Stat(p); e == nil {
+		writeJSONError(w, http.StatusBadRequest, "invalid parameter")
+		return
+	} else if !os.IsNotExist(e) {
+		http.Error(w, e.Error(), 500)
+		return
+	}
 	if !a.MaskFor(p).HasWrite() {
 		writeJSONError(w, http.StatusForbidden, "has no privilege")
 		return
@@ -518,7 +525,15 @@ func (s *Server) upload(w http.ResponseWriter, r *http.Request) {
 				http.Error(w, "invalid filename", 400)
 				return
 			}
-			if e := savePart(s.Store, path.Join(dir, name), h); e != nil {
+			target := path.Join(dir, name)
+			if _, e := s.Store.Stat(target); e == nil {
+				writeJSONError(w, http.StatusBadRequest, "invalid parameter")
+				return
+			} else if !os.IsNotExist(e) {
+				http.Error(w, e.Error(), 500)
+				return
+			}
+			if e := savePart(s.Store, target, h); e != nil {
 				http.Error(w, e.Error(), 500)
 				return
 			}
@@ -570,7 +585,15 @@ func (s *Server) upload2(w http.ResponseWriter, r *http.Request) {
 				http.Error(w, "invalid filename", 400)
 				return
 			}
-			if err := savePart(s.Store, path.Join(dir, name), h); err != nil {
+			target := path.Join(dir, name)
+			if _, e := s.Store.Stat(target); e == nil {
+				writeJSONError(w, http.StatusBadRequest, "invalid parameter")
+				return
+			} else if !os.IsNotExist(e) {
+				http.Error(w, e.Error(), 500)
+				return
+			}
+			if err := savePart(s.Store, target, h); err != nil {
 				http.Error(w, err.Error(), 500)
 				return
 			}