diff --git a/src/Libraries/Microsoft.Extensions.AI.OpenAI/CHANGELOG.md b/src/Libraries/Microsoft.Extensions.AI.OpenAI/CHANGELOG.md
index a50aea8dea3..d531f3dd445 100644
--- a/src/Libraries/Microsoft.Extensions.AI.OpenAI/CHANGELOG.md
+++ b/src/Libraries/Microsoft.Extensions.AI.OpenAI/CHANGELOG.md
@@ -4,6 +4,9 @@
 
 - Added M.E.AI to OpenAI conversions for response format types.
 - Added `ResponseTool` to `AITool` conversions.
+- Fixed the handling of `HostedCodeInterpreterTool` with Responses when no file IDs were provided.
+- Fixed an issue where requests would fail when AllowMultipleToolCalls was set with no tools provided.
+- Fixed an issue where requests would fail when an AuthorName was provided containing invalid characters.
 
 ## 9.9.0-preview.1.25458.4
 
diff --git a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs
index fb0ee48a236..33f22dda420 100644
--- a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIChatClient.cs
@@ -10,6 +10,7 @@
 using System.Runtime.CompilerServices;
 using System.Text;
 using System.Text.Json;
+using System.Text.RegularExpressions;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Shared.Diagnostics;
@@ -19,14 +20,16 @@
 #pragma warning disable CA1308 // Normalize strings to uppercase
 #pragma warning disable EA0011 // Consider removing unnecessary conditional access operator (?)
 #pragma warning disable S1067 // Expressions should not be too complex
+#pragma warning disable S2333 // Unnecessary partial
 #pragma warning disable S3011 // Reflection should not be used to increase accessibility of classes, methods, or fields
 #pragma warning disable SA1202 // Elements should be ordered by access
+#pragma warning disable SA1203 // Constants should appear before fields
 #pragma warning disable SA1204 // Static elements should appear before instance elements
 
 namespace Microsoft.Extensions.AI;
 
 /// <summary>Represents an <see cref="IChatClient"/> for an OpenAI <see cref="OpenAIClient"/> or <see cref="ChatClient"/>.</summary>
-internal sealed class OpenAIChatClient : IChatClient
+internal sealed partial class OpenAIChatClient : IChatClient
 {
     // These delegate instances are used to call the internal overloads of CompleteChatAsync and CompleteChatStreamingAsync that accept
     // a RequestOptions. These should be replaced once a better way to pass RequestOptions is available.
@@ -157,10 +160,11 @@ internal static ChatTool ToOpenAIChatTool(AIFunctionDeclaration aiFunction, Chat
                 input.Role == OpenAIClientExtensions.ChatRoleDeveloper)
             {
                 var parts = ToOpenAIChatContent(input.Contents);
+                string? name = SanitizeAuthorName(input.AuthorName);
                 yield return
-                    input.Role == ChatRole.System ? new SystemChatMessage(parts) { ParticipantName = input.AuthorName } :
-                    input.Role == OpenAIClientExtensions.ChatRoleDeveloper ? new DeveloperChatMessage(parts) { ParticipantName = input.AuthorName } :
-                    new UserChatMessage(parts) { ParticipantName = input.AuthorName };
+                    input.Role == ChatRole.System ? new SystemChatMessage(parts) { ParticipantName = name } :
+                    input.Role == OpenAIClientExtensions.ChatRoleDeveloper ? new DeveloperChatMessage(parts) { ParticipantName = name } :
+                    new UserChatMessage(parts) { ParticipantName = name };
             }
             else if (input.Role == ChatRole.Tool)
             {
@@ -233,7 +237,7 @@ internal static ChatTool ToOpenAIChatTool(AIFunctionDeclaration aiFunction, Chat
                         new(ChatMessageContentPart.CreateTextPart(string.Empty));
                 }
 
-                message.ParticipantName = input.AuthorName;
+                message.ParticipantName = SanitizeAuthorName(input.AuthorName);
                 message.Refusal = refusal;
 
                 yield return message;
@@ -568,7 +572,6 @@ private ChatCompletionOptions ToOpenAIOptions(ChatOptions? options)
         result.TopP ??= options.TopP;
         result.PresencePenalty ??= options.PresencePenalty;
         result.Temperature ??= options.Temperature;
-        result.AllowParallelToolCalls ??= options.AllowMultipleToolCalls;
         result.Seed ??= options.Seed;
 
         if (options.StopSequences is { Count: > 0 } stopSequences)
@@ -589,6 +592,11 @@ private ChatCompletionOptions ToOpenAIOptions(ChatOptions? options)
                 }
             }
 
+            if (result.Tools.Count > 0)
+            {
+                result.AllowParallelToolCalls ??= options.AllowMultipleToolCalls;
+            }
+
             if (result.ToolChoice is null && result.Tools.Count > 0)
             {
                 switch (options.ToolMode)
@@ -749,6 +757,27 @@ internal static void ConvertContentParts(ChatMessageContent content, IList<AICon
             _ => new ChatFinishReason(s),
         };
 
+    /// <summary>Sanitizes the author name to be appropriate for including as an OpenAI participant name.</summary>
+    private static string? SanitizeAuthorName(string? name)
+    {
+        if (name is not null)
+        {
+            const int MaxLength = 64;
+
+            name = InvalidAuthorNameRegex().Replace(name, string.Empty);
+            if (name.Length == 0)
+            {
+                name = null;
+            }
+            else if (name.Length > MaxLength)
+            {
+                name = name.Substring(0, MaxLength);
+            }
+        }
+
+        return name;
+    }
+
     /// <summary>POCO representing function calling info. Used to concatenation information for a single function call from across multiple streaming updates.</summary>
     private sealed class FunctionCallInfo
     {
@@ -756,4 +785,13 @@ private sealed class FunctionCallInfo
         public string? Name;
         public StringBuilder? Arguments;
     }
+
+    private const string InvalidAuthorNamePattern = @"[^a-zA-Z0-9_]+";
+#if NET
+    [GeneratedRegex(InvalidAuthorNamePattern)]
+    private static partial Regex InvalidAuthorNameRegex();
+#else
+    private static Regex InvalidAuthorNameRegex() => _invalidAuthorNameRegex;
+    private static readonly Regex _invalidAuthorNameRegex = new(InvalidAuthorNamePattern, RegexOptions.Compiled);
+#endif
 }
diff --git a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs
index d094f5f8581..aa9807d16e6 100644
--- a/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs
+++ b/src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs
@@ -418,7 +418,6 @@ private ResponseCreationOptions ToOpenAIResponseCreationOptions(ChatOptions? opt
 
         // Handle strongly-typed properties.
         result.MaxOutputTokenCount ??= options.MaxOutputTokens;
-        result.ParallelToolCallsEnabled ??= options.AllowMultipleToolCalls;
         result.PreviousResponseId ??= options.ConversationId;
         result.Temperature ??= options.Temperature;
         result.TopP ??= options.TopP;
@@ -530,6 +529,11 @@ private ResponseCreationOptions ToOpenAIResponseCreationOptions(ChatOptions? opt
                 }
             }
 
+            if (result.Tools.Count > 0)
+            {
+                result.ParallelToolCallsEnabled ??= options.AllowMultipleToolCalls;
+            }
+
             if (result.ToolChoice is null && result.Tools.Count > 0)
             {
                 switch (options.ToolMode)
diff --git a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
index bdf3b2e7c0a..eca94315fa0 100644
--- a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIChatClientTests.cs
@@ -152,6 +152,7 @@ public async Task BasicRequestResponse_NonStreaming()
 
         var response = await client.GetResponseAsync("hello", new()
         {
+            AllowMultipleToolCalls = false,
             MaxOutputTokens = 10,
             Temperature = 0.5f,
         });
@@ -658,15 +659,46 @@ public async Task StronglyTypedOptions_AllSent()
     {
         const string Input = """
             {
-                "messages":[{"role":"user","content":"hello"}],
-                "model":"gpt-4o-mini",
-                "logprobs":true,
-                "top_logprobs":42,
-                "logit_bias":{"12":34},
-                "parallel_tool_calls":false,
-                "user":"12345",
-                "metadata":{"something":"else"},
-                "store":true
+                "metadata": {
+                    "something": "else"
+                },
+                "user": "12345",
+                "messages": [
+                    {
+                        "role": "user",
+                        "content": "hello"
+                    }
+                ],
+                "model": "gpt-4o-mini",
+                "top_logprobs": 42,
+                "store": true,
+                "logit_bias": {
+                    "12": 34
+                },
+                "logprobs": true,
+                "tools": [
+                    {
+                        "type": "function",
+                        "function": {
+                            "description": "",
+                            "name": "GetPersonAge",
+                            "parameters": {
+                                "type": "object",
+                                "required": [
+                                    "name"
+                                ],
+                                "properties": {
+                                    "name": {
+                                        "type": "string"
+                                    }
+                                },
+                                "additionalProperties": false
+                            }
+                        }
+                    }
+                ],
+                "tool_choice": "auto",
+                "parallel_tool_calls": false
             }
             """;
 
@@ -694,6 +726,7 @@ public async Task StronglyTypedOptions_AllSent()
         Assert.NotNull(await client.GetResponseAsync("hello", new()
         {
             AllowMultipleToolCalls = false,
+            Tools = [AIFunctionFactory.Create((string name) => 42, "GetPersonAge")],
             RawRepresentationFactory = (c) =>
             {
                 var openAIOptions = new ChatCompletionOptions
diff --git a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs
index 7724ad98360..ed00f856667 100644
--- a/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs
+++ b/test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIConversionTests.cs
@@ -159,7 +159,7 @@ public void AsOpenAIChatMessages_ProducesExpectedOutput(bool withOptions)
         List<ChatMessage> messages =
         [
             new(ChatRole.System, "You are a helpful assistant."),
-            new(ChatRole.User, "Hello"),
+            new(ChatRole.User, "Hello") { AuthorName = "Jane" },
             new(ChatRole.Assistant,
             [
                 new TextContent("Hi there!"),
@@ -168,9 +168,9 @@ public void AsOpenAIChatMessages_ProducesExpectedOutput(bool withOptions)
                     ["param1"] = "value1",
                     ["param2"] = 42
                 }),
-            ]),
+            ]) { AuthorName = "!@#$%John Smith^*)" },
             new(ChatRole.Tool, [new FunctionResultContent("callid123", "theresult")]),
-            new(ChatRole.Assistant, "The answer is 42."),
+            new(ChatRole.Assistant, "The answer is 42.") { AuthorName = "@#$#$@$" },
         ];
 
         ChatOptions? options = withOptions ? new ChatOptions { Instructions = "You talk like a parrot." } : null;
@@ -196,6 +196,7 @@ public void AsOpenAIChatMessages_ProducesExpectedOutput(bool withOptions)
 
         UserChatMessage m1 = Assert.IsType<UserChatMessage>(convertedMessages[index + 1], exactMatch: false);
         Assert.Equal("Hello", Assert.Single(m1.Content).Text);
+        Assert.Equal("Jane", m1.ParticipantName);
 
         AssistantChatMessage m2 = Assert.IsType<AssistantChatMessage>(convertedMessages[index + 2], exactMatch: false);
         Assert.Single(m2.Content);
@@ -208,6 +209,7 @@ public void AsOpenAIChatMessages_ProducesExpectedOutput(bool withOptions)
             ["param1"] = "value1",
             ["param2"] = 42
         }), JsonSerializer.Deserialize<JsonElement>(tc.FunctionArguments.ToMemory().Span)));
+        Assert.Equal("JohnSmith", m2.ParticipantName);
 
         ToolChatMessage m3 = Assert.IsType<ToolChatMessage>(convertedMessages[index + 3], exactMatch: false);
         Assert.Equal("callid123", m3.ToolCallId);
@@ -215,6 +217,7 @@ public void AsOpenAIChatMessages_ProducesExpectedOutput(bool withOptions)
 
         AssistantChatMessage m4 = Assert.IsType<AssistantChatMessage>(convertedMessages[index + 4], exactMatch: false);
         Assert.Equal("The answer is 42.", Assert.Single(m4.Content).Text);
+        Assert.Null(m4.ParticipantName);
     }
 
     [Fact]