diff --git a/src/ConfigCat.Client.Tests/BasicConfigCatClientIntegrationTests.cs b/src/ConfigCat.Client.Tests/BasicConfigCatClientIntegrationTests.cs
index b856926b..43e0bd44 100644
--- a/src/ConfigCat.Client.Tests/BasicConfigCatClientIntegrationTests.cs
+++ b/src/ConfigCat.Client.Tests/BasicConfigCatClientIntegrationTests.cs
@@ -340,7 +340,7 @@ static void Configure(ConfigCatClientOptions options)
     public async Task Http_Timeout_Test_Async()
     {
         var response = $"{{ \"f\": {{ \"fakeKey\": {{ \"v\": \"fakeValue\", \"p\": [] ,\"r\": [] }} }} }}";
-        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake", options =>
+        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake-67890123456789012/1234567890123456789012", options =>
         {
             options.PollingMode = PollingModes.ManualPoll;
             options.Logger = ConsoleLogger;
@@ -357,7 +357,7 @@ public async Task Http_Timeout_Test_Async()
     public void Http_Timeout_Test_Sync()
     {
         var response = $"{{ \"f\": {{ \"fakeKey\": {{ \"v\": \"fakeValue\", \"p\": [] ,\"r\": [] }} }} }}";
-        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake", options =>
+        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake-67890123456789012/1234567890123456789012", options =>
         {
             options.PollingMode = PollingModes.ManualPoll;
             options.Logger = ConsoleLogger;
@@ -374,7 +374,7 @@ public async Task Ensure_MaxInitWait_Overrides_Timeout()
     {
         var now = DateTimeOffset.UtcNow;
         var response = $"{{ \"f\": {{ \"fakeKey\": {{ \"v\": \"fakeValue\", \"p\": [] ,\"r\": [] }} }} }}";
-        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake", options =>
+        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake-67890123456789012/1234567890123456789012", options =>
         {
             options.PollingMode = PollingModes.AutoPoll(maxInitWaitTime: TimeSpan.FromSeconds(1));
             options.Logger = ConsoleLogger;
@@ -390,7 +390,7 @@ public void Ensure_MaxInitWait_Overrides_Timeout_Sync()
     {
         var now = DateTimeOffset.UtcNow;
         var response = $"{{ \"f\": {{ \"fakeKey\": {{ \"v\": \"fakeValue\", \"p\": [] ,\"r\": [] }} }} }}";
-        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake", options =>
+        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake-67890123456789012/1234567890123456789012", options =>
         {
             options.PollingMode = PollingModes.AutoPoll(maxInitWaitTime: TimeSpan.FromSeconds(1));
             options.Logger = ConsoleLogger;
@@ -407,7 +407,7 @@ public void Ensure_Client_Dispose_Kill_Hanging_Http_Call()
         var defer = new ManualResetEvent(false);
         var now = DateTimeOffset.UtcNow;
         var response = $"{{ \"f\": {{ \"fakeKey\": {{ \"v\": \"fakeValue\", \"p\": [] ,\"r\": [] }} }} }}";
-        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake", options =>
+        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake-67890123456789012/1234567890123456789012", options =>
         {
             options.Logger = ConsoleLogger;
             options.HttpClientHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK, response, TimeSpan.FromSeconds(5));
@@ -426,7 +426,7 @@ public void Ensure_Client_Dispose_Kill_Hanging_Http_Call_Sync()
         var defer = new ManualResetEvent(false);
         var now = DateTimeOffset.UtcNow;
         var response = $"{{ \"f\": {{ \"fakeKey\": {{ \"v\": \"fakeValue\", \"p\": [] ,\"r\": [] }} }} }}";
-        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake", options =>
+        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake-67890123456789012/1234567890123456789012", options =>
         {
             options.Logger = ConsoleLogger;
             options.HttpClientHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK, response, TimeSpan.FromSeconds(5));
@@ -447,7 +447,7 @@ public void Ensure_Client_Dispose_Kill_Hanging_Http_Call_Sync()
     public void Ensure_Multiple_Requests_Doesnt_Interfere_In_ValueTasks()
     {
         var response = $"{{ \"f\": {{ \"fakeKey\": {{ \"v\": \"fakeValue\", \"p\": [] ,\"r\": [] }} }} }}";
-        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake", options =>
+        using IConfigCatClient manualPollClient = ConfigCatClient.Get("fake-67890123456789012/1234567890123456789012", options =>
         {
             options.Logger = ConsoleLogger;
             options.PollingMode = PollingModes.ManualPoll;
diff --git a/src/ConfigCat.Client.Tests/ConfigCatClientTests.cs b/src/ConfigCat.Client.Tests/ConfigCatClientTests.cs
index 928992b1..1b16629e 100644
--- a/src/ConfigCat.Client.Tests/ConfigCatClientTests.cs
+++ b/src/ConfigCat.Client.Tests/ConfigCatClientTests.cs
@@ -83,7 +83,7 @@ public void CreateAnInstance_WhenLazyLoadConfigurationTimeToLiveSecondsIsZero_Sh
     [DoNotParallelize]
     public void CreateAnInstance_WhenLoggerIsNull_ShouldCreateAnInstance()
     {
-        using var client = ConfigCatClient.Get("hsdrTr4sxbHdSgdhHRZds346hdgsS2vfsgf/GsdrTr4sxbHdSgdhHRZds346hdOPsSgvfsgf", options =>
+        using var client = ConfigCatClient.Get("hsdrTr4sxbHdSgdhHRZds3/GsdrTr4sxbHdSgdhHRZds3", options =>
         {
             options.Logger = null;
         });
@@ -95,7 +95,7 @@ public void CreateAnInstance_WhenLoggerIsNull_ShouldCreateAnInstance()
     [DoNotParallelize]
     public void CreateAnInstance_WithSdkKey_ShouldCreateAnInstance()
     {
-        using var _ = ConfigCatClient.Get("hsdrTr4sxbHdSgdhHRZds346hdgsS2vfsgf/GsdrTr4sxbHdSgdhHRZds346hdOPsSgvfsgf");
+        using var _ = ConfigCatClient.Get("hsdrTr4sxbHdSgdhHRZds3/GsdrTr4sxbHdSgdhHRZds3");
     }
 
     [TestMethod]
@@ -149,6 +149,10 @@ public void GetValue_EvaluateServiceThrowException_ShouldReturnDefaultValue()
 
         const string defaultValue = "Victory for the Firstborn!";
 
+        this.configServiceMock
+            .Setup(m => m.GetConfig())
+            .Throws<Exception>();
+
         this.evaluatorMock
             .Setup(m => m.Evaluate(ref It.Ref<EvaluateContext>.IsAny))
             .Throws<Exception>();
@@ -178,6 +182,10 @@ public async Task GetValueAsync_EvaluateServiceThrowException_ShouldReturnDefaul
 
         const string defaultValue = "Victory for the Firstborn!";
 
+        this.configServiceMock
+            .Setup(m => m.GetConfigAsync(It.IsAny<CancellationToken>()))
+            .Throws<Exception>();
+
         this.evaluatorMock
             .Setup(m => m.Evaluate(ref It.Ref<EvaluateContext>.IsAny))
             .Throws<Exception>();
@@ -622,6 +630,10 @@ public async Task GetAllValueDetails_ConfigServiceThrowException_ShouldReturnEmp
     {
         // Arrange
 
+        this.configServiceMock
+            .Setup(m => m.GetConfig())
+            .Throws<Exception>();
+
         this.configServiceMock
             .Setup(m => m.GetConfigAsync(It.IsAny<CancellationToken>()))
             .Throws<Exception>();
@@ -779,7 +791,7 @@ public void GetAllKeys_DeserializerThrowException_ShouldReturnsWithEmptyArray()
     {
         // Arrange
 
-        this.configServiceMock.Setup(m => m.GetConfigAsync(It.IsAny<CancellationToken>())).ReturnsAsync(ProjectConfig.Empty);
+        this.configServiceMock.Setup(m => m.GetConfig()).Returns(ProjectConfig.Empty);
         var o = new Config();
 
         IConfigCatClient instance = new ConfigCatClient(
@@ -1065,7 +1077,7 @@ public async Task ForceRefreshAsync_ShouldBeCancelable()
 
     private static IConfigCatClient CreateClientFromLocalFile(string fileName, User? defaultUser = null)
     {
-        return ConfigCatClient.Get("localhost", options =>
+        return ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(
                 Path.Combine("data", fileName),
@@ -1160,11 +1172,11 @@ void Configure(ConfigCatClientOptions options)
 
         // Act
 
-        using var client1 = ConfigCatClient.Get("test", Configure);
+        using var client1 = ConfigCatClient.Get("test-67890123456789012/1234567890123456789012", Configure);
         var warnings1 = warnings.ToArray();
 
         warnings.Clear();
-        using var client2 = ConfigCatClient.Get("test", passConfigureToSecondGet ? Configure : null);
+        using var client2 = ConfigCatClient.Get("test-67890123456789012/1234567890123456789012", passConfigureToSecondGet ? Configure : null);
         var warnings2 = warnings.ToArray();
 
         // Assert
@@ -1189,7 +1201,7 @@ public void Dispose_CachedInstanceRemoved()
     {
         // Arrange
 
-        var client1 = ConfigCatClient.Get("test", options => options.PollingMode = PollingModes.ManualPoll);
+        var client1 = ConfigCatClient.Get("test-67890123456789012/1234567890123456789012", options => options.PollingMode = PollingModes.ManualPoll);
 
         // Act
 
@@ -1211,7 +1223,7 @@ public void Dispose_CanRemoveCurrentCachedInstanceOnly()
     {
         // Arrange
 
-        var client1 = ConfigCatClient.Get("test", options => options.PollingMode = PollingModes.ManualPoll);
+        var client1 = ConfigCatClient.Get("test-67890123456789012/1234567890123456789012", options => options.PollingMode = PollingModes.ManualPoll);
 
         // Act
 
@@ -1221,7 +1233,7 @@ public void Dispose_CanRemoveCurrentCachedInstanceOnly()
 
         var instanceCount2 = ConfigCatClient.Instances.GetAliveCount();
 
-        var client2 = ConfigCatClient.Get("test", options => options.PollingMode = PollingModes.ManualPoll);
+        var client2 = ConfigCatClient.Get("test-67890123456789012/1234567890123456789012", options => options.PollingMode = PollingModes.ManualPoll);
 
         var instanceCount3 = ConfigCatClient.Instances.GetAliveCount();
 
@@ -1248,8 +1260,8 @@ public void DisposeAll_CachedInstancesRemoved()
     {
         // Arrange
 
-        var client1 = ConfigCatClient.Get("test1", options => options.PollingMode = PollingModes.AutoPoll());
-        var client2 = ConfigCatClient.Get("test2", options => options.PollingMode = PollingModes.ManualPoll);
+        var client1 = ConfigCatClient.Get("test1-7890123456789012/1234567890123456789012", options => options.PollingMode = PollingModes.AutoPoll());
+        var client2 = ConfigCatClient.Get("test2-7890123456789012/1234567890123456789012", options => options.PollingMode = PollingModes.ManualPoll);
 
         // Act
 
@@ -1283,8 +1295,8 @@ static void CreateClients(out int instanceCount)
             // because that could interfere with this test: when raising the event, the service acquires a strong reference to the client,
             // which would temporarily prevent the client from being GCd. This could break the test in the case of unlucky timing.
             // Setting maxInitWaitTime to zero prevents this because then the event is raised immediately at creation.
-            var client1 = ConfigCatClient.Get("test1", options => options.PollingMode = PollingModes.AutoPoll(maxInitWaitTime: TimeSpan.Zero));
-            var client2 = ConfigCatClient.Get("test2", options => options.PollingMode = PollingModes.ManualPoll);
+            var client1 = ConfigCatClient.Get("test1-7890123456789012/1234567890123456789012", options => options.PollingMode = PollingModes.AutoPoll(maxInitWaitTime: TimeSpan.Zero));
+            var client2 = ConfigCatClient.Get("test2-7890123456789012/1234567890123456789012", options => options.PollingMode = PollingModes.ManualPoll);
 
             instanceCount = ConfigCatClient.Instances.GetAliveCount();
 
diff --git a/src/ConfigCat.Client.Tests/OverrideTests.cs b/src/ConfigCat.Client.Tests/OverrideTests.cs
index dd82e09c..165d2575 100644
--- a/src/ConfigCat.Client.Tests/OverrideTests.cs
+++ b/src/ConfigCat.Client.Tests/OverrideTests.cs
@@ -24,7 +24,7 @@ public class OverrideTests
     [TestMethod]
     public void LocalFile()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(ComplexJsonPath, false, OverrideBehaviour.LocalOnly);
         });
@@ -39,7 +39,7 @@ public void LocalFile()
     [TestMethod]
     public async Task LocalFileAsync()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(ComplexJsonPath, false, OverrideBehaviour.LocalOnly);
         });
@@ -53,7 +53,7 @@ public async Task LocalFileAsync()
 
     public static void LocalFile_Parallel()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(ComplexJsonPath, false, OverrideBehaviour.LocalOnly);
         });
@@ -77,7 +77,7 @@ public void LocalFileAsync_Parallel()
             "stringSetting",
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(ComplexJsonPath, false, OverrideBehaviour.LocalOnly);
         });
@@ -100,7 +100,7 @@ public void LocalFileAsync_Parallel_Sync()
             "stringSetting",
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(ComplexJsonPath, false, OverrideBehaviour.LocalOnly);
         });
@@ -114,7 +114,7 @@ public void LocalFileAsync_Parallel_Sync()
     [TestMethod]
     public void LocalFile_Default_WhenErrorOccures()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile("something-not-existing", false, OverrideBehaviour.LocalOnly);
         });
@@ -126,7 +126,7 @@ public void LocalFile_Default_WhenErrorOccures()
     [TestMethod]
     public void LocalFile_Read()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(ComplexJsonPath, true, OverrideBehaviour.LocalOnly);
         });
@@ -141,7 +141,7 @@ public void LocalFile_Read()
     [TestMethod]
     public async Task LocalFileAsync_Read()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(ComplexJsonPath, true, OverrideBehaviour.LocalOnly);
         });
@@ -156,7 +156,7 @@ public async Task LocalFileAsync_Read()
     [TestMethod]
     public void LocalFile_Default_WhenErrorOccures_Reload()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile("something-not-existing", true, OverrideBehaviour.LocalOnly);
         });
@@ -168,7 +168,7 @@ public void LocalFile_Default_WhenErrorOccures_Reload()
     [TestMethod]
     public void LocalFile_Simple()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(SimpleJsonPath, false, OverrideBehaviour.LocalOnly);
         });
@@ -183,7 +183,7 @@ public void LocalFile_Simple()
     [TestMethod]
     public async Task LocalFileAsync_Simple()
     {
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(SimpleJsonPath, false, OverrideBehaviour.LocalOnly);
         });
@@ -207,7 +207,7 @@ public void LocalFile_Dictionary()
             {"stringSetting", "test"},
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, OverrideBehaviour.LocalOnly);
         });
@@ -231,7 +231,7 @@ public async Task LocalFileAsync_Dictionary()
             {"stringSetting", "test"},
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, OverrideBehaviour.LocalOnly);
         });
@@ -252,7 +252,7 @@ public void LocalOnly()
             {"nonexisting", true},
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, OverrideBehaviour.LocalOnly);
             options.PollingMode = PollingModes.ManualPoll;
@@ -276,7 +276,7 @@ public void LocalOnly_Watch()
             {"fakeKey", "test1"},
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, true, OverrideBehaviour.LocalOnly);
             options.PollingMode = PollingModes.ManualPoll;
@@ -297,7 +297,7 @@ public async Task LocalOnly_Async_Watch()
             {"fakeKey", "test1"},
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, true, OverrideBehaviour.LocalOnly);
             options.PollingMode = PollingModes.ManualPoll;
@@ -319,7 +319,7 @@ public async Task LocalOnly_Async()
             {"nonexisting", true},
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, OverrideBehaviour.LocalOnly);
             options.PollingMode = PollingModes.ManualPoll;
@@ -346,7 +346,7 @@ public void LocalOverRemote()
 
         var fakeHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK);
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, OverrideBehaviour.LocalOverRemote);
             options.HttpClientHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK, GetJsonContent("false"));
@@ -370,7 +370,7 @@ public async Task LocalOverRemote_Async()
 
         var fakeHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK);
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, OverrideBehaviour.LocalOverRemote);
             options.HttpClientHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK, GetJsonContent("false"));
@@ -394,7 +394,7 @@ public void RemoteOverLocal()
 
         var fakeHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK);
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, OverrideBehaviour.RemoteOverLocal);
             options.HttpClientHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK, GetJsonContent("false"));
@@ -418,7 +418,7 @@ public async Task RemoteOverLocal_Async()
 
         var fakeHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK);
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalDictionary(dict, OverrideBehaviour.RemoteOverLocal);
             options.HttpClientHandler = new FakeHttpClientHandler(System.Net.HttpStatusCode.OK, GetJsonContent("false"));
@@ -436,7 +436,7 @@ public async Task LocalFile_Watcher_Reload()
     {
         await CreateFileAndWriteContent(SampleFileToCreate, "initial");
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(SampleFileToCreate, true, OverrideBehaviour.LocalOnly);
         });
@@ -457,7 +457,7 @@ public async Task LocalFile_Watcher_Reload_Sync()
     {
         await CreateFileAndWriteContent(SampleFileToCreate, "initial");
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.FlagOverrides = FlagOverrides.LocalFile(SampleFileToCreate, true, OverrideBehaviour.LocalOnly);
         });
@@ -507,7 +507,7 @@ public void OverrideValueTypeMismatchShouldBeHandledCorrectly_Dictionary(object
             [key] = overrideValue is not Type valueType ? overrideValue : Activator.CreateInstance(valueType)!
         };
 
-        using var client = ConfigCatClient.Get("localhost", options =>
+        using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
         {
             options.PollingMode = PollingModes.ManualPoll;
             options.FlagOverrides = FlagOverrides.LocalDictionary(dictionary, OverrideBehaviour.LocalOnly);
@@ -569,7 +569,7 @@ public void OverrideValueTypeMismatchShouldBeHandledCorrectly_SimplifiedConfig(s
 
         try
         {
-            using var client = ConfigCatClient.Get("localhost", options =>
+            using var client = ConfigCatClient.Get("localhost-123456789012/1234567890123456789012", options =>
             {
                 options.PollingMode = PollingModes.ManualPoll;
                 options.FlagOverrides = FlagOverrides.LocalFile(filePath, autoReload: false, OverrideBehaviour.LocalOnly);
diff --git a/src/ConfigCatClient/ConfigCatClient.cs b/src/ConfigCatClient/ConfigCatClient.cs
index f0a0e1c5..594ff437 100644
--- a/src/ConfigCatClient/ConfigCatClient.cs
+++ b/src/ConfigCatClient/ConfigCatClient.cs
@@ -34,6 +34,32 @@ public sealed class ConfigCatClient : IConfigCatClient
     // which is good enough in these cases.
     private volatile User? defaultUser;
 
+    private static bool IsValidSdkKey(string sdkKey, bool customBaseUrl)
+    {
+        const string proxyPrefix = "configcat-proxy/";
+
+        if (customBaseUrl && sdkKey.Length > proxyPrefix.Length && sdkKey.StartsWith(proxyPrefix, StringComparison.Ordinal))
+        {
+            return true;
+        }
+
+        var components = sdkKey.Split('/');
+        const int keyLength = 22;
+
+        return components.Length switch
+        {
+            2 => components[0].Length == keyLength && components[1].Length == keyLength,
+            3 => components[0] == "configcat-sdk-1" && components[1].Length == keyLength && components[2].Length == keyLength,
+            _ => false
+        };
+    }
+
+    internal static string GetCacheKey(string sdkKey)
+    {
+        var key = $"{sdkKey}_{ConfigCatClientOptions.ConfigFileName}_{ProjectConfig.SerializationFormatVersion}";
+        return key.Sha1();
+    }
+
     /// <inheritdoc />
     public LogLevel LogLevel
     {
@@ -113,7 +139,7 @@ internal ConfigCatClient(IConfigService configService, IConfigCatLogger logger,
     /// <param name="sdkKey">SDK Key to access the ConfigCat config.</param>
     /// <param name="configurationAction">The action used to configure the client.</param>
     /// <exception cref="ArgumentNullException"><paramref name="sdkKey"/> is <see langword="null"/>.</exception>
-    /// <exception cref="ArgumentException"><paramref name="sdkKey"/> is an empty string.</exception>
+    /// <exception cref="ArgumentException"><paramref name="sdkKey"/> is an empty string or in an invalid format.</exception>
     public static IConfigCatClient Get(string sdkKey, Action<ConfigCatClientOptions>? configurationAction = null)
     {
         if (sdkKey is null)
@@ -129,6 +155,11 @@ public static IConfigCatClient Get(string sdkKey, Action<ConfigCatClientOptions>
         var options = new ConfigCatClientOptions();
         configurationAction?.Invoke(options);
 
+        if (!IsValidSdkKey(sdkKey, options.IsCustomBaseUrl))
+        {
+            throw new ArgumentException($"SDK Key '{sdkKey}' is invalid.", nameof(sdkKey));
+        }
+
         var instance = Instances.GetOrCreate(sdkKey, options, out var instanceAlreadyCreated);
 
         if (instanceAlreadyCreated && configurationAction is not null)
@@ -662,12 +693,6 @@ private static IConfigService DetermineConfigService(PollingMode pollingMode, Ht
         };
     }
 
-    internal static string GetCacheKey(string sdkKey)
-    {
-        var key = $"{sdkKey}_{ConfigCatClientOptions.ConfigFileName}_{ProjectConfig.SerializationFormatVersion}";
-        return key.Sha1();
-    }
-
     /// <inheritdoc />
     public void SetDefaultUser(User user)
     {