dotnet
diff --git a/‎src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.json‎
Lines changed: 32 additions & 0 deletions b/‎src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.json‎
Lines changed: 32 additions & 0 deletions
diff --git a/‎src/Libraries/Microsoft.Extensions.AI.Abstractions/Tools/HostedToolSearchTool.cs‎
Lines changed: 77 additions & 0 deletions b/‎src/Libraries/Microsoft.Extensions.AI.Abstractions/Tools/HostedToolSearchTool.cs‎
Lines changed: 77 additions & 0 deletions
diff --git a/‎src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs‎
Lines changed: 171 additions & 3 deletions b/‎src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs‎
Lines changed: 171 additions & 3 deletions
diff --git a/‎src/Shared/DiagnosticIds/DiagnosticIds.cs‎
Lines changed: 1 addition & 0 deletions b/‎src/Shared/DiagnosticIds/DiagnosticIds.cs‎
Lines changed: 1 addition & 0 deletions
@@ -2687,6 +2687,38 @@
         }
       ]
     },
+    {
+      "Type": "class Microsoft.Extensions.AI.HostedToolSearchTool : Microsoft.Extensions.AI.AITool",
+      "Stage": "Experimental",
+      "Methods": [
+        {
+          "Member": "Microsoft.Extensions.AI.HostedToolSearchTool.HostedToolSearchTool();",
+          "Stage": "Experimental"
+        },
+        {
+          "Member": "Microsoft.Extensions.AI.HostedToolSearchTool.HostedToolSearchTool(System.Collections.Generic.IReadOnlyDictionary<string, object?>? additionalProperties);",
+          "Stage": "Experimental"
+        }
+      ],
+      "Properties": [
+        {
+          "Member": "override System.Collections.Generic.IReadOnlyDictionary<string, object?> Microsoft.Extensions.AI.HostedToolSearchTool.AdditionalProperties { get; }",
+          "Stage": "Experimental"
+        },
+        {
+          "Member": "System.Collections.Generic.IList<string>? Microsoft.Extensions.AI.HostedToolSearchTool.DeferredTools { get; set; }",
+          "Stage": "Experimental"
+        },
+        {
+          "Member": "override string Microsoft.Extensions.AI.HostedToolSearchTool.Name { get; }",
+          "Stage": "Experimental"
+        },
+        {
+          "Member": "string? Microsoft.Extensions.AI.HostedToolSearchTool.Namespace { get; set; }",
+          "Stage": "Experimental"
+        }
+      ]
+    },
     {
       "Type": "sealed class Microsoft.Extensions.AI.HostedVectorStoreContent : Microsoft.Extensions.AI.AIContent",
       "Stage": "Stable",
 
@@ -0,0 +1,77 @@
+// 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>
+/// <para>
+/// 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. When included, deferred tools are not placed into the model's context
+/// upfront; instead, the model invokes tool search to surface relevant tools on demand, reducing the input
+/// tokens consumed by tool definitions the model doesn't need.
+/// </para>
+/// <para>
+/// By default, when a <see cref="HostedToolSearchTool"/> is present in the tools list, all other deferrable tools
+/// are treated as having deferred loading enabled. Use <see cref="DeferredTools"/> to control which tools have deferred loading
+/// on a per-tool basis.
+/// </para>
+/// </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;
+
+    /// <summary>
+    /// Gets or sets the list of tool names for which deferred loading should be enabled.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The default value is <see langword="null"/>, which enables deferred loading for all deferrable tools in the tools list.
+    /// </para>
+    /// <para>
+    /// When non-null, only deferrable tools whose names appear in this list will have deferred loading enabled.
+    /// </para>
+    /// </remarks>
+    public IList<string>? DeferredTools { get; set; }
+
+    /// <summary>
+    /// Gets or sets the namespace name under which deferred tools should be grouped.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// When non-null, all deferred tools are wrapped inside a <c>{"type":"namespace","name":"..."}</c>
+    /// container. Non-deferred tools remain as top-level tools.
+    /// </para>
+    /// <para>
+    /// When <see langword="null"/> (the default), deferred tools are sent as top-level tools
+    /// with <c>defer_loading</c> set individually.
+    /// </para>
+    /// </remarks>
+    public string? Namespace { get; set; }
+}
@@ -17,6 +17,7 @@
 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
@@ -701,15 +702,26 @@ private static bool IsStoredOutputDisabled(CreateResponseOptions? options, Respo
         (response is not null && response.Patch.TryGetValue("$.store"u8, out bool store) && !store);
 #pragma warning restore SCME0001
 
-    internal static ResponseTool? ToResponseTool(AITool tool, ChatOptions? options = null)
+    internal static ResponseTool? ToResponseTool(AITool tool, ChatOptions? options = null, ToolSearchLookup? toolSearchLookup = null)
     {
         switch (tool)
         {
             case ResponseToolAITool rtat:
                 return rtat.Tool;
 
             case AIFunctionDeclaration aiFunction:
-                return ToResponseTool(aiFunction, options);
+                var functionTool = ToResponseTool(aiFunction, options);
+                if ((toolSearchLookup ??= ToolSearchLookup.Create(options?.Tools)).IsDeferred(aiFunction.Name))
+                {
+                    functionTool.Patch.Set("$.defer_loading"u8, "true"u8);
+                }
+
+                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
@@ -821,6 +833,11 @@ private static bool IsStoredOutputDisabled(CreateResponseOptions? options, Respo
                         break;
                 }
 
+                if ((toolSearchLookup ??= ToolSearchLookup.Create(options?.Tools)).IsDeferred(mcpTool.ServerName))
+                {
+                    responsesMcpTool.Patch.Set("$.defer_loading"u8, "true"u8);
+                }
+
                 return responsesMcpTool;
 
             default:
@@ -843,6 +860,34 @@ internal static FunctionTool ToResponseTool(AIFunctionDeclaration aiFunction, Ch
         };
     }
 
+    /// <summary>
+    /// Builds a <c>{"type":"namespace"}</c> <see cref="ResponseTool"/> from a name and set of tools.
+    /// The OpenAI .NET SDK doesn't expose a NamespaceTool type, so we construct the JSON manually.
+    /// </summary>
+    internal static ResponseTool ToNamespaceResponseTool(string name, IEnumerable<ResponseTool> namespacedTools)
+    {
+        using var stream = new System.IO.MemoryStream();
+        using (var writer = new Utf8JsonWriter(stream))
+        {
+            writer.WriteStartObject();
+            writer.WriteString("type"u8, "namespace"u8);
+            writer.WriteString("name"u8, name);
+
+            writer.WriteStartArray("tools"u8);
+            foreach (var namespacedTool in namespacedTools)
+            {
+                var toolData = ModelReaderWriter.Write(namespacedTool, ModelReaderWriterOptions.Json, OpenAIContext.Default);
+                using var doc = JsonDocument.Parse(toolData);
+                doc.RootElement.WriteTo(writer);
+            }
+
+            writer.WriteEndArray();
+            writer.WriteEndObject();
+        }
+
+        return ModelReaderWriter.Read<ResponseTool>(BinaryData.FromBytes(stream.ToArray()), ModelReaderWriterOptions.Json, OpenAIContext.Default)!;
+    }
+
     /// <summary>Creates a <see cref="ChatRole"/> from a <see cref="MessageRole"/>.</summary>
     private static ChatRole AsChatRole(MessageRole? role) =>
         role switch
@@ -926,14 +971,45 @@ private CreateResponseOptions AsCreateResponseOptions(ChatOptions? options, out
         // Populate tools if there are any.
         if (options.Tools is { Count: > 0 } tools)
         {
+            ToolSearchLookup toolSearchLookup = ToolSearchLookup.Create(tools);
+            Dictionary<string, List<ResponseTool>>? namespaceGroups = null;
+
             foreach (AITool tool in tools)
             {
-                if (ToResponseTool(tool, options) is { } responseTool)
+                if (ToResponseTool(tool, options, toolSearchLookup) is { } responseTool)
                 {
+                    // When a namespaced HostedToolSearchTool claims this deferred tool,
+                    // collect it for later wrapping in a namespace container.
+                    string? responseToolName = responseTool is FunctionTool ft ? ft.FunctionName
+                        : responseTool is McpTool mcp ? mcp.ServerLabel
+                        : null;
+
+                    if (responseToolName is not null
+                        && toolSearchLookup.GetNamespace(responseToolName) is { } ns)
+                    {
+                        namespaceGroups ??= new(StringComparer.Ordinal);
+                        if (!namespaceGroups.TryGetValue(ns, out var group))
+                        {
+                            group = new();
+                            namespaceGroups[ns] = group;
+                        }
+
+                        group.Add(responseTool);
+                        continue;
+                    }
+
                     result.Tools.Add(responseTool);
                 }
             }
 
+            if (namespaceGroups is not null)
+            {
+                foreach (KeyValuePair<string, List<ResponseTool>> kvp in namespaceGroups)
+                {
+                    result.Tools.Add(ToNamespaceResponseTool(kvp.Key, kvp.Value));
+                }
+            }
+
             if (result.Tools.Count > 0)
             {
                 result.ParallelToolCallsEnabled ??= options.AllowMultipleToolCalls;
@@ -969,6 +1045,98 @@ private CreateResponseOptions AsCreateResponseOptions(ChatOptions? options, out
         return result;
     }
 
+    internal sealed class ToolSearchLookup
+    {
+        private static readonly ToolSearchLookup _empty = new(deferAll: false, deferredToolNames: [], namespacedToolNames: []);
+        private readonly bool _deferAll;
+        private readonly HashSet<string> _deferredToolNames;
+        private readonly Dictionary<string, string> _namespacedToolNames;
+
+        private ToolSearchLookup(bool deferAll, HashSet<string> deferredToolNames, Dictionary<string, string> namespacedToolNames)
+        {
+            _deferAll = deferAll;
+            _deferredToolNames = deferredToolNames;
+            _namespacedToolNames = namespacedToolNames;
+        }
+
+        public static ToolSearchLookup Create(IList<AITool>? tools)
+        {
+            if (tools is not { Count: > 0 })
+            {
+                return _empty;
+            }
+
+            HashSet<string> functionAndMcpToolNames = new(
+                tools.Select(
+                    static tool => tool switch
+                    {
+                        AIFunctionDeclaration aiFunction => aiFunction.Name,
+                        HostedMcpServerTool mcpTool => mcpTool.ServerName,
+                        _ => null,
+                    })
+                .OfType<string>(),
+                StringComparer.Ordinal);
+
+            if (functionAndMcpToolNames.Count == 0)
+            {
+                return _empty;
+            }
+
+            bool deferAll = false;
+            HashSet<string> deferredToolNames = new(StringComparer.Ordinal);
+            Dictionary<string, string> namespacedToolNames = new(StringComparer.Ordinal);
+            HashSet<string> unclaimedToolNames = new(functionAndMcpToolNames, StringComparer.Ordinal);
+
+            foreach (AITool tool in tools)
+            {
+                if (tool is not HostedToolSearchTool toolSearch)
+                {
+                    continue;
+                }
+
+                if (toolSearch.DeferredTools is not { } deferredTools)
+                {
+                    deferAll = true;
+                    deferredToolNames.UnionWith(functionAndMcpToolNames);
+
+                    if (toolSearch.Namespace is { } ns && unclaimedToolNames.Count > 0)
+                    {
+                        foreach (string toolName in unclaimedToolNames)
+                        {
+                            namespacedToolNames[toolName] = ns;
+                        }
+
+                        unclaimedToolNames.Clear();
+                    }
+
+                    continue;
+                }
+
+                foreach (string deferredTool in deferredTools)
+                {
+                    if (!functionAndMcpToolNames.Contains(deferredTool))
+                    {
+                        continue;
+                    }
+
+                    _ = deferredToolNames.Add(deferredTool);
+                    if (toolSearch.Namespace is { } ns && unclaimedToolNames.Remove(deferredTool))
+                    {
+                        namespacedToolNames[deferredTool] = ns;
+                    }
+                }
+            }
+
+            return new(deferAll, deferredToolNames, namespacedToolNames);
+        }
+
+        public bool IsDeferred(string toolName) =>
+            _deferAll || _deferredToolNames.Contains(toolName);
+
+        public string? GetNamespace(string toolName) =>
+            _namespacedToolNames.TryGetValue(toolName, out string? ns) ? ns : null;
+    }
+
     internal static ResponseTextFormat? ToOpenAIResponseTextFormat(ChatResponseFormat? format, ChatOptions? options = null) =>
         format switch
         {
 
@@ -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;