diff --git a/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientExtensions.cs b/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientExtensions.cs
index fcca066848e..699b98c2e02 100644
--- a/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientExtensions.cs
@@ -2,7 +2,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
 using Microsoft.Agents.AI;
 using Microsoft.Extensions.AI;
 using Microsoft.Extensions.DependencyInjection;
@@ -134,16 +133,6 @@ internal static IChatClient WithDefaultAgentMiddleware(this IChatClient chatClie
         // so FICC emits execute_tool spans on the agent source.
         chatBuilder.Use(innerClient => new DeferredOpenTelemetryChatClient(innerClient));
 
-        var agentChatClient = chatBuilder.Build(services);
-
-        if (options?.ChatOptions?.Tools is { Count: > 0 })
-        {
-            // When tools are provided in the constructor, set the tools for the whole lifecycle of the chat client
-            var functionService = agentChatClient.GetService<FunctionInvokingChatClient>();
-            Debug.Assert(functionService is not null, "FunctionInvokingChatClient should be registered in the chat client.");
-            functionService!.AdditionalTools = options.ChatOptions.Tools;
-        }
-
-        return agentChatClient;
+        return chatBuilder.Build(services);
     }
 }
diff --git a/dotnet/tests/Microsoft.Agents.AI.Anthropic.UnitTests/Extensions/AnthropicBetaServiceExtensionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.Anthropic.UnitTests/Extensions/AnthropicBetaServiceExtensionsTests.cs
index 45d027fc261..2e49e588cd1 100644
--- a/dotnet/tests/Microsoft.Agents.AI.Anthropic.UnitTests/Extensions/AnthropicBetaServiceExtensionsTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.Anthropic.UnitTests/Extensions/AnthropicBetaServiceExtensionsTests.cs
@@ -210,11 +210,11 @@ public void CreateAIAgent_WithTools_AssignsToolsCorrectly()
         // When tools are provided, ChatOptions is created but instructions remain null
         Assert.Null(agent.Instructions);
 
-        // Verify that tools are registered in the FunctionInvokingChatClient
-        var functionInvokingClient = agent.GetService<FunctionInvokingChatClient>();
-        Assert.NotNull(functionInvokingClient);
-        Assert.NotNull(functionInvokingClient.AdditionalTools);
-        Assert.Contains(functionInvokingClient.AdditionalTools, t => t is AIFunction func && func.Name == "TestFunction");
+        // Verify that tools are registered on the agent's ChatOptions, which are merged into the
+        // invocation-scoped ChatOptions on every call (see ChatClientAgent.CreateConfiguredChatOptions).
+        var agentTools = agent.GetService<ChatOptions>()?.Tools;
+        Assert.NotNull(agentTools);
+        Assert.Contains(agentTools, t => t is AIFunction func && func.Name == "TestFunction");
     }
 
     /// <summary>
@@ -310,11 +310,11 @@ public void CreateAIAgent_WithToolsAndInstructions_AssignsBothCorrectly()
         Assert.Equal("Test Agent", agent.Name);
         Assert.Equal("Test instructions", agent.Instructions);
 
-        // Verify that tools are registered in the FunctionInvokingChatClient
-        var functionInvokingClient = agent.GetService<FunctionInvokingChatClient>();
-        Assert.NotNull(functionInvokingClient);
-        Assert.NotNull(functionInvokingClient.AdditionalTools);
-        Assert.Contains(functionInvokingClient.AdditionalTools, t => t is AIFunction func && func.Name == "TestFunction");
+        // Verify that tools are registered on the agent's ChatOptions, which are merged into the
+        // invocation-scoped ChatOptions on every call (see ChatClientAgent.CreateConfiguredChatOptions).
+        var agentTools = agent.GetService<ChatOptions>()?.Tools;
+        Assert.NotNull(agentTools);
+        Assert.Contains(agentTools, t => t is AIFunction func && func.Name == "TestFunction");
     }
 
     /// <summary>
@@ -339,7 +339,7 @@ public void CreateAIAgent_WithEmptyTools_DoesNotAssignTools()
         // With empty tools and no instructions, agent instructions remain null
         Assert.Null(agent.Instructions);
 
-        // Verify that FunctionInvokingChatClient has no additional tools assigned
+        // Verify that the FunctionInvokingChatClient is not used as a fallback tool store.
         var functionInvokingClient = agent.GetService<FunctionInvokingChatClient>();
         Assert.NotNull(functionInvokingClient);
         Assert.True(functionInvokingClient.AdditionalTools is null or { Count: 0 });
diff --git a/dotnet/tests/Microsoft.Agents.AI.Anthropic.UnitTests/Extensions/AnthropicClientExtensionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.Anthropic.UnitTests/Extensions/AnthropicClientExtensionsTests.cs
index abbbe6111fa..876d71936e2 100644
--- a/dotnet/tests/Microsoft.Agents.AI.Anthropic.UnitTests/Extensions/AnthropicClientExtensionsTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.Anthropic.UnitTests/Extensions/AnthropicClientExtensionsTests.cs
@@ -284,11 +284,11 @@ public void CreateAIAgent_WithTools_AssignsToolsCorrectly()
         // When tools are provided, ChatOptions is created but instructions remain null
         Assert.Null(agent.Instructions);
 
-        // Verify that tools are registered in the FunctionInvokingChatClient
-        var functionInvokingClient = agent.GetService<FunctionInvokingChatClient>();
-        Assert.NotNull(functionInvokingClient);
-        Assert.NotNull(functionInvokingClient.AdditionalTools);
-        Assert.Contains(functionInvokingClient.AdditionalTools, t => t is AIFunction func && func.Name == "TestFunction");
+        // Verify that tools are registered on the agent's ChatOptions, which are merged into the
+        // invocation-scoped ChatOptions on every call (see ChatClientAgent.CreateConfiguredChatOptions).
+        var agentTools = agent.GetService<ChatOptions>()?.Tools;
+        Assert.NotNull(agentTools);
+        Assert.Contains(agentTools, t => t is AIFunction func && func.Name == "TestFunction");
     }
 
     /// <summary>
@@ -384,11 +384,11 @@ public void CreateAIAgent_WithToolsAndInstructions_AssignsBothCorrectly()
         Assert.Equal("Test Agent", agent.Name);
         Assert.Equal("Test instructions", agent.Instructions);
 
-        // Verify that tools are registered in the FunctionInvokingChatClient
-        var functionInvokingClient = agent.GetService<FunctionInvokingChatClient>();
-        Assert.NotNull(functionInvokingClient);
-        Assert.NotNull(functionInvokingClient.AdditionalTools);
-        Assert.Contains(functionInvokingClient.AdditionalTools, t => t is AIFunction func && func.Name == "TestFunction");
+        // Verify that tools are registered on the agent's ChatOptions, which are merged into the
+        // invocation-scoped ChatOptions on every call (see ChatClientAgent.CreateConfiguredChatOptions).
+        var agentTools = agent.GetService<ChatOptions>()?.Tools;
+        Assert.NotNull(agentTools);
+        Assert.Contains(agentTools, t => t is AIFunction func && func.Name == "TestFunction");
     }
 
     /// <summary>
@@ -413,7 +413,7 @@ public void CreateAIAgent_WithEmptyTools_DoesNotAssignTools()
         // With empty tools and no instructions, agent instructions remain null
         Assert.Null(agent.Instructions);
 
-        // Verify that FunctionInvokingChatClient has no additional tools assigned
+        // Verify that the FunctionInvokingChatClient is not used as a fallback tool store.
         var functionInvokingClient = agent.GetService<FunctionInvokingChatClient>();
         Assert.NotNull(functionInvokingClient);
         Assert.True(functionInvokingClient.AdditionalTools is null or { Count: 0 });
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientExtensionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientExtensionsTests.cs
index 04937c23e8e..f6fc856e0b8 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientExtensionsTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientExtensionsTests.cs
@@ -109,6 +109,32 @@ public void CreateAIAgent_WithExistingFunctionInvokingChatClient_ConfiguresConcu
         Assert.True(functionInvokingClient.AllowConcurrentInvocation);
     }
 
+    [Fact]
+    public void CreateAIAgent_SharedChatClient_DoesNotLeakToolsBetweenAgents()
+    {
+        // Arrange: a single pre-decorated IChatClient shared by two independently-constructed agents,
+        // one "privileged" and one "public", each with their own distinct tool.
+        var chatClientMock = new Mock<IChatClient>();
+        var sharedChatClient = chatClientMock.Object.AsBuilder().UseFunctionInvocation().Build();
+
+        AITool publicTool = AIFunctionFactory.Create(() => "public", name: "public_read");
+        AITool privilegedTool = AIFunctionFactory.Create(() => "privileged", name: "privileged_write");
+
+        // Act: construct the low-privilege agent first, then the privileged agent on the same shared client.
+        var publicAgent = sharedChatClient.AsAIAgent(tools: [publicTool]);
+        var privilegedAgent = sharedChatClient.AsAIAgent(tools: [privilegedTool]);
+
+        // Assert: neither agent mutated the shared FunctionInvokingChatClient's AdditionalTools, so
+        // constructing the privileged agent cannot overwrite/leak tools into the public agent's execution scope.
+        var functionInvokingClient = sharedChatClient.GetService<FunctionInvokingChatClient>();
+        Assert.NotNull(functionInvokingClient);
+        Assert.True(functionInvokingClient.AdditionalTools is null or { Count: 0 });
+
+        // Each agent's own configured tools remain scoped to itself.
+        Assert.Equal([publicTool], publicAgent.ChatOptions!.Tools);
+        Assert.Equal([privilegedTool], privilegedAgent.ChatOptions!.Tools);
+    }
+
     [Fact]
     public void CreateAIAgent_WithNullClient_Throws()
     {