Workaround OpenAI assistant's RunCreationOption's tools override (#6512)

In OpenAIAssistantChatClient, as part of providing ChatOptions.Tools to the service, the only API available to do so is ToolsOverride, which ends up replacing rather than augmenting any tools defined at the assistant level. To work around that, this changes the implementation to retrieve the assistant's tools and send them as part of the override, effectively implementing a merge rather than override.

Author: stephentoub

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

@@ -44,6 +44,9 @@ internal sealed partial class OpenAIAssistantChatClient : IChatClient
     /// <summary>The thread ID to use if none is supplied in <see cref="ChatOptions.ConversationId"/>.</summary>
     private readonly string? _defaultThreadId;
 
+    /// <summary>List of tools associated with the assistant.</summary>
+    private IReadOnlyList<ToolDefinition>? _assistantTools;
+
     /// <summary>Initializes a new instance of the <see cref="OpenAIAssistantChatClient"/> class for the specified <see cref="AssistantClient"/>.</summary>
     public OpenAIAssistantChatClient(AssistantClient assistantClient, string assistantId, string? defaultThreadId)
     {
@@ -83,7 +86,7 @@ public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
         _ = Throw.IfNull(messages);
 
         // Extract necessary state from messages and options.
-        (RunCreationOptions runOptions, List<FunctionResultContent>? toolResults) = CreateRunOptions(messages, options);
+        (RunCreationOptions runOptions, List<FunctionResultContent>? toolResults) = await CreateRunOptionsAsync(messages, options, cancellationToken).ConfigureAwait(false);
 
         // Get the thread ID.
         string? threadId = options?.ConversationId ?? _defaultThreadId;
@@ -238,8 +241,8 @@ void IDisposable.Dispose()
     /// Creates the <see cref="RunCreationOptions"/> to use for the request and extracts any function result contents 
     /// that need to be submitted as tool results.
     /// </summary>
-    private (RunCreationOptions RunOptions, List<FunctionResultContent>? ToolResults) CreateRunOptions(
-        IEnumerable<ChatMessage> messages, ChatOptions? options)
+    private async ValueTask<(RunCreationOptions RunOptions, List<FunctionResultContent>? ToolResults)> CreateRunOptionsAsync(
+        IEnumerable<ChatMessage> messages, ChatOptions? options, CancellationToken cancellationToken)
     {
         // Create the options instance to populate, either a fresh or using one the caller provides.
         RunCreationOptions runOptions =
@@ -257,6 +260,24 @@ void IDisposable.Dispose()
 
             if (options.Tools is { Count: > 0 } tools)
             {
+                // If the caller has provided any tool overrides, we'll assume they don't want to use the assistant's tools.
+                // But if they haven't, the only way we can provide our tools is via an override, whereas we'd really like to
+                // just add them. To handle that, we'll get all of the assistant's tools and add them to the override list
+                // along with our tools.
+                if (runOptions.ToolsOverride.Count == 0)
+                {
+                    if (_assistantTools is null)
+                    {
+                        var assistant = await _client.GetAssistantAsync(_assistantId, cancellationToken).ConfigureAwait(false);
+                        _assistantTools = assistant.Value.Tools;
+                    }
+
+                    foreach (var tool in _assistantTools)
+                    {
+                        runOptions.ToolsOverride.Add(tool);
+                    }
+                }
+
                 // The caller can provide tools in the supplied ThreadAndRunOptions. Augment it with any supplied via ChatOptions.Tools.
                 foreach (AITool tool in tools)
                 {
@@ -290,7 +311,6 @@ void IDisposable.Dispose()
                         runOptions.ToolConstraint = ToolConstraint.None;
                         break;
 
-                    case null:
                     case AutoChatToolMode:
                         runOptions.ToolConstraint = ToolConstraint.Auto;
                         break;