diff --git a/flake.nix b/flake.nix
index be11a0f..07827cd 100644
--- a/flake.nix
+++ b/flake.nix
@@ -60,6 +60,31 @@
           })
           // {
             format = treefmt.config.build.check self;
+            fetcher-assert-npm =
+              let
+                # force download debian for consistent hash across systems
+                binaryTargetBySystem = {
+                  x86_64-linux = "debian";
+                  aarch64-linux = "debian";
+                  x86_64-darwin = "debian";
+                  aarch64-darwin = "debian";
+                };
+                prisma-legacy = prisma-factory {
+                  inherit pkgs binaryTargetBySystem;
+                  hash = "sha256-R9PG286KQTbzF0r/PPcShUkMiYam2prRh/JICjmhCZA=";
+                };
+                prisma-new =
+                  lockName: lockFile:
+                  prisma-factory {
+                    inherit pkgs binaryTargetBySystem;
+                    hash = "sha256-R9PG286KQTbzF0r/PPcShUkMiYam2prRh/JICjmhCZA=";
+                    ${lockName} = lockFile;
+                  };
+              in
+              assert
+                (prisma-legacy.fromNpmLock ./npm/package-lock.json).env
+                == (prisma-new "npmLock" ./npm/package-lock.json).env;
+              pkgs.hello;
           };
         packages.default =
           (prisma-factory {