diff --git a/dotnet/samples/GettingStarted/AgentProviders/Agent_With_OpenAIAssistants/Program.cs b/dotnet/samples/GettingStarted/AgentProviders/Agent_With_OpenAIAssistants/Program.cs
index 9a91e43d37..b39a21f71f 100644
--- a/dotnet/samples/GettingStarted/AgentProviders/Agent_With_OpenAIAssistants/Program.cs
+++ b/dotnet/samples/GettingStarted/AgentProviders/Agent_With_OpenAIAssistants/Program.cs
@@ -5,6 +5,8 @@
 // WARNING: The Assistants API is deprecated and will be shut down.
 // For more information see the OpenAI documentation: https://platform.openai.com/docs/assistants/migration
 
+#pragma warning disable CS0618 // Type or member is obsolete - OpenAI Assistants API is deprecated but still used in this sample
+
 using Microsoft.Agents.AI;
 using OpenAI;
 
diff --git a/dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIAssistantClientExtensions.cs b/dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIAssistantClientExtensions.cs
index fb464fdc39..f103a7fc1c 100644
--- a/dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIAssistantClientExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.OpenAI/Extensions/OpenAIAssistantClientExtensions.cs
@@ -30,6 +30,7 @@ public static class OpenAIAssistantClientExtensions
     /// <param name="clientFactory">Provides a way to customize the creation of the underlying <see cref="IChatClient"/> used by the agent.</param>
     /// <param name="services">An optional <see cref="IServiceProvider"/> to use for resolving services required by the <see cref="AIFunction"/> instances being invoked.</param>
     /// <returns>A <see cref="ChatClientAgent"/> instance that can be used to perform operations on the assistant.</returns>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static ChatClientAgent GetAIAgent(
         this AssistantClient assistantClient,
         ClientResult<Assistant> assistantClientResult,
@@ -54,6 +55,7 @@ public static ChatClientAgent GetAIAgent(
     /// <param name="clientFactory">Provides a way to customize the creation of the underlying <see cref="IChatClient"/> used by the agent.</param>
     /// <param name="services">An optional <see cref="IServiceProvider"/> to use for resolving services required by the <see cref="AIFunction"/> instances being invoked.</param>
     /// <returns>A <see cref="ChatClientAgent"/> instance that can be used to perform operations on the assistant.</returns>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static ChatClientAgent GetAIAgent(
         this AssistantClient assistantClient,
         Assistant assistantMetadata,
@@ -102,6 +104,7 @@ public static ChatClientAgent GetAIAgent(
     /// <param name="services">An optional <see cref="IServiceProvider"/> to use for resolving services required by the <see cref="AIFunction"/> instances being invoked.</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ChatClientAgent"/> instance that can be used to perform operations on the assistant agent.</returns>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static ChatClientAgent GetAIAgent(
         this AssistantClient assistantClient,
         string agentId,
@@ -134,6 +137,7 @@ public static ChatClientAgent GetAIAgent(
     /// <param name="services">An optional <see cref="IServiceProvider"/> to use for resolving services required by the <see cref="AIFunction"/> instances being invoked.</param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
     /// <returns>A <see cref="ChatClientAgent"/> instance that can be used to perform operations on the assistant agent.</returns>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static async Task<ChatClientAgent> GetAIAgentAsync(
         this AssistantClient assistantClient,
         string agentId,
@@ -166,6 +170,7 @@ public static async Task<ChatClientAgent> GetAIAgentAsync(
     /// <param name="services">An optional <see cref="IServiceProvider"/> to use for resolving services required by the <see cref="AIFunction"/> instances being invoked.</param>
     /// <returns>A <see cref="ChatClientAgent"/> instance that can be used to perform operations on the assistant.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="assistantClientResult"/> or <paramref name="options"/> is <see langword="null"/>.</exception>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static ChatClientAgent GetAIAgent(
         this AssistantClient assistantClient,
         ClientResult<Assistant> assistantClientResult,
@@ -191,6 +196,7 @@ public static ChatClientAgent GetAIAgent(
     /// <param name="services">An optional <see cref="IServiceProvider"/> to use for resolving services required by the <see cref="AIFunction"/> instances being invoked.</param>
     /// <returns>A <see cref="ChatClientAgent"/> instance that can be used to perform operations on the assistant.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="assistantMetadata"/> or <paramref name="options"/> is <see langword="null"/>.</exception>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static ChatClientAgent GetAIAgent(
         this AssistantClient assistantClient,
         Assistant assistantMetadata,
@@ -252,6 +258,7 @@ public static ChatClientAgent GetAIAgent(
     /// <returns>A <see cref="ChatClientAgent"/> instance that can be used to perform operations on the assistant agent.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="assistantClient"/> or <paramref name="options"/> is <see langword="null"/>.</exception>
     /// <exception cref="ArgumentException"><paramref name="agentId"/> is empty or whitespace.</exception>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static ChatClientAgent GetAIAgent(
         this AssistantClient assistantClient,
         string agentId,
@@ -291,6 +298,7 @@ public static ChatClientAgent GetAIAgent(
     /// <returns>A <see cref="ChatClientAgent"/> instance that can be used to perform operations on the assistant agent.</returns>
     /// <exception cref="ArgumentNullException"><paramref name="assistantClient"/> or <paramref name="options"/> is <see langword="null"/>.</exception>
     /// <exception cref="ArgumentException"><paramref name="agentId"/> is empty or whitespace.</exception>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static async Task<ChatClientAgent> GetAIAgentAsync(
         this AssistantClient assistantClient,
         string agentId,
@@ -333,6 +341,7 @@ public static async Task<ChatClientAgent> GetAIAgentAsync(
     /// <returns>An <see cref="ChatClientAgent"/> instance backed by the OpenAI Assistant service.</returns>
     /// <exception cref="ArgumentNullException">Thrown when <paramref name="client"/> or <paramref name="model"/> is <see langword="null"/>.</exception>
     /// <exception cref="ArgumentException">Thrown when <paramref name="model"/> is empty or whitespace.</exception>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static ChatClientAgent CreateAIAgent(
         this AssistantClient client,
         string model,
@@ -371,6 +380,7 @@ public static ChatClientAgent CreateAIAgent(
     /// <returns>An <see cref="ChatClientAgent"/> instance backed by the OpenAI Assistant service.</returns>
     /// <exception cref="ArgumentNullException">Thrown when <paramref name="client"/> or <paramref name="model"/> or <paramref name="options"/> is <see langword="null"/>.</exception>
     /// <exception cref="ArgumentException">Thrown when <paramref name="model"/> is empty or whitespace.</exception>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static ChatClientAgent CreateAIAgent(
         this AssistantClient client,
         string model,
@@ -437,6 +447,7 @@ public static ChatClientAgent CreateAIAgent(
     /// <returns>An <see cref="ChatClientAgent"/> instance backed by the OpenAI Assistant service.</returns>
     /// <exception cref="ArgumentNullException">Thrown when <paramref name="client"/> or <paramref name="model"/> is <see langword="null"/>.</exception>
     /// <exception cref="ArgumentException">Thrown when <paramref name="model"/> is empty or whitespace.</exception>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static async Task<ChatClientAgent> CreateAIAgentAsync(
         this AssistantClient client,
         string model,
@@ -477,6 +488,7 @@ await client.CreateAIAgentAsync(model,
     /// <returns>An <see cref="ChatClientAgent"/> instance backed by the OpenAI Assistant service.</returns>
     /// <exception cref="ArgumentNullException">Thrown when <paramref name="client"/> or <paramref name="model"/> is <see langword="null"/>.</exception>
     /// <exception cref="ArgumentException">Thrown when <paramref name="model"/> is empty or whitespace.</exception>
+    [Obsolete("The Assistants API has been deprecated. Please use the Responses API instead.")]
     public static async Task<ChatClientAgent> CreateAIAgentAsync(
         this AssistantClient client,
         string model,
diff --git a/dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIAssistantClientExtensionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIAssistantClientExtensionsTests.cs
index 26f855d59e..c75d72cf34 100644
--- a/dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIAssistantClientExtensionsTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.OpenAI.UnitTests/Extensions/OpenAIAssistantClientExtensionsTests.cs
@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // Type or member is obsolete - This is intentional as we are testing deprecated methods
+
 using System;
 using System.ClientModel;
 using System.ClientModel.Primitives;
diff --git a/dotnet/tests/OpenAIAssistant.IntegrationTests/OpenAIAssistantClientExtensionsTests.cs b/dotnet/tests/OpenAIAssistant.IntegrationTests/OpenAIAssistantClientExtensionsTests.cs
index f1373bd98b..0c8d6b80dd 100644
--- a/dotnet/tests/OpenAIAssistant.IntegrationTests/OpenAIAssistantClientExtensionsTests.cs
+++ b/dotnet/tests/OpenAIAssistant.IntegrationTests/OpenAIAssistantClientExtensionsTests.cs
@@ -1,5 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
+#pragma warning disable CS0618 // Type or member is obsolete - Testing deprecated OpenAI Assistants API extension methods
+
 using System;
 using System.Diagnostics;
 using System.IO;