test: verify load_dotenv_files override behavior

tests/basic/test_main.py

@@ -14,7 +14,7 @@ from prompt_toolkit.output import DummyOutput
 from aider.coders import Coder
 from aider.dump import dump  # noqa: F401
 from aider.io import InputOutput
-from aider.main import check_gitignore, main, setup_git
+from aider.main import check_gitignore, load_dotenv_files, main, setup_git
 from aider.utils import GitTemporaryDirectory, IgnorantTemporaryDirectory, make_repo
 
 
@@ -1275,6 +1275,65 @@ class TestMain(TestCase):
         for call in mock_io_instance.tool_warning.call_args_list:
             self.assertNotIn("Cost estimates may be inaccurate", call[0][0])
 
+    def test_load_dotenv_files_override(self):
+        with GitTemporaryDirectory() as git_dir:
+            git_dir = Path(git_dir)
+
+            # Create fake home and .aider directory
+            fake_home = git_dir / "fake_home"
+            fake_home.mkdir()
+            aider_dir = fake_home / ".aider"
+            aider_dir.mkdir()
+
+            # Create oauth keys file
+            oauth_keys_file = aider_dir / "oauth-keys.env"
+            oauth_keys_file.write_text("OAUTH_VAR=oauth_val\nSHARED_VAR=oauth_shared\n")
+
+            # Create git root .env file
+            git_root_env = git_dir / ".env"
+            git_root_env.write_text("GIT_VAR=git_val\nSHARED_VAR=git_shared\n")
+
+            # Create CWD .env file in a subdir
+            cwd_subdir = git_dir / "subdir"
+            cwd_subdir.mkdir()
+            cwd_env = cwd_subdir / ".env"
+            cwd_env.write_text("CWD_VAR=cwd_val\nSHARED_VAR=cwd_shared\n")
+
+            # Change to subdir
+            original_cwd = os.getcwd()
+            os.chdir(cwd_subdir)
+
+            # Clear relevant env vars before test
+            for var in ["OAUTH_VAR", "SHARED_VAR", "GIT_VAR", "CWD_VAR"]:
+                if var in os.environ:
+                    del os.environ[var]
+
+            with patch("pathlib.Path.home", return_value=fake_home):
+                loaded_files = load_dotenv_files(str(git_dir), None)
+
+                # Assert files were loaded in expected order (oauth first)
+                self.assertIn(str(oauth_keys_file.resolve()), loaded_files)
+                self.assertIn(str(git_root_env.resolve()), loaded_files)
+                self.assertIn(str(cwd_env.resolve()), loaded_files)
+                self.assertLess(
+                    loaded_files.index(str(oauth_keys_file.resolve())),
+                    loaded_files.index(str(git_root_env.resolve())),
+                )
+                self.assertLess(
+                    loaded_files.index(str(git_root_env.resolve())),
+                    loaded_files.index(str(cwd_env.resolve())),
+                )
+
+                # Assert environment variables reflect the override order
+                self.assertEqual(os.environ.get("OAUTH_VAR"), "oauth_val")
+                self.assertEqual(os.environ.get("GIT_VAR"), "git_val")
+                self.assertEqual(os.environ.get("CWD_VAR"), "cwd_val")
+                # SHARED_VAR should be overridden by the last loaded file (cwd .env)
+                self.assertEqual(os.environ.get("SHARED_VAR"), "cwd_shared")
+
+            # Restore CWD
+            os.chdir(original_cwd)
+
     @patch("aider.main.InputOutput")
     def test_cache_without_stream_no_warning(self, MockInputOutput):
         mock_io_instance = MockInputOutput.return_value