diff --git a/nixos/lib/eval-config.nix b/nixos/lib/eval-config.nix
index 8bab3752073f..ad028013f65f 100644
--- a/nixos/lib/eval-config.nix
+++ b/nixos/lib/eval-config.nix
@@ -65,6 +65,14 @@ let
   withWarnings = x:
     lib.warnIf (evalConfigArgs?extraArgs) "The extraArgs argument to eval-config.nix is deprecated. Please set config._module.args instead."
     lib.warnIf (evalConfigArgs?check) "The check argument to eval-config.nix is deprecated. Please set config._module.check instead."
+    lib.warnIf (specialArgs?pkgs) ''
+      You have set specialArgs.pkgs, which means that options like nixpkgs.config
+      and nixpkgs.overlays will be ignored. If you wish to reuse an already created
+      pkgs, which you know is configured correctly for this NixOS configuration,
+      please import the `nixosModules.pkgsReadOnly` module from the nixpkgs flake or
+      `(modulesPath + "/misc/nixpkgs/read-only.nix"), and set `{ nixpkgs.pkgs = <your pkgs>; }`.
+      This properly disables the ignored options to prevent future surprises.
+    ''
     x;
 
   legacyModules =