diff --git a/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs b/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
index 0a8673dc91d..9c0506a2307 100644
--- a/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs
@@ -205,6 +205,16 @@ public int MaximumConsecutiveErrorsPerRequest
         set => _maximumConsecutiveErrorsPerRequest = Throw.IfLessThan(value, 0);
     }
 
+    /// <summary>Gets or sets a collection of additional tools the client is able to invoke.</summary>
+    /// <remarks>
+    /// These will not impact the requests sent by the <see cref="FunctionInvokingChatClient"/>, which will pass through the
+    /// <see cref="ChatOptions.Tools" /> unmodified. However, if the inner client requests the invocation of a tool
+    /// that was not in <see cref="ChatOptions.Tools" />, this <see cref="AdditionalTools"/> collection will also be consulted
+    /// to look for a corresponding tool to invoke. This is useful when the service may have been pre-configured to be aware
+    /// of certain tools that aren't also sent on each individual request.
+    /// </remarks>
+    public IList<AITool>? AdditionalTools { get; set; }
+
     /// <summary>Gets or sets a delegate used to invoke <see cref="AIFunction"/> instances.</summary>
     /// <remarks>
     /// By default, the protected <see cref="InvokeFunctionAsync"/> method is called for each <see cref="AIFunction"/> to be invoked,
@@ -250,7 +260,7 @@ public override async Task<ChatResponse> GetResponseAsync(
 
             // Any function call work to do? If yes, ensure we're tracking that work in functionCallContents.
             bool requiresFunctionInvocation =
-                options?.Tools is { Count: > 0 } &&
+                (options?.Tools is { Count: > 0 } || AdditionalTools is { Count: > 0 }) &&
                 iteration < MaximumIterationsPerRequest &&
                 CopyFunctionCalls(response.Messages, ref functionCallContents);
 
@@ -288,7 +298,7 @@ public override async Task<ChatResponse> GetResponseAsync(
 
             // Add the responses from the function calls into the augmented history and also into the tracked
             // list of response messages.
-            var modeAndMessages = await ProcessFunctionCallsAsync(augmentedHistory, options!, functionCallContents!, iteration, consecutiveErrorCount, isStreaming: false, cancellationToken);
+            var modeAndMessages = await ProcessFunctionCallsAsync(augmentedHistory, options, functionCallContents!, iteration, consecutiveErrorCount, isStreaming: false, cancellationToken);
             responseMessages.AddRange(modeAndMessages.MessagesAdded);
             consecutiveErrorCount = modeAndMessages.NewConsecutiveErrorCount;
 
@@ -297,7 +307,7 @@ public override async Task<ChatResponse> GetResponseAsync(
                 break;
             }
 
-            UpdateOptionsForNextIteration(ref options!, response.ConversationId);
+            UpdateOptionsForNextIteration(ref options, response.ConversationId);
         }
 
         Debug.Assert(responseMessages is not null, "Expected to only be here if we have response messages.");
@@ -367,7 +377,7 @@ public override async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseA
 
             // If there are no tools to call, or for any other reason we should stop, return the response.
             if (functionCallContents is not { Count: > 0 } ||
-                options?.Tools is not { Count: > 0 } ||
+                (options?.Tools is not { Count: > 0 } && AdditionalTools is not { Count: > 0 }) ||
                 iteration >= _maximumIterationsPerRequest)
             {
                 break;
@@ -535,9 +545,16 @@ private static bool CopyFunctionCalls(
         return any;
     }
 
-    private static void UpdateOptionsForNextIteration(ref ChatOptions options, string? conversationId)
+    private static void UpdateOptionsForNextIteration(ref ChatOptions? options, string? conversationId)
     {
-        if (options.ToolMode is RequiredChatToolMode)
+        if (options is null)
+        {
+            if (conversationId is not null)
+            {
+                options = new() { ConversationId = conversationId };
+            }
+        }
+        else if (options.ToolMode is RequiredChatToolMode)
         {
             // We have to reset the tool mode to be non-required after the first iteration,
             // as otherwise we'll be in an infinite loop.
@@ -566,7 +583,7 @@ private static void UpdateOptionsForNextIteration(ref ChatOptions options, strin
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.</param>
     /// <returns>A value indicating how the caller should proceed.</returns>
     private async Task<(bool ShouldTerminate, int NewConsecutiveErrorCount, IList<ChatMessage> MessagesAdded)> ProcessFunctionCallsAsync(
-        List<ChatMessage> messages, ChatOptions options, List<FunctionCallContent> functionCallContents, int iteration, int consecutiveErrorCount,
+        List<ChatMessage> messages, ChatOptions? options, List<FunctionCallContent> functionCallContents, int iteration, int consecutiveErrorCount,
         bool isStreaming, CancellationToken cancellationToken)
     {
         // We must add a response for every tool call, regardless of whether we successfully executed it or not.
@@ -695,13 +712,13 @@ private void ThrowIfNoFunctionResultsAdded(IList<ChatMessage>? messages)
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.</param>
     /// <returns>A value indicating how the caller should proceed.</returns>
     private async Task<FunctionInvocationResult> ProcessFunctionCallAsync(
-        List<ChatMessage> messages, ChatOptions options, List<FunctionCallContent> callContents,
+        List<ChatMessage> messages, ChatOptions? options, List<FunctionCallContent> callContents,
         int iteration, int functionCallIndex, bool captureExceptions, bool isStreaming, CancellationToken cancellationToken)
     {
         var callContent = callContents[functionCallIndex];
 
         // Look up the AIFunction for the function call. If the requested function isn't available, send back an error.
-        AIFunction? aiFunction = options.Tools!.OfType<AIFunction>().FirstOrDefault(t => t.Name == callContent.Name);
+        AIFunction? aiFunction = FindAIFunction(options?.Tools, callContent.Name) ?? FindAIFunction(AdditionalTools, callContent.Name);
         if (aiFunction is null)
         {
             return new(terminate: false, FunctionInvocationStatus.NotFound, callContent, result: null, exception: null);
@@ -746,6 +763,23 @@ private async Task<FunctionInvocationResult> ProcessFunctionCallAsync(
             callContent,
             result,
             exception: null);
+
+        static AIFunction? FindAIFunction(IList<AITool>? tools, string functionName)
+        {
+            if (tools is not null)
+            {
+                int count = tools.Count;
+                for (int i = 0; i < count; i++)
+                {
+                    if (tools[i] is AIFunction function && function.Name == functionName)
+                    {
+                        return function;
+                    }
+                }
+            }
+
+            return null;
+        }
     }
 
     /// <summary>Creates one or more response messages for function invocation results.</summary>
diff --git a/src/Libraries/Microsoft.Extensions.AI/Microsoft.Extensions.AI.json b/src/Libraries/Microsoft.Extensions.AI/Microsoft.Extensions.AI.json
index 59ed3d32fab..3e3f0426dd1 100644
--- a/src/Libraries/Microsoft.Extensions.AI/Microsoft.Extensions.AI.json
+++ b/src/Libraries/Microsoft.Extensions.AI/Microsoft.Extensions.AI.json
@@ -515,6 +515,10 @@
         }
       ],
       "Properties": [
+        {
+          "Member": "System.Collections.Generic.IList<Microsoft.Extensions.AI.AITool>? Microsoft.Extensions.AI.FunctionInvokingChatClient.AdditionalTools { get; set; }",
+          "Stage": "Stable"
+        },
         {
           "Member": "bool Microsoft.Extensions.AI.FunctionInvokingChatClient.AllowConcurrentInvocation { get; set; }",
           "Stage": "Stable"
diff --git a/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs b/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
index b4ce2f1546c..08cb5ee5760 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs
@@ -39,6 +39,7 @@ public void Ctor_HasExpectedDefaults()
         Assert.Equal(40, client.MaximumIterationsPerRequest);
         Assert.Equal(3, client.MaximumConsecutiveErrorsPerRequest);
         Assert.Null(client.FunctionInvoker);
+        Assert.Null(client.AdditionalTools);
     }
 
     [Fact]
@@ -67,6 +68,11 @@ public void Properties_Roundtrip()
         Func<FunctionInvocationContext, CancellationToken, ValueTask<object?>> invoker = (ctx, ct) => new ValueTask<object?>("test");
         client.FunctionInvoker = invoker;
         Assert.Same(invoker, client.FunctionInvoker);
+
+        Assert.Null(client.AdditionalTools);
+        IList<AITool> additionalTools = [AIFunctionFactory.Create(() => "Additional Tool")];
+        client.AdditionalTools = additionalTools;
+        Assert.Same(additionalTools, client.AdditionalTools);
     }
 
     [Fact]
@@ -99,6 +105,73 @@ public async Task SupportsSingleFunctionCallPerRequestAsync()
         await InvokeAndAssertStreamingAsync(options, plan);
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task SupportsToolsProvidedByAdditionalTools(bool provideOptions)
+    {
+        ChatOptions? options = provideOptions ?
+            new() { Tools = [AIFunctionFactory.Create(() => "Shouldn't be invoked", "ChatOptionsFunc")] } :
+            null;
+
+        Func<ChatClientBuilder, ChatClientBuilder> configure = builder =>
+            builder.UseFunctionInvocation(configure: c => c.AdditionalTools =
+            [
+                AIFunctionFactory.Create(() => "Result 1", "Func1"),
+                AIFunctionFactory.Create((int i) => $"Result 2: {i}", "Func2"),
+                AIFunctionFactory.Create((int i) => { }, "VoidReturn"),
+            ]);
+
+        List<ChatMessage> plan =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId1", result: "Result 1")]),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId2", result: "Result 2: 42")]),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId3", "VoidReturn", arguments: new Dictionary<string, object?> { { "i", 43 } })]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId3", result: "Success: Function completed.")]),
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        await InvokeAndAssertAsync(options, plan, configurePipeline: configure);
+
+        await InvokeAndAssertStreamingAsync(options, plan, configurePipeline: configure);
+    }
+
+    [Fact]
+    public async Task PrefersToolsProvidedByChatOptions()
+    {
+        ChatOptions options = new()
+        {
+            Tools = [AIFunctionFactory.Create(() => "Result 1", "Func1")]
+        };
+
+        Func<ChatClientBuilder, ChatClientBuilder> configure = builder =>
+            builder.UseFunctionInvocation(configure: c => c.AdditionalTools =
+            [
+                AIFunctionFactory.Create(() => "Should never be invoked", "Func1"),
+                AIFunctionFactory.Create((int i) => $"Result 2: {i}", "Func2"),
+                AIFunctionFactory.Create((int i) => { }, "VoidReturn"),
+            ]);
+
+        List<ChatMessage> plan =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId1", result: "Result 1")]),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId2", result: "Result 2: 42")]),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId3", "VoidReturn", arguments: new Dictionary<string, object?> { { "i", 43 } })]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId3", result: "Success: Function completed.")]),
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        await InvokeAndAssertAsync(options, plan, configurePipeline: configure);
+
+        await InvokeAndAssertStreamingAsync(options, plan, configurePipeline: configure);
+    }
+
     [Theory]
     [InlineData(false)]
     [InlineData(true)]
@@ -1002,7 +1075,7 @@ public override void Post(SendOrPostCallback d, object? state)
     }
 
     private static async Task<List<ChatMessage>> InvokeAndAssertAsync(
-        ChatOptions options,
+        ChatOptions? options,
         List<ChatMessage> plan,
         List<ChatMessage>? expected = null,
         Func<ChatClientBuilder, ChatClientBuilder>? configurePipeline = null,
@@ -1102,7 +1175,7 @@ private static UsageDetails CreateRandomUsage()
     }
 
     private static async Task<List<ChatMessage>> InvokeAndAssertStreamingAsync(
-        ChatOptions options,
+        ChatOptions? options,
         List<ChatMessage> plan,
         List<ChatMessage>? expected = null,
         Func<ChatClientBuilder, ChatClientBuilder>? configurePipeline = null,
diff --git a/test/Libraries/Microsoft.Extensions.AI.Tests/Microsoft.Extensions.AI.Tests.csproj b/test/Libraries/Microsoft.Extensions.AI.Tests/Microsoft.Extensions.AI.Tests.csproj
index e4f17abb179..c07f3056054 100644
--- a/test/Libraries/Microsoft.Extensions.AI.Tests/Microsoft.Extensions.AI.Tests.csproj
+++ b/test/Libraries/Microsoft.Extensions.AI.Tests/Microsoft.Extensions.AI.Tests.csproj
@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <NoWarn>$(NoWarn);CA1063;CA1861;SA1130;VSTHRD003</NoWarn>
+    <NoWarn>$(NoWarn);CA1063;CA1861;S104;SA1130;VSTHRD003</NoWarn>
     <NoWarn>$(NoWarn);MEAI001</NoWarn>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>