diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index bb968c0..11d0e31 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -12,5 +12,8 @@ jobs:
       - uses: actions/checkout@v3
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@main
+        with:
+          extra-conf: |
+            experimental-features = nix-command flakes impure-derivations ca-derivations
       - name: Test
         run: ./test/test.sh
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..750baeb
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+result
+result-*
diff --git a/flake.nix b/flake.nix
index 3d4720b..94d88a8 100644
--- a/flake.nix
+++ b/flake.nix
@@ -57,7 +57,7 @@
               perSystemPaths ++ flakePaths;
 
           # Build result organized by system
-          result = builtins.listToAttrs (map (sys:
+          buildResultForSystem = sys:
             let
               allPaths = collectPathsForSystem sys;
               uniquePaths = lib.unique allPaths;
@@ -67,11 +67,35 @@
                 in if name == null then acc else acc // { "${name}" = path; }
               ) { } outPaths;
             in
-              { name = sys; value = { inherit outPaths byName; }; }
-          ) build-systems);
+              { inherit allPaths outPaths byName; };
+
+          resultsBySystem = map (sys: { name = sys; value = buildResultForSystem sys; }) build-systems;
+
+          result = builtins.listToAttrs (map (item:
+            {
+              name = item.name;
+              value = { inherit (item.value) outPaths byName; };
+            }
+          ) resultsBySystem);
+
+          hasImpure = drv:
+            if lib.isDerivation drv
+            then drv.__impure or false
+            else false;
+
+          isImpure = lib.any (item: lib.any hasImpure item.value.allPaths) resultsBySystem;
         in
         {
-          default = pkgs.writeText "devour-output.json" (builtins.toJSON result);
+          default =
+            let
+              drv = pkgs.writeText "devour-output.json" (builtins.toJSON result);
+            in
+              if isImpure
+              then
+                lib.overrideDerivation drv (old: {
+                  __impure = true;
+                })
+              else drv;
         }
       );
     };
diff --git a/test/impure_flake/flake.nix b/test/impure_flake/flake.nix
new file mode 100644
index 0000000..78bb35a
--- /dev/null
+++ b/test/impure_flake/flake.nix
@@ -0,0 +1,18 @@
+{
+  outputs = { self }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin" ];
+      forAllSystems = f: builtins.listToAttrs (map (name: { inherit name; value = f name; }) systems);
+    in
+    {
+      packages = forAllSystems (system: {
+        impure = derivation {
+          name = "impure";
+          inherit system;
+          builder = "/bin/sh";
+          args = ["-c" "echo impure > $out"];
+          __impure = true;
+        };
+      });
+    };
+}
diff --git a/test/test.sh b/test/test.sh
index 194c004..6acb73f 100755
--- a/test/test.sh
+++ b/test/test.sh
@@ -7,4 +7,7 @@ nix build .. --override-input flake github:srid/haskell-multi-nix/c85563721c3886
 
 diff expected result
 
-rm -f result
\ No newline at end of file
+rm -f result
+
+# Run impure flake tests
+./test_impure.sh
diff --git a/test/test_impure.sh b/test/test_impure.sh
new file mode 100755
index 0000000..3ccde64
--- /dev/null
+++ b/test/test_impure.sh
@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+set -euxo pipefail
+
+# Navigate to test directory
+cd "$(dirname "$0")"
+
+echo "Running impure flake test..."
+
+nix build .. --override-input flake path:./impure_flake
+
+impure=$(nix eval ..#default.__impure --override-input flake path:./impure_flake)
+
+if [[ $impure == true ]]; then
+    echo "PASS: Derivation has __impure set to true."
+else
+    echo "FAIL: Derivation does NOT have __impure set to true."
+    exit 1
+fi
+
+echo "Impure flake test passed!"