diff --git a/cells/repo/flake.lock b/cells/repo/flake.lock
index b99a60f..d6fd692 100644
--- a/cells/repo/flake.lock
+++ b/cells/repo/flake.lock
@@ -69,12 +69,31 @@
         "type": "gitlab"
       }
     },
+    "nixtest-lib": {
+      "locked": {
+        "dir": "lib",
+        "lastModified": 1765728058,
+        "narHash": "sha256-V3FXECl1oTxEtGteNz3o3GJs/X8asSn1TxRpZ2F+htU=",
+        "owner": "TECHNOFAB",
+        "repo": "nixtest",
+        "rev": "2477ad31ae3aa4134e1bb5eeddbebe0cb64ccb57",
+        "type": "gitlab"
+      },
+      "original": {
+        "dir": "lib",
+        "owner": "TECHNOFAB",
+        "ref": "v1.2.1",
+        "repo": "nixtest",
+        "type": "gitlab"
+      }
+    },
     "root": {
       "inputs": {
         "devshell": "devshell",
         "devtools-lib": "devtools-lib",
         "nix-gitlab-ci-lib": "nix-gitlab-ci-lib",
         "nixmkdocs": "nixmkdocs",
+        "nixtest-lib": "nixtest-lib",
         "soonix-lib": "soonix-lib",
         "treefmt-nix": "treefmt-nix"
       }
diff --git a/cells/repo/flake.nix b/cells/repo/flake.nix
index 46a3c1d..8180fcc 100644
--- a/cells/repo/flake.nix
+++ b/cells/repo/flake.nix
@@ -5,6 +5,7 @@
     soonix-lib.url = "gitlab:TECHNOFAB/soonix?dir=lib";
     nix-gitlab-ci-lib.url = "gitlab:TECHNOFAB/nix-gitlab-ci/3.1.2?dir=lib";
     devtools-lib.url = "gitlab:rensa-nix/devtools?dir=lib";
+    nixtest-lib.url = "gitlab:TECHNOFAB/nixtest/v1.2.1?dir=lib";
     treefmt-nix = {
       url = "github:numtide/treefmt-nix";
       flake = false;
@@ -18,6 +19,8 @@
       doclib = i.nixmkdocs.lib {inherit (i.parent) pkgs;};
       soonix = i.soonix-lib.lib {inherit (i.parent) pkgs;};
       cilib = i.nix-gitlab-ci-lib.lib {inherit (i.parent) pkgs;};
+      ntlib = i.nixtest-lib.lib {inherit (i.parent) pkgs;};
+      rensa = import "${i.parent.self}/lib" {inherit (i.parent.pkgs) lib;};
       treefmt = import i.treefmt-nix;
     };
 }
diff --git a/cells/repo/tests.nix b/cells/repo/tests.nix
new file mode 100644
index 0000000..fa75087
--- /dev/null
+++ b/cells/repo/tests.nix
@@ -0,0 +1,10 @@
+{inputs, ...}: let
+  inherit (inputs) pkgs ntlib rensa;
+in {
+  tests = ntlib.mkNixtest {
+    modules = ntlib.autodiscover {dir = "${inputs.self}/tests";};
+    args = {
+      inherit pkgs ntlib rensa;
+    };
+  };
+}
diff --git a/flake.nix b/flake.nix
index a47fa5a..52b2326 100644
--- a/flake.nix
+++ b/flake.nix
@@ -24,12 +24,14 @@
         (simple "docs")
         (simple "ci")
         (simple "soonix")
+        (simple "tests")
       ];
     } {
       packages = rensa.select inputs.self [
         ["repo" "docs"]
         ["repo" "ci" "packages"]
         ["repo" "soonix" "packages"]
+        ["repo" "tests"]
       ];
     };
 }
diff --git a/tests/builder_test.nix b/tests/builder_test.nix
new file mode 100644
index 0000000..a520b43
--- /dev/null
+++ b/tests/builder_test.nix
@@ -0,0 +1,90 @@
+{
+  pkgs,
+  ntlib,
+  rensa,
+  ...
+}: {
+  suites."Builder" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "has system outputs";
+        expected = true;
+        actual = let
+          testFlake = rensa.build {
+            inputs = {};
+            cellsFrom = ../cells;
+            cellBlocks = with rensa.blocks; [
+              (simple "test")
+            ];
+            systems = ["x86_64-linux"];
+          };
+        in
+          builtins.hasAttr "x86_64-linux" testFlake;
+      }
+      {
+        name = "has ren metadata";
+        expected = true;
+        actual = let
+          testFlake = rensa.build {
+            inputs = {};
+            cellsFrom = ../cells;
+            cellBlocks = with rensa.blocks; [
+              (simple "test")
+            ];
+            systems = ["x86_64-linux"];
+          };
+        in
+          builtins.hasAttr "__ren" testFlake;
+      }
+      {
+        name = "merges outputs";
+        expected = {
+          packages = {
+            test-package = "merged";
+          };
+        };
+        actual = let
+          testFlake =
+            rensa.buildWith {
+              inputs = {};
+              cellsFrom = ../cells;
+              cellBlocks = with rensa.blocks; [
+                (simple "test")
+              ];
+              systems = ["x86_64-linux"];
+            } {
+              packages = {
+                test-package = "merged";
+              };
+            };
+        in {
+          packages = testFlake.packages;
+        };
+      }
+      {
+        name = "eval test";
+        type = "script";
+        script = ''
+          ${ntlib.helpers.path [pkgs.gnugrep pkgs.coreutils pkgs.nix]}
+          ${ntlib.helpers.scriptHelpers}
+
+          result=$(nix eval --impure --expr '
+            let
+              rensa = import ${../lib} {lib = import ${pkgs.path}/lib;};
+              testFlake = rensa.build {
+                inputs = { test = "value"; };
+                cellsFrom = ${../cells};
+                cellBlocks = [];
+                systems = [ "x86_64-linux" ];
+              };
+            in
+              testFlake.__ren.__schema
+          ')
+
+          assert "$result == \"v0\"" "should contain version"
+        '';
+      }
+    ];
+  };
+}
diff --git a/tests/filter_test.nix b/tests/filter_test.nix
new file mode 100644
index 0000000..f45f1a3
--- /dev/null
+++ b/tests/filter_test.nix
@@ -0,0 +1,51 @@
+{rensa, ...}: {
+  suites."Filter" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "filter by cell";
+        expected = {
+          x86_64-linux = {};
+        };
+        actual = let
+          testFlake = {
+            __ren = {
+              cells = ["repo" "other"];
+            };
+            x86_64-linux = {
+              repo = {
+                docs = {value = "docs-value";};
+                ci = {value = "ci-value";};
+              };
+              other = {
+                packages = {value = "other-packages";};
+              };
+            };
+          };
+        in
+          rensa.filter (_: cell: cell == "repo") testFlake [["*" "*"]];
+      }
+      {
+        name = "filter by block";
+        expected = {
+          x86_64-linux = {};
+        };
+        actual = let
+          testFlake = {
+            __ren = {
+              cells = ["repo"];
+            };
+            x86_64-linux = {
+              repo = {
+                docs = {value = "docs-value";};
+                ci = {value = "ci-value";};
+                packages = {value = "packages-value";};
+              };
+            };
+          };
+        in
+          rensa.filter (block: _: block == "docs") testFlake [["repo" "*"]];
+      }
+    ];
+  };
+}
diff --git a/tests/loader_test.nix b/tests/loader_test.nix
new file mode 100644
index 0000000..9732293
--- /dev/null
+++ b/tests/loader_test.nix
@@ -0,0 +1,57 @@
+{
+  pkgs,
+  ntlib,
+  rensa,
+  ...
+}: {
+  suites."Loader" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "cell sibling access";
+        expected = {
+          hello = "world";
+        };
+        actual = let
+          testFlake = rensa.build {
+            inputs = {};
+            cellsFrom = ../cells;
+            cellBlocks = with rensa.blocks; [
+              (simple "test")
+            ];
+            systems = ["x86_64-linux"];
+          };
+        in
+          testFlake.x86_64-linux.test.test;
+      }
+      {
+        name = "load file";
+        type = "script";
+        script = ''
+          ${ntlib.helpers.path [pkgs.gnugrep pkgs.coreutils pkgs.nix]}
+          ${ntlib.helpers.scriptHelpers}
+
+          mkdir -p "cells/testcell"
+          echo '{ hello = "world"; }' > "cells/testcell/packages.nix"
+
+          cat > "flake.nix" << 'EOF'
+          {
+            outputs = inputs: let
+              rensa = import ${../lib} { lib = import "${pkgs.path}/lib"; };
+            in rensa.build {
+              inputs = {};
+              cellsFrom = ./cells;
+              cellBlocks = with rensa.blocks; [ (simple "packages") ];
+              systems = [ "x86_64-linux" ];
+            };
+          }
+          EOF
+
+          result=$(nix eval --impure .#x86_64-linux.testcell.packages.hello)
+
+          assert "$result == \"world\"" "should equal to world"
+        '';
+      }
+    ];
+  };
+}
diff --git a/tests/select_test.nix b/tests/select_test.nix
new file mode 100644
index 0000000..823e101
--- /dev/null
+++ b/tests/select_test.nix
@@ -0,0 +1,49 @@
+{rensa, ...}: {
+  suites."Select" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "single block";
+        expected = {
+          x86_64-linux = {value = "test-value";};
+        };
+        actual = let
+          testFlake = {
+            __ren = {
+              cells = ["repo"];
+            };
+            x86_64-linux = {
+              repo = {
+                docs = {value = "test-value";};
+                ci = {packages = {value = "ci-packages";};};
+              };
+            };
+          };
+        in
+          rensa.select testFlake [["repo" "docs"]];
+      }
+      {
+        name = "multiple blocks";
+        expected = {
+          x86_64-linux = {value = "ci-packages";};
+        };
+        actual = let
+          testFlake = {
+            __ren = {
+              cells = ["repo"];
+            };
+            x86_64-linux = {
+              repo = {
+                ci = {packages = {value = "ci-packages";};};
+                other = {value = "ignored";};
+              };
+            };
+          };
+        in
+          rensa.select testFlake [
+            ["repo" "ci" "packages"]
+          ];
+      }
+    ];
+  };
+}