diff --git a/tests/test_auth/test_cookie_persistence.py b/tests/test_auth/test_cookie_persistence.py
new file mode 100644
index 0000000..5b2f417
--- /dev/null
+++ b/tests/test_auth/test_cookie_persistence.py
@@ -0,0 +1,136 @@
+"""Tests for Cookie persistence functionality
+
+Tests CookieManager class for saving and loading cookies from ~/.kb_env file.
+"""
+
+import stat
+import pytest
+from pathlib import Path
+
+from kb_workflow.auth.cookie_manager import CookieManager
+
+
+class TestCookieManager:
+    """Test suite for CookieManager class"""
+
+    def test_save_cookie_creates_file(self, tmp_path: Path):
+        """Test that saving cookie creates the env file"""
+        # Arrange
+        env_file = tmp_path / ".kb_env"
+        manager = CookieManager(env_path=env_file)
+        cookie_str = "z_c0=token123; _zap=zap_value"
+
+        # Act
+        manager.save_cookie(cookie_str)
+
+        # Assert
+        assert env_file.exists()
+        content = env_file.read_text(encoding="utf-8")
+        assert 'ZHIHU_COOKIE="z_c0=token123; _zap=zap_value"' in content
+
+    def test_save_cookie_updates_existing(self, tmp_path: Path):
+        """Test that saving cookie updates existing ZHIHU_COOKIE line"""
+        # Arrange
+        env_file = tmp_path / ".kb_env"
+        env_file.write_text('ZHIHU_COOKIE="old_cookie"\n', encoding="utf-8")
+        manager = CookieManager(env_path=env_file)
+        new_cookie = "z_c0=new_token"
+
+        # Act
+        manager.save_cookie(new_cookie)
+
+        # Assert
+        content = env_file.read_text(encoding="utf-8")
+        assert 'ZHIHU_COOKIE="z_c0=new_token"' in content
+        assert "old_cookie" not in content
+
+    def test_save_cookie_appends_new(self, tmp_path: Path):
+        """Test that saving cookie appends new line when ZHIHU_COOKIE doesn't exist"""
+        # Arrange
+        env_file = tmp_path / ".kb_env"
+        env_file.write_text("OTHER_VAR=value\n", encoding="utf-8")
+        manager = CookieManager(env_path=env_file)
+        cookie_str = "z_c0=token"
+
+        # Act
+        manager.save_cookie(cookie_str)
+
+        # Assert
+        content = env_file.read_text(encoding="utf-8")
+        assert "OTHER_VAR=value" in content
+        assert 'ZHIHU_COOKIE="z_c0=token"' in content
+
+    def test_save_cookie_preserves_other_lines(self, tmp_path: Path):
+        """Test that saving cookie preserves other configuration lines"""
+        # Arrange
+        env_file = tmp_path / ".kb_env"
+        env_file.write_text(
+            "KB_BASE_PATH=/path/to/kb\nOTHER_VAR=value\n",
+            encoding="utf-8"
+        )
+        manager = CookieManager(env_path=env_file)
+        cookie_str = "z_c0=token"
+
+        # Act
+        manager.save_cookie(cookie_str)
+
+        # Assert
+        content = env_file.read_text(encoding="utf-8")
+        assert "KB_BASE_PATH=/path/to/kb" in content
+        assert "OTHER_VAR=value" in content
+        assert 'ZHIHU_COOKIE="z_c0=token"' in content
+
+    def test_save_cookie_sets_file_permissions(self, tmp_path: Path):
+        """Test that saved file has 600 permissions"""
+        # Arrange
+        env_file = tmp_path / ".kb_env"
+        manager = CookieManager(env_path=env_file)
+        cookie_str = "z_c0=token"
+
+        # Act
+        manager.save_cookie(cookie_str)
+
+        # Assert
+        file_stat = env_file.stat()
+        file_mode = stat.filemode(file_stat.st_mode)
+        # Check owner read/write only (600)
+        assert (file_stat.st_mode & 0o777) == 0o600, f"Expected 600, got {oct(file_stat.st_mode & 0o777)}"
+
+    def test_load_cookie_returns_saved(self, tmp_path: Path):
+        """Test that loading cookie returns previously saved value"""
+        # Arrange
+        env_file = tmp_path / ".kb_env"
+        manager = CookieManager(env_path=env_file)
+        cookie_str = "z_c0=token123; _zap=zap_value"
+
+        # Act
+        manager.save_cookie(cookie_str)
+        loaded = manager.load_cookie()
+
+        # Assert
+        assert loaded == "z_c0=token123; _zap=zap_value"
+
+    def test_load_cookie_empty_if_not_exists(self, tmp_path: Path):
+        """Test that loading cookie returns empty string if file doesn't exist"""
+        # Arrange
+        env_file = tmp_path / ".kb_env"
+        manager = CookieManager(env_path=env_file)
+
+        # Act
+        loaded = manager.load_cookie()
+
+        # Assert
+        assert loaded == ""
+
+    def test_load_cookie_empty_if_key_not_found(self, tmp_path: Path):
+        """Test that loading cookie returns empty string if key not in file"""
+        # Arrange
+        env_file = tmp_path / ".kb_env"
+        env_file.write_text("OTHER_VAR=value\n", encoding="utf-8")
+        manager = CookieManager(env_path=env_file)
+
+        # Act
+        loaded = manager.load_cookie()
+
+        # Assert
+        assert loaded == ""