Make a number of improvements to the OpenAI serialization helpers.

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

@@ -3,6 +3,7 @@
 
 using System;
 using System.ClientModel.Primitives;
+using System.Text.Json;
 
 namespace Microsoft.Extensions.AI;
 
@@ -23,6 +24,12 @@ public static TModel Deserialize<TModel>(BinaryData data)
         return JsonModelDeserializationWitness<TModel>.Value.Create(data, ModelReaderWriterOptions.Json);
     }
 
+    public static TModel Deserialize<TModel>(ref Utf8JsonReader reader)
+        where TModel : IJsonModel<TModel>, new()
+    {
+        return JsonModelDeserializationWitness<TModel>.Value.Create(ref reader, ModelReaderWriterOptions.Json);
+    }
+
     private sealed class JsonModelDeserializationWitness<TModel>
         where TModel : IJsonModel<TModel>, new()
     {

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

@@ -1,13 +1,21 @@
 // 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 System.ComponentModel;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using OpenAI.Chat;
+
+#pragma warning disable CA1034 // Nested types should not be visible
 
 namespace Microsoft.Extensions.AI;
 
 /// <summary>
 /// Represents an OpenAI chat completion request deserialized as Microsoft.Extension.AI models.
 /// </summary>
+[JsonConverter(typeof(Converter))]
 public sealed class OpenAIChatCompletionRequest
 {
     /// <summary>
@@ -29,4 +37,33 @@ public sealed class OpenAIChatCompletionRequest
     /// Gets the model id requested by the chat completion.
     /// </summary>
     public string? ModelId { get; init; }
+
+    /// <summary>
+    /// Converts an OpenAIChatCompletionRequest object to and from JSON.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public sealed class Converter : JsonConverter<OpenAIChatCompletionRequest>
+    {
+        /// <summary>
+        /// Reads and converts the JSON to type OpenAIChatCompletionRequest.
+        /// </summary>
+        /// <param name="reader">The reader.</param>
+        /// <param name="typeToConvert">The type to convert.</param>
+        /// <param name="options">The serializer options.</param>
+        /// <returns>The converted OpenAIChatCompletionRequest object.</returns>
+        public override OpenAIChatCompletionRequest? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            ChatCompletionOptions chatCompletionOptions = JsonModelHelpers.Deserialize<ChatCompletionOptions>(ref reader);
+            return OpenAIModelMappers.FromOpenAIChatCompletionRequest(chatCompletionOptions);
+        }
+
+        /// <summary>
+        /// Writes the specified value as JSON.
+        /// </summary>
+        /// <param name="writer">The writer.</param>
+        /// <param name="value">The value to write.</param>
+        /// <param name="options">The serializer options.</param>
+        public override void Write(Utf8JsonWriter writer, OpenAIChatCompletionRequest value, JsonSerializerOptions options) =>
+            throw new NotSupportedException("Request body serialization is not supported.");
+    }
 }

src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIModelMapper.ChatCompletion.cs

@@ -17,6 +17,7 @@
 #pragma warning disable S103 // Lines should not be too long
 #pragma warning disable CA1859 // Use concrete types when possible for improved performance
 #pragma warning disable S1067 // Expressions should not be too complex
+#pragma warning disable S3440 // Variables should not be checked against the values they're about to be assigned
 
 namespace Microsoft.Extensions.AI;
 
@@ -57,7 +58,7 @@ public static OpenAI.Chat.ChatCompletion ToOpenAIChatCompletion(ChatCompletion c
         return OpenAIChatModelFactory.ChatCompletion(
             id: chatCompletion.CompletionId ?? CreateCompletionId(),
             model: chatCompletion.ModelId,
-            createdAt: chatCompletion.CreatedAt ?? default,
+            createdAt: chatCompletion.CreatedAt ?? DateTimeOffset.UtcNow,
             role: ToOpenAIChatRole(chatCompletion.Message.Role).Value,
             finishReason: ToOpenAIFinishReason(chatCompletion.FinishReason),
             content: new(ToOpenAIChatContent(chatCompletion.Message.Contents)),
@@ -148,7 +149,12 @@ public static ChatOptions FromOpenAIOptions(OpenAI.Chat.ChatCompletionOptions? o
 
         if (options is not null)
         {
-            result.ModelId = _getModelIdAccessor.Invoke(options, null)?.ToString();
+            result.ModelId = _getModelIdAccessor.Invoke(options, null)?.ToString() switch
+            {
+                null or "" => null,
+                var modelId => modelId,
+            };
+
             result.FrequencyPenalty = options.FrequencyPenalty;
             result.MaxOutputTokens = options.MaxOutputTokenCount;
             result.TopP = options.TopP;

src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIModelMappers.StreamingChatCompletion.cs

@@ -48,9 +48,9 @@ internal static partial class OpenAIModelMappers
             yield return OpenAIChatModelFactory.StreamingChatCompletionUpdate(
                 completionId: chatCompletionUpdate.CompletionId ?? CreateCompletionId(),
                 model: chatCompletionUpdate.ModelId,
-                createdAt: chatCompletionUpdate.CreatedAt ?? default,
+                createdAt: chatCompletionUpdate.CreatedAt ?? DateTimeOffset.UtcNow,
                 role: ToOpenAIChatRole(chatCompletionUpdate.Role),
-                finishReason: ToOpenAIFinishReason(chatCompletionUpdate.FinishReason),
+                finishReason: chatCompletionUpdate.FinishReason is null ? null : ToOpenAIFinishReason(chatCompletionUpdate.FinishReason),
                 contentUpdate: [.. ToOpenAIChatContent(chatCompletionUpdate.Contents)],
                 toolCallUpdates: toolCallUpdates,
                 refusalUpdate: chatCompletionUpdate.AdditionalProperties.GetValueOrDefault<string>(nameof(OpenAI.Chat.StreamingChatCompletionUpdate.RefusalUpdate)),

test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAISerializationTests.cs

@@ -85,6 +85,38 @@ public static async Task RequestDeserialization_SimpleMessage_Stream()
         Assert.Null(textContent.AdditionalProperties);
     }
 
+    [Fact]
+    public static void RequestDeserialization_SimpleMessage_JsonSerializer()
+    {
+        const string RequestJson = """
+            {"messages":[{"role":"user","content":"hello"}],"model":"gpt-4o-mini","max_completion_tokens":20,"stream":true,"stream_options":{"include_usage":true},"temperature":0.5}
+            """;
+
+        OpenAIChatCompletionRequest? request = JsonSerializer.Deserialize<OpenAIChatCompletionRequest>(RequestJson);
+
+        Assert.NotNull(request);
+        Assert.True(request.Stream);
+        Assert.Equal("gpt-4o-mini", request.ModelId);
+
+        Assert.NotNull(request.Options);
+        Assert.Equal("gpt-4o-mini", request.Options.ModelId);
+        Assert.Equal(0.5f, request.Options.Temperature);
+        Assert.Equal(20, request.Options.MaxOutputTokens);
+        Assert.Null(request.Options.TopK);
+        Assert.Null(request.Options.TopP);
+        Assert.Null(request.Options.StopSequences);
+        Assert.Null(request.Options.AdditionalProperties);
+        Assert.Null(request.Options.Tools);
+
+        ChatMessage message = Assert.Single(request.Messages);
+        Assert.Equal(ChatRole.User, message.Role);
+        AIContent content = Assert.Single(message.Contents);
+        TextContent textContent = Assert.IsType<TextContent>(content);
+        Assert.Equal("hello", textContent.Text);
+        Assert.Null(textContent.RawRepresentation);
+        Assert.Null(textContent.AdditionalProperties);
+    }
+
     [Fact]
     public static async Task RequestDeserialization_MultipleMessages()
     {
@@ -614,13 +646,13 @@ static async IAsyncEnumerable<StreamingChatCompletionUpdate> CreateStreamingComp
         string result = Encoding.UTF8.GetString(stream.ToArray());
 
         AssertSseEqual("""
-            data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 0","tool_calls":[],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"finish_reason":"stop","index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk"}
+            data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 0","tool_calls":[],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk"}
 
-            data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 1","tool_calls":[],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"finish_reason":"stop","index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk"}
+            data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 1","tool_calls":[],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk"}
 
-            data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 2","tool_calls":[{"index":0,"id":"callId","type":"function","function":{"name":"MyCoolFunc","arguments":"{\r\n  \u0022arg1\u0022: 42,\r\n  \u0022arg2\u0022: \u0022str\u0022\r\n}"}}],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"finish_reason":"stop","index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk"}
+            data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 2","tool_calls":[{"index":0,"id":"callId","type":"function","function":{"name":"MyCoolFunc","arguments":"{\r\n  \u0022arg1\u0022: 42,\r\n  \u0022arg2\u0022: \u0022str\u0022\r\n}"}}],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk"}
 
-            data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 3","tool_calls":[],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"finish_reason":"stop","index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk"}
+            data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 3","tool_calls":[],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk"}
 
             data: {"id":"chatcmpl-ADymNiWWeqCJqHNFXiI1QtRcLuXcl","choices":[{"delta":{"content":"Streaming update 4","tool_calls":[],"role":"assistant"},"logprobs":{"content":[],"refusal":[]},"finish_reason":"stop","index":0}],"created":1727888631,"model":"gpt-4o-mini-2024-07-18","system_fingerprint":"fp_f85bea6784","object":"chat.completion.chunk","usage":{"completion_tokens":9,"prompt_tokens":8,"total_tokens":17,"completion_tokens_details":{"audio_tokens":2,"reasoning_tokens":90},"prompt_tokens_details":{"audio_tokens":1,"cached_tokens":13}}}