diff --git a/.gitignore b/.gitignore
index 0a89a9401..dad9cb625 100644
--- a/.gitignore
+++ b/.gitignore
@@ -291,6 +291,7 @@ Thumbs.db
 
 # ignore local cert
 /lib/PuppeteerSharp.TestServer/testCert.cer
+/lib/PuppeteerSharp.TestServer/testCert.pem
 /lib/PuppeteerSharp.TestServer/testKey.pem
 /lib/PuppeteerSharp.Tests/Screenshots/
 
diff --git a/lib/PuppeteerSharp.Tests/UtilitiesTests/CookiePartitionKeyConverterTests.cs b/lib/PuppeteerSharp.Tests/UtilitiesTests/CookiePartitionKeyConverterTests.cs
new file mode 100644
index 000000000..c434125d5
--- /dev/null
+++ b/lib/PuppeteerSharp.Tests/UtilitiesTests/CookiePartitionKeyConverterTests.cs
@@ -0,0 +1,117 @@
+using System.Text.Json;
+using NUnit.Framework;
+
+namespace PuppeteerSharp.Tests.UtilitiesTests
+{
+    /// <summary>
+    /// Unit tests for CookiePartitionKeyConverter. These are standalone tests that don't require browsers.
+    /// </summary>
+    [TestFixture]
+    public class CookiePartitionKeyConverterTests
+    {
+        [Test]
+        public void ShouldSerializeAndDeserializeAsString()
+        {
+            // Arrange
+            var cookie = new CookieParam
+            {
+                Name = "test",
+                Value = "value",
+                PartitionKey = "https://example.com"
+            };
+
+            // Act - Serialize to JSON
+            var json = JsonSerializer.Serialize(cookie);
+
+            // Assert - Verify it's serialized as a string
+            Assert.That(json, Does.Contain("\"PartitionKey\":\"https://example.com\""));
+
+            // Act - Deserialize back
+            var deserialized = JsonSerializer.Deserialize<CookieParam>(json);
+
+            // Assert - Verify round-trip works
+            Assert.That(deserialized.PartitionKey, Is.EqualTo("https://example.com"));
+        }
+
+        [Test]
+        public void ShouldDeserializeCdpObjectFormat()
+        {
+            // Arrange - CDP returns object format
+            var cdpJson = @"{""Name"":""test"",""Value"":""value"",""PartitionKey"":{""topLevelSite"":""https://example.com"",""hasCrossSiteAncestor"":false}}";
+
+            // Act
+            var cookie = JsonSerializer.Deserialize<CookieParam>(cdpJson);
+
+            // Assert - Should extract the topLevelSite value
+            Assert.That(cookie.PartitionKey, Is.EqualTo("https://example.com"));
+        }
+
+        [Test]
+        public void ShouldDeserializeStringFormat()
+        {
+            // Arrange - User-saved format (simple string)
+            var stringJson = @"{""Name"":""test"",""Value"":""value"",""PartitionKey"":""https://example.com""}";
+
+            // Act
+            var cookie = JsonSerializer.Deserialize<CookieParam>(stringJson);
+
+            // Assert
+            Assert.That(cookie.PartitionKey, Is.EqualTo("https://example.com"));
+        }
+
+        [Test]
+        public void ShouldHandleNullPartitionKey()
+        {
+            // Arrange
+            var cookie = new CookieParam
+            {
+                Name = "test",
+                Value = "value",
+                PartitionKey = null
+            };
+
+            // Act
+            var json = JsonSerializer.Serialize(cookie);
+            var deserialized = JsonSerializer.Deserialize<CookieParam>(json);
+
+            // Assert
+            Assert.That(deserialized.PartitionKey, Is.Null);
+        }
+
+        [Test]
+        public void ShouldSupportRoundTripSerializationToFile()
+        {
+            // Arrange - Simulate getting cookies from browser
+            var originalCookies = new[]
+            {
+                new CookieParam
+                {
+                    Name = "cookie1",
+                    Value = "value1",
+                    Domain = "example.com",
+                    PartitionKey = "https://example.com"
+                },
+                new CookieParam
+                {
+                    Name = "cookie2",
+                    Value = "value2",
+                    Domain = "test.com",
+                    PartitionKey = "https://test.com"
+                }
+            };
+
+            // Act - Serialize to JSON (simulating saving to file)
+            var json = JsonSerializer.Serialize(originalCookies);
+
+            // Act - Deserialize from JSON (simulating loading from file)
+            var loadedCookies = JsonSerializer.Deserialize<CookieParam[]>(json);
+
+            // Assert - Verify all data is preserved
+            Assert.That(loadedCookies, Has.Length.EqualTo(2));
+            Assert.That(loadedCookies[0].Name, Is.EqualTo("cookie1"));
+            Assert.That(loadedCookies[0].PartitionKey, Is.EqualTo("https://example.com"));
+            Assert.That(loadedCookies[1].Name, Is.EqualTo("cookie2"));
+            Assert.That(loadedCookies[1].PartitionKey, Is.EqualTo("https://test.com"));
+        }
+    }
+}
diff --git a/lib/PuppeteerSharp/Helpers/Json/CookiePartitionKeyConverter.cs b/lib/PuppeteerSharp/Helpers/Json/CookiePartitionKeyConverter.cs
index 5a4c2584a..d254f9865 100644
--- a/lib/PuppeteerSharp/Helpers/Json/CookiePartitionKeyConverter.cs
+++ b/lib/PuppeteerSharp/Helpers/Json/CookiePartitionKeyConverter.cs
@@ -18,6 +18,12 @@ internal sealed class CookiePartitionKeyConverter : JsonConverter<string>
             Type objectType,
             JsonSerializerOptions options)
         {
+            // Handle both string format (for user serialization) and object format (from CDP)
+            if (reader.TokenType == JsonTokenType.String)
+            {
+                return reader.GetString();
+            }
+
             JsonNode? node = JsonNode.Parse(ref reader);
 
             return node?["topLevelSite"]?.GetValue<string>() ?? null;
@@ -29,12 +35,15 @@ public override void Write(
             string value,
             JsonSerializerOptions options)
         {
-            if (value != null && writer != null)
+            // Write as a simple string for user serialization/deserialization
+            // This allows cookies to be easily saved to and loaded from files
+            if (value != null)
+            {
+                writer.WriteStringValue(value);
+            }
+            else
             {
-                writer.WriteStartObject("partitionKey");
-                writer.WriteString("topLevelSite", value);
-                writer.WriteBoolean("hasCrossSiteAncestor", false);
-                writer.WriteEndObject();
+                writer.WriteNullValue();
             }
         }
     }