diff --git a/pkgs/sdk/server/contract-tests/Representations.cs b/pkgs/sdk/server/contract-tests/Representations.cs
index 9efeb5ce7..77e91f81c 100644
--- a/pkgs/sdk/server/contract-tests/Representations.cs
+++ b/pkgs/sdk/server/contract-tests/Representations.cs
@@ -208,12 +208,27 @@ public class SdkConfigPersistentCacheParams
     public class SdkConfigDataInitializerParams
     {
         public SdkConfigPollingParams Polling { get; set; }
+        public SdkConfigFileDataParams File { get; set; }
     }
 
     public class SdkConfigDataSynchronizerParams
     {
         public SdkConfigStreamingParams Streaming { get; set; }
         public SdkConfigPollingParams Polling { get; set; }
+        public SdkConfigFileDataParams File { get; set; }
+        public SdkConfigTestDataParams TestData { get; set; }
+    }
+
+    public class SdkConfigFileDataParams
+    {
+        public string[] FilePaths { get; set; }
+        public bool? Persist { get; set; }
+    }
+
+    public class SdkConfigTestDataParams
+    {
+        public Dictionary<string, LdValue> Flags { get; set; }
+        public bool? Persist { get; set; }
     }
 
     public class SdkConfigPollingParams
@@ -244,6 +259,7 @@ public class CommandParams
         public RegisterFlagChangeListenerParams RegisterFlagChangeListener { get; set; }
         public RegisterFlagValueChangeListenerParams RegisterFlagValueChangeListener { get; set; }
         public UnregisterListenerParams UnregisterListener { get; set; }
+        public UpdateTestDataParams UpdateTestData { get; set; }
     }
 
     public class ContextComparisonPairParams
@@ -304,6 +320,12 @@ public class UnregisterListenerParams
         public string ListenerId { get; set; }
     }
 
+    public class UpdateTestDataParams
+    {
+        public string FlagKey { get; set; }
+        public LdValue Value { get; set; }
+    }
+
     public class ListenerNotification
     {
         public string ListenerId { get; set; }
diff --git a/pkgs/sdk/server/contract-tests/SdkClientEntity.cs b/pkgs/sdk/server/contract-tests/SdkClientEntity.cs
index 2007cca9d..861c7fe6f 100644
--- a/pkgs/sdk/server/contract-tests/SdkClientEntity.cs
+++ b/pkgs/sdk/server/contract-tests/SdkClientEntity.cs
@@ -23,6 +23,9 @@ public class SdkClientEntity
         private readonly Logger _log;
         private readonly Dictionary<string, EventHandler<FlagChangeEvent>> _flagListeners =
             new Dictionary<string, EventHandler<FlagChangeEvent>>();
+        // The test data source of this client, if the configuration has a testData synchronizer. The
+        // harness sends at most one.
+        private TestData _testData;
 
         public SdkClientEntity(
             SdkConfigParams sdkParams,
@@ -170,6 +173,21 @@ public bool DoCommand(CommandParams command, out object response)
                     break;
                 }
 
+                case "updateTestData":
+                {
+                    if (_testData == null)
+                    {
+                        throw new InvalidOperationException("the client has no test data source");
+                    }
+                    var p = command.UpdateTestData;
+                    if (p?.FlagKey == null)
+                    {
+                        throw new ArgumentException("updateTestData requires a flag key");
+                    }
+                    _testData.Update(_testData.Flag(p.FlagKey).ValueForAll(p.Value));
+                    break;
+                }
+
                 default:
                     return false;
             }
@@ -376,7 +394,7 @@ private ContextBuildResponse DoContextConvert(ContextConvertParams p)
             }
         }
 
-        private static Configuration BuildSdkConfig(SdkConfigParams sdkParams, ILogAdapter logAdapter, string tag)
+        private Configuration BuildSdkConfig(SdkConfigParams sdkParams, ILogAdapter logAdapter, string tag)
         {
             var builder = Configuration.Builder(sdkParams.Credential);
 
@@ -617,6 +635,10 @@ private static Configuration BuildSdkConfig(SdkConfigParams sdkParams, ILogAdapt
                             }
                             initializers.Add(pollingBuilder);
                         }
+                        else if (initializer.File != null)
+                        {
+                            initializers.Add(CreateFileDataSource(initializer.File, false));
+                        }
                     }
                     if (initializers.Count > 0)
                     {
@@ -721,7 +743,45 @@ private static FileOverrideSourceBuilder BuildOverrideSource(SdkConfigOverridesP
             return source;
         }
 
-        private static IComponentConfigurer<IDataSource> CreateSynchronizer(
+        private static FileDataSourceBuilder CreateFileDataSource(SdkConfigFileDataParams p, bool autoUpdate)
+        {
+            // A file data source with no files would load an empty data set, which hides LaunchDarkly data
+            // in memory or, when persisted, empties the store.
+            if (p.FilePaths == null || p.FilePaths.Length == 0 || p.FilePaths.Any(string.IsNullOrEmpty))
+            {
+                throw new ArgumentException("a file data source requires at least one file path");
+            }
+            var builder = FileData.DataSource().FilePaths(p.FilePaths).AutoUpdate(autoUpdate);
+            if (p.Persist.HasValue)
+            {
+                builder.ShouldPersist(p.Persist.Value);
+            }
+            return builder;
+        }
+
+        private TestData TestDataFor(SdkConfigTestDataParams p)
+        {
+            // The updateTestData command addresses the client's test data source, so there is only one.
+            if (_testData != null)
+            {
+                throw new ArgumentException("the configuration has more than one testData synchronizer");
+            }
+            _testData = TestData.DataSource();
+            if (p.Persist.HasValue)
+            {
+                _testData.ShouldPersist(p.Persist.Value);
+            }
+            if (p.Flags != null)
+            {
+                foreach (var flag in p.Flags)
+                {
+                    _testData.Update(_testData.Flag(flag.Key).ValueForAll(flag.Value));
+                }
+            }
+            return _testData;
+        }
+
+        private IComponentConfigurer<IDataSource> CreateSynchronizer(
             SdkConfigDataSynchronizerParams synchronizer,
             string payloadFilter)
         {
@@ -763,6 +823,15 @@ private static IComponentConfigurer<IDataSource> CreateSynchronizer(
                 }
                 return streamingBuilder;
             }
+            else if (synchronizer.File != null)
+            {
+                // A file synchronizer reloads the files when they change.
+                return CreateFileDataSource(synchronizer.File, true);
+            }
+            else if (synchronizer.TestData != null)
+            {
+                return TestDataFor(synchronizer.TestData);
+            }
             return null;
         }
 
diff --git a/pkgs/sdk/server/contract-tests/TestService.cs b/pkgs/sdk/server/contract-tests/TestService.cs
index 9ce3be0b2..20d66e88f 100644
--- a/pkgs/sdk/server/contract-tests/TestService.cs
+++ b/pkgs/sdk/server/contract-tests/TestService.cs
@@ -52,7 +52,10 @@ public class Webapp
             "instance-id",
             "retry-conformance-fdv1-streaming",
             "retry-conformance-fdv1-polling",
-            "flag-overrides"
+            "flag-overrides",
+            "file-data-source",
+            "test-data-source",
+            "data-source-persist-opt-in"
         };
 
         public readonly Handler Handler;