diff --git a/cmd/nixgen/main.go b/cmd/nixgen/main.go
index 907f172..c76bc45 100644
--- a/cmd/nixgen/main.go
+++ b/cmd/nixgen/main.go
@@ -32,7 +32,7 @@ func maine() error {
 		}
 		return nil
 	}
-	fs.Func("format", "Output format", formatFunc)
+	fs.Func("format", "Output format [json, yaml, toml]: default 'json'", formatFunc)
 	fs.Func("f", "--format", formatFunc)
 	if err := fs.Parse(os.Args[1:]); err != nil {
 		return err