Add HostedToolSearchTool and SearchableAIFunctionDeclaration for tool search / deferred loading support

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

@@ -3,15 +3,18 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
+using Microsoft.Shared.DiagnosticIds;
 using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Extensions.AI;
 
 /// <summary>
 /// Provides an optional base class for an <see cref="AIFunctionDeclaration"/> that passes through calls to another instance.
 /// </summary>
-internal class DelegatingAIFunctionDeclaration : AIFunctionDeclaration // could be made public in the future if there's demand
+[Experimental(DiagnosticIds.Experiments.AIToolSearch, UrlFormat = DiagnosticIds.UrlFormat)]
+public class DelegatingAIFunctionDeclaration : AIFunctionDeclaration
 {
     /// <summary>
     /// Initializes a new instance of the <see cref="DelegatingAIFunctionDeclaration"/> class as a wrapper around <paramref name="innerFunction"/>.

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

@@ -0,0 +1,62 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Extensions.AI;
+
+/// <summary>
+/// Represents an <see cref="AIFunctionDeclaration"/> that signals to supporting AI services that deferred
+/// loading should be used when tool search is enabled. Only the function's name and description are sent initially;
+/// the full JSON schema is loaded on demand by the service when the model selects this tool.
+/// </summary>
+/// <remarks>
+/// This class is a marker/decorator that signals to a supporting provider that the function should be
+/// sent with deferred loading (only name and description upfront). Use <see cref="CreateToolSet"/> to create
+/// a complete tool list including a <see cref="HostedToolSearchTool"/> and wrapped functions.
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AIToolSearch, UrlFormat = DiagnosticIds.UrlFormat)]
+public sealed class SearchableAIFunctionDeclaration : DelegatingAIFunctionDeclaration
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="SearchableAIFunctionDeclaration"/> class.
+    /// </summary>
+    /// <param name="innerFunction">The <see cref="AIFunctionDeclaration"/> represented by this instance.</param>
+    /// <param name="namespaceName">An optional namespace for grouping related tools in the tool search index.</param>
+    /// <exception cref="System.ArgumentNullException"><paramref name="innerFunction"/> is <see langword="null"/>.</exception>
+    public SearchableAIFunctionDeclaration(AIFunctionDeclaration innerFunction, string? namespaceName = null)
+        : base(innerFunction)
+    {
+        Namespace = namespaceName;
+    }
+
+    /// <summary>Gets the optional namespace this function belongs to, for grouping related tools in the tool search index.</summary>
+    public string? Namespace { get; }
+
+    /// <summary>
+    /// Creates a complete tool list with a <see cref="HostedToolSearchTool"/> and the given functions wrapped as <see cref="SearchableAIFunctionDeclaration"/>.
+    /// </summary>
+    /// <param name="functions">The functions to include as searchable tools.</param>
+    /// <param name="namespaceName">An optional namespace for grouping related tools.</param>
+    /// <param name="toolSearchProperties">Any additional properties to pass to the <see cref="HostedToolSearchTool"/>.</param>
+    /// <returns>A list of <see cref="AITool"/> instances ready for use in <see cref="ChatOptions.Tools"/>.</returns>
+    /// <exception cref="System.ArgumentNullException"><paramref name="functions"/> is <see langword="null"/>.</exception>
+    public static IList<AITool> CreateToolSet(
+        IEnumerable<AIFunctionDeclaration> functions,
+        string? namespaceName = null,
+        IReadOnlyDictionary<string, object?>? toolSearchProperties = null)
+    {
+        _ = Throw.IfNull(functions);
+
+        var tools = new List<AITool> { new HostedToolSearchTool(toolSearchProperties) };
+        foreach (var fn in functions)
+        {
+            tools.Add(new SearchableAIFunctionDeclaration(fn, namespaceName));
+        }
+
+        return tools;
+    }
+}

src/Libraries/Microsoft.Extensions.AI.Abstractions/Tools/HostedMcpServerTool.cs

@@ -3,6 +3,8 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
 using Microsoft.Shared.Diagnostics;
 
 namespace Microsoft.Extensions.AI;
@@ -143,4 +145,20 @@ private static string ValidateUrl(Uri serverAddress)
     /// </para>
     /// </remarks>
     public IDictionary<string, string>? Headers { get; set; }
+
+    /// <summary>
+    /// Gets or sets a value indicating whether tools from this MCP server should use deferred loading when tool search is enabled.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// When set to <see langword="true"/> and a <see cref="HostedToolSearchTool"/> is present in <see cref="ChatOptions.Tools"/>,
+    /// the provider will signal to the AI service that tools from this MCP server should be deferred — only their names and
+    /// descriptions are sent initially, and full schemas are loaded on demand when the model selects them.
+    /// </para>
+    /// <para>
+    /// The default value is <see langword="false"/>.
+    /// </para>
+    /// </remarks>
+    [Experimental(DiagnosticIds.Experiments.AIToolSearch, UrlFormat = DiagnosticIds.UrlFormat)]
+    public bool DeferLoadingTools { get; set; }
 }

src/Libraries/Microsoft.Extensions.AI.Abstractions/Tools/HostedToolSearchTool.cs

@@ -0,0 +1,38 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.Shared.DiagnosticIds;
+
+namespace Microsoft.Extensions.AI;
+
+/// <summary>Represents a hosted tool that can be specified to an AI service to enable it to search for and selectively load tool definitions on demand.</summary>
+/// <remarks>
+/// This tool does not itself implement tool search. It is a marker that can be used to inform a service
+/// that tool search should be enabled, reducing token usage by deferring full tool schema loading until the model requests it.
+/// </remarks>
+[Experimental(DiagnosticIds.Experiments.AIToolSearch, UrlFormat = DiagnosticIds.UrlFormat)]
+public class HostedToolSearchTool : AITool
+{
+    /// <summary>Any additional properties associated with the tool.</summary>
+    private IReadOnlyDictionary<string, object?>? _additionalProperties;
+
+    /// <summary>Initializes a new instance of the <see cref="HostedToolSearchTool"/> class.</summary>
+    public HostedToolSearchTool()
+    {
+    }
+
+    /// <summary>Initializes a new instance of the <see cref="HostedToolSearchTool"/> class.</summary>
+    /// <param name="additionalProperties">Any additional properties associated with the tool.</param>
+    public HostedToolSearchTool(IReadOnlyDictionary<string, object?>? additionalProperties)
+    {
+        _additionalProperties = additionalProperties;
+    }
+
+    /// <inheritdoc />
+    public override string Name => "tool_search";
+
+    /// <inheritdoc />
+    public override IReadOnlyDictionary<string, object?> AdditionalProperties => _additionalProperties ?? base.AdditionalProperties;
+}

src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs

@@ -18,6 +18,7 @@
 using System.Threading.Tasks;
 using Microsoft.Shared.DiagnosticIds;
 using Microsoft.Shared.Diagnostics;
+using OpenAI;
 using OpenAI.Responses;
 
 #pragma warning disable S1226 // Method parameters, caught exceptions and foreach variables' initial values should not be ignored
@@ -702,7 +703,22 @@ private static bool IsStoredOutputDisabled(CreateResponseOptions? options, Respo
                 return rtat.Tool;
 
             case AIFunctionDeclaration aiFunction:
-                return ToResponseTool(aiFunction, options);
+                var functionTool = ToResponseTool(aiFunction, options);
+                if (tool.GetService<SearchableAIFunctionDeclaration>() is { } searchable)
+                {
+                    functionTool.Patch.Set("$.defer_loading"u8, "true"u8);
+                    if (searchable.Namespace is { } ns)
+                    {
+                        functionTool.Patch.Set("$.namespace"u8, JsonSerializer.SerializeToUtf8Bytes(ns, OpenAIJsonContext.Default.String).AsSpan());
+                    }
+                }
+
+                return functionTool;
+
+            case HostedToolSearchTool:
+                // Workaround: The OpenAI .NET SDK doesn't yet expose a ToolSearchTool type.
+                // See https://github.com/openai/openai-dotnet/issues/1053
+                return ModelReaderWriter.Read<ResponseTool>(BinaryData.FromString("""{"type": "tool_search"}"""), ModelReaderWriterOptions.Json, OpenAIContext.Default)!;
 
             case HostedWebSearchTool webSearchTool:
                 return new WebSearchTool
@@ -814,6 +830,11 @@ private static bool IsStoredOutputDisabled(CreateResponseOptions? options, Respo
                         break;
                 }
 
+                if (mcpTool.DeferLoadingTools)
+                {
+                    responsesMcpTool.Patch.Set("$.defer_loading"u8, "true"u8);
+                }
+
                 return responsesMcpTool;
 
             default:

src/Shared/DiagnosticIds/DiagnosticIds.cs

@@ -58,6 +58,7 @@ internal static class Experiments
         internal const string AIResponseContinuations = AIExperiments;
         internal const string AICodeInterpreter = AIExperiments;
         internal const string AIWebSearch = AIExperiments;
+        internal const string AIToolSearch = AIExperiments;
         internal const string AIRealTime = AIExperiments;
         internal const string AIFiles = AIExperiments;
 

test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Functions/SearchableAIFunctionDeclarationTests.cs

@@ -0,0 +1,108 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using Xunit;
+
+namespace Microsoft.Extensions.AI.Functions;
+
+public class SearchableAIFunctionDeclarationTests
+{
+    [Fact]
+    public void Constructor_NullFunction_ThrowsArgumentNullException()
+    {
+        Assert.Throws<ArgumentNullException>("innerFunction", () => new SearchableAIFunctionDeclaration(null!));
+    }
+
+    [Fact]
+    public void Constructor_DelegatesToInnerFunction_Properties()
+    {
+        var inner = AIFunctionFactory.Create(() => 42, "MyFunc", "My description");
+        var wrapper = new SearchableAIFunctionDeclaration(inner);
+
+        Assert.Equal(inner.Name, wrapper.Name);
+        Assert.Equal(inner.Description, wrapper.Description);
+        Assert.Equal(inner.JsonSchema, wrapper.JsonSchema);
+        Assert.Equal(inner.ReturnJsonSchema, wrapper.ReturnJsonSchema);
+        Assert.Same(inner.AdditionalProperties, wrapper.AdditionalProperties);
+        Assert.Equal(inner.ToString(), wrapper.ToString());
+    }
+
+    [Fact]
+    public void Namespace_DefaultIsNull()
+    {
+        var inner = AIFunctionFactory.Create(() => 42);
+        var wrapper = new SearchableAIFunctionDeclaration(inner);
+
+        Assert.Null(wrapper.Namespace);
+    }
+
+    [Fact]
+    public void Namespace_Roundtrips()
+    {
+        var inner = AIFunctionFactory.Create(() => 42);
+        var wrapper = new SearchableAIFunctionDeclaration(inner, namespaceName: "myNamespace");
+
+        Assert.Equal("myNamespace", wrapper.Namespace);
+    }
+
+    [Fact]
+    public void GetService_ReturnsSelf()
+    {
+        var inner = AIFunctionFactory.Create(() => 42);
+        var wrapper = new SearchableAIFunctionDeclaration(inner);
+
+        Assert.Same(wrapper, wrapper.GetService<SearchableAIFunctionDeclaration>());
+    }
+
+    [Fact]
+    public void CreateToolSet_NullFunctions_Throws()
+    {
+        Assert.Throws<ArgumentNullException>("functions", () => SearchableAIFunctionDeclaration.CreateToolSet(null!));
+    }
+
+    [Fact]
+    public void CreateToolSet_ReturnsHostedToolSearchToolFirst_ThenWrappedFunctions()
+    {
+        var f1 = AIFunctionFactory.Create(() => 1, "F1");
+        var f2 = AIFunctionFactory.Create(() => 2, "F2");
+
+        var tools = SearchableAIFunctionDeclaration.CreateToolSet([f1, f2]);
+
+        Assert.Equal(3, tools.Count);
+        Assert.IsType<HostedToolSearchTool>(tools[0]);
+        Assert.Empty(tools[0].AdditionalProperties);
+
+        var s1 = Assert.IsType<SearchableAIFunctionDeclaration>(tools[1]);
+        Assert.Equal("F1", s1.Name);
+        Assert.Null(s1.Namespace);
+
+        var s2 = Assert.IsType<SearchableAIFunctionDeclaration>(tools[2]);
+        Assert.Equal("F2", s2.Name);
+        Assert.Null(s2.Namespace);
+    }
+
+    [Fact]
+    public void CreateToolSet_WithNamespace_AppliesNamespaceToAll()
+    {
+        var f1 = AIFunctionFactory.Create(() => 1, "F1");
+
+        var tools = SearchableAIFunctionDeclaration.CreateToolSet([f1], namespaceName: "ns");
+
+        var s1 = Assert.IsType<SearchableAIFunctionDeclaration>(tools[1]);
+        Assert.Equal("ns", s1.Namespace);
+    }
+
+    [Fact]
+    public void CreateToolSet_WithAdditionalProperties_PassesToHostedToolSearchTool()
+    {
+        var props = new Dictionary<string, object?> { ["key"] = "value" };
+        var f1 = AIFunctionFactory.Create(() => 1, "F1");
+
+        var tools = SearchableAIFunctionDeclaration.CreateToolSet([f1], toolSearchProperties: props);
+
+        var toolSearch = Assert.IsType<HostedToolSearchTool>(tools[0]);
+        Assert.Same(props, toolSearch.AdditionalProperties);
+    }
+}

test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Tools/HostedMcpServerToolTests.cs

@@ -24,6 +24,7 @@ public void Constructor_PropsDefault()
         Assert.Null(tool.AllowedTools);
         Assert.Null(tool.ApprovalMode);
         Assert.Null(tool.Headers);
+        Assert.False(tool.DeferLoadingTools);
     }
 
     [Fact]
@@ -96,6 +97,12 @@ public void Constructor_Roundtrips()
         Assert.NotNull(tool.Headers);
         Assert.Single(tool.Headers);
         Assert.Equal("value1", tool.Headers["X-Custom-Header"]);
+
+        Assert.False(tool.DeferLoadingTools);
+        tool.DeferLoadingTools = true;
+        Assert.True(tool.DeferLoadingTools);
+        tool.DeferLoadingTools = false;
+        Assert.False(tool.DeferLoadingTools);
     }
 
     [Fact]

test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Tools/HostedToolSearchToolTests.cs

@@ -0,0 +1,38 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using Xunit;
+
+namespace Microsoft.Extensions.AI;
+
+public class HostedToolSearchToolTests
+{
+    [Fact]
+    public void Constructor_Roundtrips()
+    {
+        var tool = new HostedToolSearchTool();
+        Assert.Equal("tool_search", tool.Name);
+        Assert.Empty(tool.Description);
+        Assert.Empty(tool.AdditionalProperties);
+        Assert.Equal(tool.Name, tool.ToString());
+    }
+
+    [Fact]
+    public void Constructor_AdditionalProperties_Roundtrips()
+    {
+        var props = new Dictionary<string, object?> { ["key"] = "value" };
+        var tool = new HostedToolSearchTool(props);
+
+        Assert.Equal("tool_search", tool.Name);
+        Assert.Same(props, tool.AdditionalProperties);
+    }
+
+    [Fact]
+    public void Constructor_NullAdditionalProperties_UsesEmpty()
+    {
+        var tool = new HostedToolSearchTool(null);
+
+        Assert.Empty(tool.AdditionalProperties);
+    }
+}