AIFunctionFactory: add a flag for disabling return schema generation.

src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs

@@ -615,7 +615,7 @@ public static ReflectionAIFunctionDescriptor GetOrCreate(MethodInfo method, AIFu
             serializerOptions.MakeReadOnly();
             ConcurrentDictionary<DescriptorKey, ReflectionAIFunctionDescriptor> innerCache = _descriptorCache.GetOrCreateValue(serializerOptions);
 
-            DescriptorKey key = new(method, options.Name, options.Description, options.ConfigureParameterBinding, options.MarshalResult, schemaOptions);
+            DescriptorKey key = new(method, options.Name, options.Description, options.ConfigureParameterBinding, options.MarshalResult, options.ExcludeResultSchema, schemaOptions);
             if (innerCache.TryGetValue(key, out ReflectionAIFunctionDescriptor? descriptor))
             {
                 return descriptor;
@@ -690,7 +690,7 @@ private ReflectionAIFunctionDescriptor(DescriptorKey key, JsonSerializerOptions
             Name = key.Name ?? GetFunctionName(key.Method);
             Description = key.Description ?? key.Method.GetCustomAttribute<DescriptionAttribute>(inherit: true)?.Description ?? string.Empty;
             JsonSerializerOptions = serializerOptions;
-            ReturnJsonSchema = returnType is null ? null : AIJsonUtilities.CreateJsonSchema(
+            ReturnJsonSchema = returnType is null || key.ExcludeResultSchema ? null : AIJsonUtilities.CreateJsonSchema(
                 returnType,
                 description: key.Method.ReturnParameter.GetCustomAttribute<DescriptionAttribute>(inherit: true)?.Description,
                 serializerOptions: serializerOptions,
@@ -1036,6 +1036,7 @@ private record struct DescriptorKey(
             string? Description,
             Func<ParameterInfo, AIFunctionFactoryOptions.ParameterBindingOptions>? GetBindParameterOptions,
             Func<object?, Type?, CancellationToken, ValueTask<object?>>? MarshalResult,
+            bool ExcludeResultSchema,
             AIJsonSchemaCreateOptions SchemaOptions);
     }
 

src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactoryOptions.cs

@@ -106,6 +106,19 @@ public AIFunctionFactoryOptions()
     /// </remarks>
     public Func<object?, Type?, CancellationToken, ValueTask<object?>>? MarshalResult { get; set; }
 
+    /// <summary>
+    /// Gets or sets a value indicating whether a schema should be created for the function's result type, if possible, and included as <see cref="AIFunction.ReturnJsonSchema" />.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The default value is <see langword="false"/>.
+    /// </para>
+    /// <para>
+    /// When set to <see langword="true"/>, results in the produced <see cref="AIFunction.ReturnJsonSchema"/> to always be <see langword="null"/>.
+    /// </para>
+    /// </remarks>
+    public bool ExcludeResultSchema { get; set; }
+
     /// <summary>Provides configuration options produced by the <see cref="ConfigureParameterBinding"/> delegate.</summary>
     public readonly record struct ParameterBindingOptions
     {

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

@@ -303,6 +303,10 @@
           "Member": "string? Microsoft.Extensions.AI.AIFunctionFactoryOptions.Description { get; set; }",
           "Stage": "Stable"
         },
+        {
+          "Member": "bool Microsoft.Extensions.AI.AIFunctionFactoryOptions.ExcludeResultSchema { get; set; }",
+          "Stage": "Stable"
+        },
         {
           "Member": "Microsoft.Extensions.AI.AIJsonSchemaCreateOptions? Microsoft.Extensions.AI.AIFunctionFactoryOptions.JsonSchemaCreateOptions { get; set; }",
           "Stage": "Stable"

test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs

@@ -274,6 +274,7 @@ public void AIFunctionFactoryOptions_DefaultValues()
         Assert.Null(options.SerializerOptions);
         Assert.Null(options.JsonSchemaCreateOptions);
         Assert.Null(options.ConfigureParameterBinding);
+        Assert.False(options.ExcludeResultSchema);
     }
 
     [Fact]
@@ -300,6 +301,21 @@ public async Task AIFunctionFactoryOptions_SupportsSkippingParameters()
         Assert.Contains("test42", result.ToString());
     }
 
+    [Fact]
+    public void AIFunctionFactoryOptions_SupportsSkippingReturnSchema()
+    {
+        AIFunction func = AIFunctionFactory.Create(
+            (string firstParameter, int secondParameter) => firstParameter + secondParameter,
+            new()
+            {
+                ExcludeResultSchema = true,
+            });
+
+        Assert.Contains("firstParameter", func.JsonSchema.ToString());
+        Assert.Contains("secondParameter", func.JsonSchema.ToString());
+        Assert.Null(func.ReturnJsonSchema);
+    }
+
     [Fact]
     public async Task AIFunctionArguments_SatisfiesParameters()
     {