diff --git a/src/ConfigCat.Client.Tests/HttpConfigFetcherTests.cs b/src/ConfigCat.Client.Tests/HttpConfigFetcherTests.cs
index a223e0fc..225c754b 100644
--- a/src/ConfigCat.Client.Tests/HttpConfigFetcherTests.cs
+++ b/src/ConfigCat.Client.Tests/HttpConfigFetcherTests.cs
@@ -1,10 +1,15 @@
 using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
 using System.Net;
 using System.Net.Http.Headers;
 using System.Threading;
 using System.Threading.Tasks;
+using ConfigCat.Client.Configuration;
 using ConfigCat.Client.Tests.Helpers;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Moq;
 
 namespace ConfigCat.Client.Tests;
 
@@ -232,4 +237,78 @@ public async Task HttpConfigFetcher_FetchAsync_PendingOperationShouldBeJoined(bo
             Assert.IsNull(configFetcher.pendingFetch);
         }
     }
+
+    [TestMethod]
+    public async Task CustomConfigFetcher_Success()
+    {
+        // Arrange
+
+        var configJson = File.ReadAllText(Path.Combine(new ConfigLocation.LocalFile("data", "sample_v5.json").GetRealLocation()));
+
+        var responseHeader = new[]
+        {
+            new KeyValuePair<string, string>("CF-RAY", "CF-12345"),
+            new KeyValuePair<string, string>("ETag", "\"abc\""),
+        };
+
+        var configFetcherMock = new Mock<IConfigCatConfigFetcher>();
+        configFetcherMock
+            .Setup(m => m.FetchAsync(It.IsAny<FetchRequest>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new FetchResponse(HttpStatusCode.OK, reasonPhrase: null, responseHeader, configJson));
+
+        using var client = new ConfigCatClient("test-67890123456789012/1234567890123456789012", new ConfigCatClientOptions
+        {
+            ConfigFetcher = configFetcherMock.Object
+        });
+
+        // Act
+
+        var value = await client.GetValueAsync("stringDefaultCat", "");
+
+        // Assert
+
+        Assert.AreEqual("Cat", value);
+    }
+
+    [TestMethod]
+    public async Task CustomConfigFetcher_Failure()
+    {
+        // Arrange
+
+        var logEvents = new List<LogEvent>();
+        var logger = LoggingHelper.CreateCapturingLogger(logEvents, LogLevel.Info);
+
+        var responseHeader = new[]
+        {
+            new KeyValuePair<string, string>("ETag", "\"abc\""),
+            new KeyValuePair<string, string>("CF-RAY", "CF-12345"),
+        };
+
+        var configFetcherMock = new Mock<IConfigCatConfigFetcher>();
+        configFetcherMock
+            .Setup(m => m.FetchAsync(It.IsAny<FetchRequest>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync(new FetchResponse(HttpStatusCode.Forbidden, "Forbidden", responseHeader));
+
+        using var client = new ConfigCatClient("test-67890123456789012/1234567890123456789012", new ConfigCatClientOptions
+        {
+            ConfigFetcher = configFetcherMock.Object,
+            Logger = logger.AsWrapper()
+        });
+
+        // Act
+
+        await client.ForceRefreshAsync();
+
+        // Assert
+
+        var errors = logEvents.Where(evt => evt.EventId == 1100).ToArray();
+        Assert.AreEqual(1, errors.Length);
+
+        var error = errors[0].Message;
+        Assert.AreEqual(1, error.ArgValues.Length);
+        Assert.IsTrue(error.ArgValues[0] is string);
+
+        var rayId = (string)error.ArgValues[0]!;
+        StringAssert.Contains(errors[0].Message.InvariantFormattedMessage, rayId);
+    }
 }