diff --git a/modules/jj.nix b/modules/jj.nix
--- a/modules/jj.nix
+++ b/modules/jj.nix
@@ -48,6 +48,7 @@ "mix"
             "format"
             "--stdin-filename"
             "$path"
+            "-"
           ];
           patterns = [
             "glob:**/*.ex"
@@ -59,6 +60,9 @@ nix-fmt = {
           command = [
             "nix"
             "fmt"
+            "--"
+            "--stdin"
+            "$path"
           ];
           patterns = [ "glob:**/*.nix" ];
         };