diff --git a/overlay.nix b/overlay.nix
index 782b8b9..6af1703 100644
--- a/overlay.nix
+++ b/overlay.nix
@@ -7,6 +7,14 @@ final: prev: {
   makeSquashFs = prev.callPackage ./lib/make-squashfs {};
   makeHpcDist = final.callPackage ./lib/make-hpc-dist {};
 
+  instrumentedFetch = drv: drv.overrideAttrs (afinal: aprev: {
+    postFetch = (aprev.postFetch or "") + ''
+      printf "FETCH_HASH:%s:FETCH_HASH" "$(\
+        ${final.lib.getExe final.nix} --extra-experimental-features "nix-command" \
+          hash path --sri "$out")"
+    '';
+  });
+
   ghidra_headless = final.ghidra.lib;
 
   # stuff that tracks upstream