Add FunctionInvokingChatClient.FunctionInvoker delegate

src/Libraries/Microsoft.Extensions.AI/ChatCompletion/FunctionInvokingChatClient.cs

@@ -205,6 +205,15 @@ public int MaximumConsecutiveErrorsPerRequest
         set => _maximumConsecutiveErrorsPerRequest = Throw.IfLessThan(value, 0);
     }
 
+    /// <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,
+    /// invoking the instance and returning its result. If this delegate is set to a non-<see langword="null"/> value,
+    /// <see cref="InvokeFunctionAsync"/> will replace its normal invocation with a call to this delegate, enabling
+    /// this delegate to assume all invocation handling of the function.
+    /// </remarks>
+    public Func<FunctionInvocationContext, CancellationToken, ValueTask<object?>>? FunctionInvoker { get; set; }
+
     /// <inheritdoc/>
     public override async Task<ChatResponse> GetResponseAsync(
         IEnumerable<ChatMessage> messages, ChatOptions? options = null, CancellationToken cancellationToken = default)
@@ -872,7 +881,9 @@ FunctionResultContent CreateFunctionResultContent(FunctionInvocationResult resul
     {
         _ = Throw.IfNull(context);
 
-        return context.Function.InvokeAsync(context.Arguments, cancellationToken);
+        return FunctionInvoker is { } invoker ?
+            invoker(context, cancellationToken) :
+            context.Function.InvokeAsync(context.Arguments, cancellationToken);
     }
 
     private static TimeSpan GetElapsedTime(long startingTimestamp) =>

src/Libraries/Microsoft.Extensions.AI/Microsoft.Extensions.AI.json

@@ -527,6 +527,10 @@
           "Member": "System.IServiceProvider? Microsoft.Extensions.AI.FunctionInvokingChatClient.FunctionInvocationServices { get; }",
           "Stage": "Stable"
         },
+        {
+          "Member": "System.Func<Microsoft.Extensions.AI.FunctionInvocationContext, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<object?>>? Microsoft.Extensions.AI.FunctionInvokingChatClient.FunctionInvoker { get; set; }",
+          "Stage": "Stable"
+        },
         {
           "Member": "bool Microsoft.Extensions.AI.FunctionInvokingChatClient.IncludeDetailedErrors { get; set; }",
           "Stage": "Stable"

test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientTests.cs

@@ -6,6 +6,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.DependencyInjection;
@@ -37,6 +38,35 @@ public void Ctor_HasExpectedDefaults()
         Assert.False(client.IncludeDetailedErrors);
         Assert.Equal(10, client.MaximumIterationsPerRequest);
         Assert.Equal(3, client.MaximumConsecutiveErrorsPerRequest);
+        Assert.Null(client.FunctionInvoker);
+    }
+
+    [Fact]
+    public void Properties_Roundtrip()
+    {
+        using TestChatClient innerClient = new();
+        using FunctionInvokingChatClient client = new(innerClient);
+
+        Assert.False(client.AllowConcurrentInvocation);
+        client.AllowConcurrentInvocation = true;
+        Assert.True(client.AllowConcurrentInvocation);
+
+        Assert.False(client.IncludeDetailedErrors);
+        client.IncludeDetailedErrors = true;
+        Assert.True(client.IncludeDetailedErrors);
+
+        Assert.Equal(10, client.MaximumIterationsPerRequest);
+        client.MaximumIterationsPerRequest = 5;
+        Assert.Equal(5, client.MaximumIterationsPerRequest);
+
+        Assert.Equal(3, client.MaximumConsecutiveErrorsPerRequest);
+        client.MaximumConsecutiveErrorsPerRequest = 1;
+        Assert.Equal(1, client.MaximumConsecutiveErrorsPerRequest);
+
+        Assert.Null(client.FunctionInvoker);
+        Func<FunctionInvocationContext, CancellationToken, ValueTask<object?>> invoker = (ctx, ct) => new ValueTask<object?>("test");
+        client.FunctionInvoker = invoker;
+        Assert.Same(invoker, client.FunctionInvoker);
     }
 
     [Fact]
@@ -208,6 +238,49 @@ public async Task ConcurrentInvocationOfParallelCallsDisabledByDefaultAsync()
         await InvokeAndAssertStreamingAsync(options, plan);
     }
 
+    [Fact]
+    public async Task FunctionInvokerDelegateOverridesHandlingAsync()
+    {
+        var options = new ChatOptions
+        {
+            Tools =
+            [
+                AIFunctionFactory.Create(() => "Result 1", "Func1"),
+                AIFunctionFactory.Create((int i) => $"Result 2: {i}", "Func2"),
+                AIFunctionFactory.Create((int i) => { }, "VoidReturn"),
+            ]
+        };
+
+        List<ChatMessage> plan =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId1", result: "Result 1 from delegate")]),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId2", "Func2", arguments: new Dictionary<string, object?> { { "i", 42 } })]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId2", result: "Result 2: 42 from delegate")]),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId3", "VoidReturn", arguments: new Dictionary<string, object?> { { "i", 43 } })]),
+            new ChatMessage(ChatRole.Tool, [new FunctionResultContent("callId3", result: "Success: Function completed.")]),
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        Func<ChatClientBuilder, ChatClientBuilder> configure = b => b.Use(
+            s => new FunctionInvokingChatClient(s)
+            {
+                FunctionInvoker = async (ctx, cancellationToken) =>
+                {
+                    Assert.NotNull(ctx);
+                    var result = await ctx.Function.InvokeAsync(ctx.Arguments, cancellationToken);
+                    return result is JsonElement e ?
+                        JsonSerializer.SerializeToElement($"{e.GetString()} from delegate", AIJsonUtilities.DefaultOptions) :
+                        result;
+                }
+            });
+
+        await InvokeAndAssertAsync(options, plan, configurePipeline: configure);
+
+        await InvokeAndAssertStreamingAsync(options, plan, configurePipeline: configure);
+    }
+
     [Fact]
     public async Task ContinuesWithSuccessfulCallsUntilMaximumIterations()
     {