diff --git a/codeagent-wrapper/agent_config_test.go b/codeagent-wrapper/agent_config_test.go
index 0a8e9ad..a3602be 100644
--- a/codeagent-wrapper/agent_config_test.go
+++ b/codeagent-wrapper/agent_config_test.go
@@ -188,6 +188,15 @@ func TestOpencodeBackend_BuildArgs(t *testing.T) {
 			t.Errorf("got %v, want %v", got, want)
 		}
 	})
+
+	t.Run("stdin mode omits dash", func(t *testing.T) {
+		cfg := &Config{Mode: "new"}
+		got := backend.BuildArgs(cfg, "-")
+		want := []string{"run", "--format", "json"}
+		if !reflect.DeepEqual(got, want) {
+			t.Errorf("got %v, want %v", got, want)
+		}
+	})
 }
 
 func TestOpencodeBackend_Interface(t *testing.T) {
diff --git a/codeagent-wrapper/backend.go b/codeagent-wrapper/backend.go
index d5c5473..5a73410 100644
--- a/codeagent-wrapper/backend.go
+++ b/codeagent-wrapper/backend.go
@@ -204,7 +204,10 @@ func (OpencodeBackend) BuildArgs(cfg *Config, targetArg string) []string {
 	if cfg.Mode == "resume" && cfg.SessionID != "" {
 		args = append(args, "-s", cfg.SessionID)
 	}
-	args = append(args, "--format", "json", targetArg)
+	args = append(args, "--format", "json")
+	if targetArg != "-" {
+		args = append(args, targetArg)
+	}
 	return args
 }