diff --git a/codeagent-wrapper/backend.go b/codeagent-wrapper/backend.go
index 5a0c79a..aed8617 100644
--- a/codeagent-wrapper/backend.go
+++ b/codeagent-wrapper/backend.go
@@ -74,7 +74,7 @@ func (GeminiBackend) BuildArgs(cfg *Config, targetArg string) []string {
 
 	if cfg.Mode == "resume" {
 		if cfg.SessionID != "" {
-			args = append(args, "--session-id", cfg.SessionID)
+			args = append(args, "-r", cfg.SessionID)
 		}
 	} else {
 		args = append(args, "-C", workdir)
diff --git a/codeagent-wrapper/backend_test.go b/codeagent-wrapper/backend_test.go
index ffc69da..a74e35c 100644
--- a/codeagent-wrapper/backend_test.go
+++ b/codeagent-wrapper/backend_test.go
@@ -66,7 +66,7 @@ func TestClaudeBuildArgs_GeminiAndCodexModes(t *testing.T) {
 		backend := GeminiBackend{}
 		cfg := &Config{Mode: "resume", SessionID: "sid-999"}
 		got := backend.BuildArgs(cfg, "resume")
-		want := []string{"-o", "stream-json", "-y", "--session-id", "sid-999", "-p", "resume"}
+		want := []string{"-o", "stream-json", "-y", "-r", "sid-999", "-p", "resume"}
 		if !reflect.DeepEqual(got, want) {
 			t.Fatalf("got %v, want %v", got, want)
 		}