diff --git a/nix/ci.nix b/nix/ci.nix
index 91f1a0f..4cf9309 100644
--- a/nix/ci.nix
+++ b/nix/ci.nix
@@ -1,6 +1,37 @@
-args@{ pkgs ? import ./default.nix args, system ? null, ... }:
+let
+  sources = import ./sources.nix;
+  lib = import (sources."nixpkgs" + "/lib");
+  inherit (import sources."project.nix" { inherit lib; }) dimension;
+in
 
-{
-  inherit (pkgs) arion tests;
-  doc = pkgs.recurseIntoAttrs (import ../doc { inherit pkgs; });
-}
+dimension "Nixpkgs version" {
+    "nixos-19_03" = {
+      nixpkgsSource = "nixpkgs";
+    };
+    "nixos-unstable" = {
+      nixpkgsSource = "nixos-unstable";
+
+      # Broken on unstable, wontfix because doc tooling will be changed.
+      # TODO: reenable
+      enableDoc = false;
+    };
+  } (
+    _name: { nixpkgsSource, enableDoc ? true }:
+
+
+      dimension "System" {
+        "x86_64-linux" = {};
+        # TODO: darwin
+        # "x86_64-darwin" = { enableNixOSTests = false; };
+      } (
+        system: {}:
+          let
+            pkgs = import ./. { inherit system; nixpkgsSrc = sources.${nixpkgsSource}; };
+          in
+          {
+            inherit (pkgs) arion tests;
+          } // lib.optionalAttrs enableDoc {
+            doc = pkgs.recurseIntoAttrs (import ../doc { inherit pkgs; });
+          }
+      )
+  )
diff --git a/nix/default.nix b/nix/default.nix
index 12e5b95..7393d12 100644
--- a/nix/default.nix
+++ b/nix/default.nix
@@ -1,9 +1,6 @@
-/**
- * This is the entry-point for all nix execution in this project.
- */
 { sources ? import ./sources.nix
 , nixpkgsSrc ? sources.nixpkgs
-, system ? null
+, system ? builtins.currentSystem
 , ...
 }:
 
@@ -15,6 +12,5 @@ import nixpkgsSrc ({
     # all the packages are defined there:
     (import ./overlay.nix)
   ];
-} // (if system == null then {} else {
   inherit system;
-}))
+})
diff --git a/nix/overlay.nix b/nix/overlay.nix
index 5c3fa01..be6a8e1 100644
--- a/nix/overlay.nix
+++ b/nix/overlay.nix
@@ -8,7 +8,7 @@ let
 in
 {
 
-  arion = import ./arion.nix { pkgs = self; };
+  inherit (import ./.. { pkgs = self; }) arion;
   tests = super.callPackage ../tests {};
   doc = super.callPackage ../doc {};
 
diff --git a/nix/sources.json b/nix/sources.json
index efa1ae1..fc22e3a 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -11,6 +11,19 @@
         "url": "https://github.com/nmattia/niv/archive/1dd094156b249586b66c16200ecfd365c7428dc0.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
+    "nixos-unstable": {
+        "branch": "nixos-unstable",
+        "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
+        "homepage": "https://github.com/NixOS/nixpkgs",
+        "owner": "NixOS",
+        "repo": "nixpkgs-channels",
+        "rev": "cb4332e3eb6dfdb653f1fc7397a0292df228a533",
+        "sha256": "1722wphznqhpfny08rcy19l85r2l893ckjc3h1vfivj6aj64fwjr",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs-channels/archive/cb4332e3eb6dfdb653f1fc7397a0292df228a533.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz",
+        "version": ""
+    },
     "nixpkgs": {
         "branch": "nixos-19.03",
         "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
@@ -22,5 +35,17 @@
         "type": "tarball",
         "url": "https://github.com/NixOS/nixpkgs-channels/archive/6420e2649fa9e267481fb78e602022dab9d1dcd1.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "project.nix": {
+        "branch": "master",
+        "description": "A configuration manager for your projects",
+        "homepage": null,
+        "owner": "hercules-ci",
+        "repo": "project.nix",
+        "rev": "33e5f3cb25feff4ccd00f8c60a05976e2ee01802",
+        "sha256": "0c3q3il5h6q3ms8m6da51knvjsfvpz12sh3a3av4d2a5ikm5ncl1",
+        "type": "tarball",
+        "url": "https://github.com/hercules-ci/project.nix/archive/33e5f3cb25feff4ccd00f8c60a05976e2ee01802.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }