diff --git a/shell.nix b/shell.nix
index 10e427a..f6f7a2c 100644
--- a/shell.nix
+++ b/shell.nix
@@ -7,6 +7,7 @@ mkShell {
     python3.pkgs.mypy
     python3.pkgs.black
     python3.pkgs.flake8
+    ruff
     shellcheck
     direnv
   ];
diff --git a/tests/conftest.py b/tests/conftest.py
index 43c8c83..8a849bf 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 pytest_plugins = [
     "direnv_project",
     "root",
diff --git a/tests/direnv_project.py b/tests/direnv_project.py
index 784eaaa..124b7fd 100644
--- a/tests/direnv_project.py
+++ b/tests/direnv_project.py
@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 import shutil
 import textwrap
 from dataclasses import dataclass
@@ -20,9 +18,10 @@ class DirenvProject:
     def envrc(self) -> Path:
         return self.dir / ".envrc"
 
-    def setup_envrc(self, content: str) -> None:
+    def setup_envrc(self, content: str, strict_env: bool) -> None:
         text = textwrap.dedent(
             f"""
+        {'strict_env' if strict_env else ''}
         source {self.nix_direnv}
         {content}
         """
diff --git a/tests/procs.py b/tests/procs.py
index 7f8cf5e..ea837d4 100644
--- a/tests/procs.py
+++ b/tests/procs.py
@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 import subprocess
 from pathlib import Path
 from typing import IO, Any, List, Optional, Union
diff --git a/tests/root.py b/tests/root.py
index 087c307..b3f0914 100644
--- a/tests/root.py
+++ b/tests/root.py
@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 from pathlib import Path
 
 import pytest
diff --git a/tests/test_gc.py b/tests/test_gc.py
index 012ef54..e46fcfd 100644
--- a/tests/test_gc.py
+++ b/tests/test_gc.py
@@ -1,9 +1,8 @@
-#!/usr/bin/env python2
-
 import subprocess
 import sys
 import unittest
 
+import pytest
 from direnv_project import DirenvProject
 from procs import run
 
@@ -60,18 +59,21 @@ def common_test_clean(direnv_project: DirenvProject) -> None:
     assert len(profiles) == 1
 
 
-def test_use_nix(direnv_project: DirenvProject) -> None:
-    direnv_project.setup_envrc("use nix")
+@pytest.mark.parametrize("strict_env", [False, True])
+def test_use_nix(direnv_project: DirenvProject, strict_env: bool) -> None:
+    direnv_project.setup_envrc("use nix", strict_env=strict_env)
     common_test(direnv_project)
 
     direnv_project.setup_envrc(
-        "use nix --argstr shellHook 'echo Executing hijacked shellHook.'"
+        "use nix --argstr shellHook 'echo Executing hijacked shellHook.'",
+        strict_env=strict_env,
     )
     common_test_clean(direnv_project)
 
 
-def test_use_flake(direnv_project: DirenvProject) -> None:
-    direnv_project.setup_envrc("use flake")
+@pytest.mark.parametrize("strict_env", [False, True])
+def test_use_flake(direnv_project: DirenvProject, strict_env: bool) -> None:
+    direnv_project.setup_envrc("use flake", strict_env=strict_env)
     common_test(direnv_project)
     inputs = list((direnv_project.dir / ".direnv/flake-inputs").iterdir())
     # should only contain our flake-utils flake
@@ -82,7 +84,7 @@ def test_use_flake(direnv_project: DirenvProject) -> None:
     for symlink in inputs:
         assert symlink.is_dir()
 
-    direnv_project.setup_envrc("use flake --impure")
+    direnv_project.setup_envrc("use flake --impure", strict_env=strict_env)
     common_test_clean(direnv_project)
 
 
diff --git a/tests/test_use_nix.py b/tests/test_use_nix.py
index 8144a62..0c79fa5 100644
--- a/tests/test_use_nix.py
+++ b/tests/test_use_nix.py
@@ -1,11 +1,10 @@
-#!/usr/bin/env python2
-
 import os
 import subprocess
 import sys
 import unittest
 from typing import Optional
 
+import pytest
 from direnv_project import DirenvProject
 from procs import run
 
@@ -30,25 +29,29 @@ def direnv_exec(
     assert "renewed cache" in out.stderr
 
 
-def test_attrs(direnv_project: DirenvProject) -> None:
-    direnv_project.setup_envrc("use nix -A subshell")
+@pytest.mark.parametrize("strict_env", [False, True])
+def test_attrs(direnv_project: DirenvProject, strict_env: bool) -> None:
+    direnv_project.setup_envrc("use nix -A subshell", strict_env=strict_env)
     direnv_exec(direnv_project, "echo $THIS_IS_A_SUBSHELL")
 
 
-def test_no_nix_path(direnv_project: DirenvProject) -> None:
-    direnv_project.setup_envrc("use nix --argstr someArg OK")
+@pytest.mark.parametrize("strict_env", [False, True])
+def test_no_nix_path(direnv_project: DirenvProject, strict_env: bool) -> None:
+    direnv_project.setup_envrc("use nix --argstr someArg OK", strict_env=strict_env)
     env = os.environ.copy()
     del env["NIX_PATH"]
     direnv_exec(direnv_project, "echo $SHOULD_BE_SET", env=env)
 
 
-def test_args(direnv_project: DirenvProject) -> None:
-    direnv_project.setup_envrc("use nix --argstr someArg OK")
+@pytest.mark.parametrize("strict_env", [False, True])
+def test_args(direnv_project: DirenvProject, strict_env: bool) -> None:
+    direnv_project.setup_envrc("use nix --argstr someArg OK", strict_env=strict_env)
     direnv_exec(direnv_project, "echo $SHOULD_BE_SET")
 
 
-def test_no_files(direnv_project: DirenvProject) -> None:
-    direnv_project.setup_envrc("use nix -p hello")
+@pytest.mark.parametrize("strict_env", [False, True])
+def test_no_files(direnv_project: DirenvProject, strict_env: bool) -> None:
+    direnv_project.setup_envrc("use nix -p hello", strict_env=strict_env)
     out = run(
         ["direnv", "status"],
         stderr=subprocess.PIPE,