diff --git a/Test/DurableTask.AzureServiceFabric.Tests/RemoteOrchestrationTypeBinderTests.cs b/Test/DurableTask.AzureServiceFabric.Tests/RemoteOrchestrationTypeBinderTests.cs
new file mode 100644
index 000000000..89e62bf00
--- /dev/null
+++ b/Test/DurableTask.AzureServiceFabric.Tests/RemoteOrchestrationTypeBinderTests.cs
@@ -0,0 +1,100 @@
+//  ----------------------------------------------------------------------------------
+//  Copyright Microsoft Corporation
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//  http://www.apache.org/licenses/LICENSE-2.0
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//  ----------------------------------------------------------------------------------
+
+namespace DurableTask.AzureServiceFabric.Tests
+{
+    using System;
+    using System.Collections.Generic;
+    using DurableTask.AzureServiceFabric.Remote;
+    using DurableTask.Core;
+    using DurableTask.Core.History;
+    using Microsoft.VisualStudio.TestTools.UnitTesting;
+    using Newtonsoft.Json;
+
+    [TestClass]
+    public class RemoteOrchestrationTypeBinderTests
+    {
+        // Mirrors the production formatter settings configured in RemoteOrchestrationServiceClient.PutJsonAsync.
+        static readonly JsonSerializerSettings RoundTripSettings = new JsonSerializerSettings
+        {
+            TypeNameHandling = TypeNameHandling.Auto,
+            SerializationBinder = new RemoteOrchestrationTypeBinder()
+        };
+
+        [TestMethod]
+        public void RoundTripsTaskMessageWithPolymorphicHistoryEvent()
+        {
+            var original = new TaskMessage
+            {
+                SequenceNumber = 7,
+                OrchestrationInstance = new OrchestrationInstance
+                {
+                    InstanceId = "instance-1",
+                    ExecutionId = "execution-1"
+                },
+                Event = new ExecutionStartedEvent(eventId: -1, input: "input")
+                {
+                    Name = "Orchestration",
+                    Version = "1.0",
+                    Tags = new Dictionary<string, string> { ["tag1"] = "value1" }
+                }
+            };
+
+            string json = JsonConvert.SerializeObject(original, RoundTripSettings);
+            var deserialized = JsonConvert.DeserializeObject<TaskMessage>(json, RoundTripSettings);
+
+            Assert.IsInstanceOfType(deserialized.Event, typeof(ExecutionStartedEvent));
+            var startedEvent = (ExecutionStartedEvent)deserialized.Event;
+            Assert.AreEqual("Orchestration", startedEvent.Name);
+            Assert.AreEqual("input", startedEvent.Input);
+            Assert.AreEqual("value1", startedEvent.Tags["tag1"]);
+        }
+
+        [TestMethod]
+        public void AllowsTypesFromDurableTaskCoreAssembly()
+        {
+            var binder = new RemoteOrchestrationTypeBinder();
+            Type bound = binder.BindToType(
+                typeof(TaskMessage).Assembly.GetName().Name,
+                typeof(TaskMessage).FullName);
+            Assert.AreEqual(typeof(TaskMessage), bound);
+        }
+
+        [TestMethod]
+        public void AllowsTypesFromDurableTaskAzureServiceFabricAssembly()
+        {
+            var binder = new RemoteOrchestrationTypeBinder();
+            Type bound = binder.BindToType(
+                typeof(TaskMessageItem).Assembly.GetName().Name,
+                typeof(TaskMessageItem).FullName);
+            Assert.AreEqual(typeof(TaskMessageItem), bound);
+        }
+
+        [TestMethod]
+        public void RejectsNonAllowlistedRootType()
+        {
+            string json = "{\"$type\":\"System.IO.FileInfo, System.Private.CoreLib\",\"OriginalPath\":\"c:\\\\evil\"}";
+            Assert.ThrowsException<JsonSerializationException>(
+                () => JsonConvert.DeserializeObject<object>(json, RoundTripSettings));
+        }
+
+        [TestMethod]
+        public void RejectsNonAllowlistedNestedType()
+        {
+            string json = "{\"$type\":\"DurableTask.Core.History.ExecutionStartedEvent, DurableTask.Core\","
+                + "\"Tags\":{\"$type\":\"System.Collections.Generic.SortedDictionary`2[[System.String, System.Private.CoreLib],[System.String, System.Private.CoreLib]], System.Collections\"}}";
+            Assert.ThrowsException<JsonSerializationException>(
+                () => JsonConvert.DeserializeObject<HistoryEvent>(json, RoundTripSettings));
+        }
+    }
+}
diff --git a/src/DurableTask.AzureServiceFabric/Remote/RemoteOrchestrationServiceClient.cs b/src/DurableTask.AzureServiceFabric/Remote/RemoteOrchestrationServiceClient.cs
index 2143dd414..e43db2ab6 100644
--- a/src/DurableTask.AzureServiceFabric/Remote/RemoteOrchestrationServiceClient.cs
+++ b/src/DurableTask.AzureServiceFabric/Remote/RemoteOrchestrationServiceClient.cs
@@ -305,7 +305,15 @@ private async Task PutJsonAsync(string instanceId, string fragment, object @obje
         {
             var mediaFormatter = new JsonMediaTypeFormatter()
             {
-                SerializerSettings = new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.All }
+                // TypeNameHandling.Auto is sufficient: it emits $type only for polymorphic members
+                // (e.g., HistoryEvent Event on TaskMessage). The strict binder restricts the resolvable
+                // type set to DurableTask.* assemblies as defense in depth in case this formatter is
+                // ever reused for deserialization.
+                SerializerSettings = new JsonSerializerSettings
+                {
+                    TypeNameHandling = TypeNameHandling.Auto,
+                    SerializationBinder = new RemoteOrchestrationTypeBinder(),
+                }
             };
 
             using (var result = await this.ExecuteRequestWithRetriesAsync(
diff --git a/src/DurableTask.AzureServiceFabric/Remote/RemoteOrchestrationTypeBinder.cs b/src/DurableTask.AzureServiceFabric/Remote/RemoteOrchestrationTypeBinder.cs
new file mode 100644
index 000000000..d141077da
--- /dev/null
+++ b/src/DurableTask.AzureServiceFabric/Remote/RemoteOrchestrationTypeBinder.cs
@@ -0,0 +1,66 @@
+//  ----------------------------------------------------------------------------------
+//  Copyright Microsoft Corporation
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//  http://www.apache.org/licenses/LICENSE-2.0
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//  ----------------------------------------------------------------------------------
+
+namespace DurableTask.AzureServiceFabric.Remote
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Reflection;
+    using DurableTask.Core;
+    using DurableTask.Core.Serializing;
+    using Newtonsoft.Json;
+
+    /// <summary>
+    /// Strict <see cref="Newtonsoft.Json.Serialization.ISerializationBinder"/> used by
+    /// <see cref="RemoteOrchestrationServiceClient"/> when configuring the JSON formatter that
+    /// serializes orchestration RPC payloads (<c>TaskMessage</c>, <c>CreateTaskOrchestrationParameters</c>,
+    /// etc.). Only types defined in <c>DurableTask.Core</c> and <c>DurableTask.AzureServiceFabric</c>,
+    /// plus <see cref="Dictionary{TKey, TValue}"/> of strings, are accepted; any other <c>$type</c>
+    /// token is rejected with a <see cref="JsonSerializationException"/>.
+    /// </summary>
+    /// <remarks>
+    /// The formatter that uses this binder is only consumed for outbound serialization in
+    /// <see cref="RemoteOrchestrationServiceClient"/>; the binder is provided as defense in depth so
+    /// that the same settings remain safe if reused for deserialization.
+    /// </remarks>
+    internal sealed class RemoteOrchestrationTypeBinder : PackageUpgradeSerializationBinder
+    {
+        static readonly Assembly DurableTaskCoreAssembly = typeof(TaskMessage).Assembly;
+        static readonly Assembly DurableTaskAzureServiceFabricAssembly = typeof(RemoteOrchestrationTypeBinder).Assembly;
+
+        /// <inheritdoc />
+        public override Type BindToType(string assemblyName, string typeName)
+        {
+            Type resolved = base.BindToType(assemblyName, typeName);
+
+            if (resolved == null || !IsAllowed(resolved))
+            {
+                throw new JsonSerializationException(
+                    $"Type '{typeName}' from assembly '{assemblyName}' is not permitted by the remote orchestration serialization binder.");
+            }
+
+            return resolved;
+        }
+
+        static bool IsAllowed(Type type)
+        {
+            // Allow types defined in DurableTask.Core (TaskMessage, HistoryEvent subclasses,
+            // OrchestrationInstance, etc.) and DurableTask.AzureServiceFabric (CreateTaskOrchestrationParameters,
+            // PurgeOrchestrationHistoryParameters, etc.), plus Dictionary<string, string> for the
+            // IDictionary<string, string> Tags members on history events.
+            return type.Assembly == DurableTaskCoreAssembly
+                || type.Assembly == DurableTaskAzureServiceFabricAssembly
+                || type == typeof(Dictionary<string, string>);
+        }
+    }
+}