diff --git a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs
index d9f43069490..24387aceb62 100644
--- a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs
@@ -582,22 +582,25 @@ private static ChatCompletionOptions ToOpenAIOptions(ChatOptions? options)
                     }
                 }
 
-                switch (options.ToolMode)
+                if (result.Tools.Count > 0)
                 {
-                    case NoneChatToolMode:
-                        result.ToolChoice = ChatToolChoice.CreateNoneChoice();
-                        break;
-
-                    case AutoChatToolMode:
-                    case null:
-                        result.ToolChoice = ChatToolChoice.CreateAutoChoice();
-                        break;
-
-                    case RequiredChatToolMode required:
-                        result.ToolChoice = required.RequiredFunctionName is null ?
-                            ChatToolChoice.CreateRequiredChoice() :
-                            ChatToolChoice.CreateFunctionChoice(required.RequiredFunctionName);
-                        break;
+                    switch (options.ToolMode)
+                    {
+                        case NoneChatToolMode:
+                            result.ToolChoice = ChatToolChoice.CreateNoneChoice();
+                            break;
+
+                        case AutoChatToolMode:
+                        case null:
+                            result.ToolChoice = ChatToolChoice.CreateAutoChoice();
+                            break;
+
+                        case RequiredChatToolMode required:
+                            result.ToolChoice = required.RequiredFunctionName is null ?
+                                ChatToolChoice.CreateRequiredChoice() :
+                                ChatToolChoice.CreateFunctionChoice(required.RequiredFunctionName);
+                            break;
+                    }
                 }
             }
 
diff --git a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
index 78dc920f8cb..2d7ec89b149 100644
--- a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
@@ -773,6 +773,91 @@ public async Task FunctionCallContent_NonStreaming()
         Assert.Equal("fp_f85bea6784", response.AdditionalProperties[nameof(ChatCompletion.SystemFingerprint)]);
     }
 
+    [Fact]
+    public async Task UnavailableBuiltInFunctionCall_NonStreaming()
+    {
+        const string Input = """
+            {
+                "messages": [
+                    {
+                        "role": "user",
+                        "content": "What day is it?"
+                    }
+                ],
+                "model": "gpt-4o-mini"
+            }
+            """;
+
+        const string Output = """
+            {
+              "id": "chatcmpl-ADydKhrSKEBWJ8gy0KCIU74rN3Hmk",
+              "object": "chat.completion",
+              "created": 1727894702,
+              "model": "gpt-4o-mini-2024-07-18",
+              "choices": [
+                {
+                  "index": 0,
+                  "message": {
+                    "role": "assistant",
+                    "content": "December 31, 2023",
+                    "refusal": null
+                  },
+                  "logprobs": null,
+                  "finish_reason": "stop"
+                }
+              ],
+              "usage": {
+                "prompt_tokens": 61,
+                "completion_tokens": 16,
+                "total_tokens": 77,
+                "prompt_tokens_details": {
+                  "cached_tokens": 13
+                },
+                "completion_tokens_details": {
+                  "reasoning_tokens": 90
+                }
+              },
+              "system_fingerprint": "fp_f85bea6784"
+            }
+            """;
+
+        using VerbatimHttpHandler handler = new(Input, Output);
+        using HttpClient httpClient = new(handler);
+        using IChatClient client = CreateChatClient(httpClient, "gpt-4o-mini");
+
+        var response = await client.GetResponseAsync("What day is it?", new()
+        {
+            Tools = [new HostedWebSearchTool()],
+        });
+        Assert.NotNull(response);
+
+        Assert.Equal("December 31, 2023", response.Text);
+        Assert.Equal("gpt-4o-mini-2024-07-18", response.ModelId);
+        Assert.Equal(ChatRole.Assistant, response.Messages.Single().Role);
+        Assert.Equal(DateTimeOffset.FromUnixTimeSeconds(1_727_894_702), response.CreatedAt);
+        Assert.Equal(ChatFinishReason.Stop, response.FinishReason);
+        Assert.NotNull(response.Usage);
+        Assert.Equal(61, response.Usage.InputTokenCount);
+        Assert.Equal(16, response.Usage.OutputTokenCount);
+        Assert.Equal(77, response.Usage.TotalTokenCount);
+
+        Assert.Equal(new Dictionary<string, long>
+        {
+            { "InputTokenDetails.AudioTokenCount", 0 },
+            { "InputTokenDetails.CachedTokenCount", 13 },
+            { "OutputTokenDetails.ReasoningTokenCount", 90 },
+            { "OutputTokenDetails.AudioTokenCount", 0 },
+            { "OutputTokenDetails.AcceptedPredictionTokenCount", 0 },
+            { "OutputTokenDetails.RejectedPredictionTokenCount", 0 },
+        }, response.Usage.AdditionalCounts);
+
+        Assert.Single(response.Messages.Single().Contents);
+        TextContent fcc = Assert.IsType<TextContent>(response.Messages.Single().Contents[0]);
+
+        Assert.NotNull(response.AdditionalProperties);
+        Assert.Equal("fp_f85bea6784", response.AdditionalProperties[nameof(ChatCompletion.SystemFingerprint)]);
+    }
+
     [Fact]
     public async Task FunctionCallContent_Streaming()
     {