diff --git a/flake.lock b/flake.lock
index 51b3cd6ca2..11d47b8cac 100644
--- a/flake.lock
+++ b/flake.lock
@@ -147,6 +147,65 @@
         "type": "github"
       }
     },
+    "hydra": {
+      "inputs": {
+        "nix": "nix",
+        "nixpkgs": [
+          "hydra",
+          "nix",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1646878427,
+        "narHash": "sha256-KtbrofMtN8GlM7D+n90kixr7QpSlVmdN+vK5CA/aRzc=",
+        "owner": "NixOS",
+        "repo": "hydra",
+        "rev": "28b682b85b7efc5cf7974065792a1f22203a5927",
+        "type": "github"
+      },
+      "original": {
+        "id": "hydra",
+        "type": "indirect"
+      }
+    },
+    "lowdown-src": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1633514407,
+        "narHash": "sha256-Dw32tiMjdK9t3ETl5fzGrutQTzh2rufgZV4A/BbxuD4=",
+        "owner": "kristapsdz",
+        "repo": "lowdown",
+        "rev": "d2c2b44ff6c27b936ec27358a2653caaef8f73b8",
+        "type": "github"
+      },
+      "original": {
+        "owner": "kristapsdz",
+        "repo": "lowdown",
+        "type": "github"
+      }
+    },
+    "nix": {
+      "inputs": {
+        "lowdown-src": "lowdown-src",
+        "nixpkgs": "nixpkgs",
+        "nixpkgs-regression": "nixpkgs-regression"
+      },
+      "locked": {
+        "lastModified": 1643066034,
+        "narHash": "sha256-xEPeMcNJVOeZtoN+d+aRwolpW8mFSEQx76HTRdlhPhg=",
+        "owner": "NixOS",
+        "repo": "nix",
+        "rev": "a1cd7e58606a41fcf62bf8637804cf8306f17f62",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "2.6.0",
+        "repo": "nix",
+        "type": "github"
+      }
+    },
     "nix-tools": {
       "flake": false,
       "locked": {
@@ -163,6 +222,21 @@
         "type": "github"
       }
     },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1632864508,
+        "narHash": "sha256-d127FIvGR41XbVRDPVvozUPQ/uRHbHwvfyKHwEt5xFM=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "82891b5e2c2359d7e58d08849e4c89511ab94234",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-21.05-small",
+        "type": "indirect"
+      }
+    },
     "nixpkgs-2003": {
       "locked": {
         "lastModified": 1620055814,
@@ -211,6 +285,21 @@
         "type": "github"
       }
     },
+    "nixpkgs-regression": {
+      "locked": {
+        "lastModified": 1643052045,
+        "narHash": "sha256-uGJ0VXIhWKGXxkeNnq4TvV3CIOkUJ3PAoLZ3HMzNVMw=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "215d4d0fd80ca5163643b03a33fde804a29cc1e2",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "rev": "215d4d0fd80ca5163643b03a33fde804a29cc1e2",
+        "type": "indirect"
+      }
+    },
     "nixpkgs-unstable": {
       "locked": {
         "lastModified": 1644486793,
@@ -255,6 +344,7 @@
         "ghc-8.6.5-iohk": "ghc-8.6.5-iohk",
         "hackage": "hackage",
         "hpc-coveralls": "hpc-coveralls",
+        "hydra": "hydra",
         "nix-tools": "nix-tools",
         "nixpkgs": [
           "nixpkgs-unstable"
diff --git a/flake.nix b/flake.nix
index 1a368b96a5..9165746764 100644
--- a/flake.nix
+++ b/flake.nix
@@ -8,6 +8,7 @@
     nixpkgs-2111 = { url = "github:NixOS/nixpkgs/nixpkgs-21.11-darwin"; };
     nixpkgs-unstable = { url = "github:NixOS/nixpkgs/nixpkgs-unstable"; };
     flake-utils = { url = "github:numtide/flake-utils"; };
+    hydra.url = "hydra";
     hackage = {
       url = "github:input-output-hk/hackage.nix";
       flake = false;
diff --git a/lib/call-cabal-project-to-nix.nix b/lib/call-cabal-project-to-nix.nix
index 5461486712..2a4b670e37 100644
--- a/lib/call-cabal-project-to-nix.nix
+++ b/lib/call-cabal-project-to-nix.nix
@@ -103,15 +103,13 @@ let
   ghc = ghc';
   subDir' = src.origSubDir or "";
   subDir = pkgs.lib.strings.removePrefix "/" subDir';
-  maybeCleanedSource =
-    if haskellLib.canCleanSource src
-      then (haskellLib.cleanSourceWith {
-        name = if name != null then "${name}-root-cabal-files" else "source-root-cabal-files";
-        src = src.origSrc or src;
-        filter = path: type: (!(src ? filter) || src.filter path type) && (
-          type == "directory" ||
-          pkgs.lib.any (i: (pkgs.lib.hasSuffix i path)) [ ".cabal" "package.yaml" ]); })
-      else src.origSrc or src;
+
+  cleanedSource = haskellLib.cleanSourceWith {
+    name = if name != null then "${name}-root-cabal-files" else "source-root-cabal-files";
+    src = src.origSrc or src;
+    filter = path: type: (!(src ? filter) || src.filter path type) && (
+      type == "directory" ||
+      pkgs.lib.any (i: (pkgs.lib.hasSuffix i path)) [ ".cabal" "package.yaml" ]); };
 
   # Using origSrcSubDir bypasses any cleanSourceWith so that it will work when
   # access to the store is restricted.  If origSrc was already in the store
@@ -473,17 +471,17 @@ let
   } ''
     tmp=$(mktemp -d)
     cd $tmp
-    # if maybeCleanedSource is empty, this means it's a new
+    # if cleanedSource is empty, this means it's a new
     # project where the files haven't been added to the git
     # repo yet. We fail early and provide a useful error
     # message to prevent headaches (#290).
-    if [ -z "$(ls -A ${maybeCleanedSource})" ]; then
+    if [ -z "$(ls -A ${cleanedSource})" ]; then
       echo "cleaned source is empty. Did you forget to 'git add -A'?"
       ${pkgs.lib.optionalString (__length fixedProject.sourceRepos == 0) ''
         exit 1
       ''}
     else
-      cp -r ${maybeCleanedSource}/* .
+      cp -r ${cleanedSource}/* .
     fi
     chmod +w -R .
     # Decide what to do for each `package.yaml` file.
diff --git a/overlays/default.nix b/overlays/default.nix
index 6a37002324..92157724b3 100644
--- a/overlays/default.nix
+++ b/overlays/default.nix
@@ -10,6 +10,7 @@ let
     bootstrap = import ./bootstrap.nix;
     ghc = import ./ghc.nix;
     ghc-packages = import ./ghc-packages.nix;
+    hydra = import ./hydra.nix args;
     darwin = import ./darwin.nix;
     windows = import ./windows.nix;
     armv6l-linux = import ./armv6l-linux.nix;
@@ -56,6 +57,7 @@ let
     gobject-introspection
     hix
     eval-packages
+    hydra
     # Restore nixpkgs haskell and haskellPackages
     (_: prev: { inherit (prev.haskell-nix-prev) haskell haskellPackages; })
   ];
diff --git a/overlays/hydra.nix b/overlays/hydra.nix
new file mode 100644
index 0000000000..7ba918fd91
--- /dev/null
+++ b/overlays/hydra.nix
@@ -0,0 +1,5 @@
+{ sources, ...}:
+
+final: prev: {
+  hydra-unstable = sources.hydra.defaultPackage.${prev.system};
+}
diff --git a/scripts/check-hydra.nix b/scripts/check-hydra.nix
index 4a56058aae..110c83cb82 100644
--- a/scripts/check-hydra.nix
+++ b/scripts/check-hydra.nix
@@ -8,6 +8,7 @@ writeScript "check-hydra.sh" ''
   set -euo pipefail
 
   export PATH="${makeBinPath [ coreutils time gnutar gzip hydra-unstable jq gitMinimal ]}"
+  export NIX_PATH=
 
   echo '~~~ Evaluating release.nix with --arg ifdLevel '$1
   command time --format '%e' -o eval-time.txt \
@@ -15,7 +16,7 @@ writeScript "check-hydra.sh" ''
       --option allowed-uris "https://github.com/NixOS https://github.com/input-output-hk" \
       --arg supportedSystems '[ builtins.currentSystem ]' \
       --arg ifdLevel $1 \
-      -I . release.nix > eval.json
+      -I $(realpath .) release.nix > eval.json
   EVAL_EXIT_CODE="$?"
   if [ "$EVAL_EXIT_CODE" != 0 ]
   then