Surface OpenAI-compatible reasoning_content as TextReasoningContent (#7295)

Author: stephentoub

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

@@ -374,6 +374,13 @@ internal static async IAsyncEnumerable<ChatResponseUpdate> FromOpenAIStreamingCh
                 ConvertContentParts(update.ContentUpdate, responseUpdate.Contents);
             }
 
+            // Check for reasoning content from OpenAI-compatible endpoints (e.g. DeepSeek, vLLM, OpenRouter)
+            // that surface it via non-standard fields in the response JSON.
+            if (TryGetReasoningDelta(update, out string? reasoningText))
+            {
+                responseUpdate.Contents.Add(new TextReasoningContent(reasoningText));
+            }
+
             if (update.OutputAudioUpdate is { } audioUpdate)
             {
                 responseUpdate.Contents.Add(new DataContent(audioUpdate.AudioBytesUpdate.ToMemory(), GetOutputAudioMimeType(options))
@@ -493,6 +500,13 @@ internal static ChatResponse FromOpenAIChatCompletion(ChatCompletion openAICompl
             }
         }
 
+        // Check for reasoning content from OpenAI-compatible endpoints (e.g. DeepSeek, vLLM, OpenRouter)
+        // that surface it via non-standard fields in the response JSON.
+        if (TryGetReasoningMessage(openAICompletion, out string? reasoningText))
+        {
+            returnMessage.Contents.Add(new TextReasoningContent(reasoningText));
+        }
+
         // Output audio is handled separately from message content parts.
         if (openAICompletion.OutputAudio is ChatOutputAudio audio)
         {
@@ -822,6 +836,16 @@ private sealed class FunctionCallInfo
         public StringBuilder? Arguments;
     }
 
+#pragma warning disable SCME0001 // JsonPatch is experimental
+    /// <summary>Tries to extract reasoning text from a streaming chat completion update's Patch.</summary>
+    private static bool TryGetReasoningDelta(StreamingChatCompletionUpdate update, [NotNullWhen(true)] out string? reasoningText)
+        => update.Patch.TryGetValue("$.choices[0].delta.reasoning_content"u8, out reasoningText) && reasoningText is not null;
+
+    /// <summary>Tries to extract reasoning text from a non-streaming chat completion's Patch.</summary>
+    private static bool TryGetReasoningMessage(ChatCompletion completion, [NotNullWhen(true)] out string? reasoningText)
+        => completion.Patch.TryGetValue("$.choices[0].message.reasoning_content"u8, out reasoningText) && reasoningText is not null;
+#pragma warning restore SCME0001
+
     private const string InvalidAuthorNamePattern = @"[^a-zA-Z0-9_]+";
 #if NET
     [GeneratedRegex(InvalidAuthorNamePattern)]

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

@@ -1907,4 +1907,107 @@ public async Task ReasoningOptions_None_ProducesNoReasoningEffortInJson()
             Reasoning = new ReasoningOptions { Effort = ReasoningEffort.None }
         }));
     }
+
+    [Fact]
+    public async Task ReasoningContent_NonStreaming_SurfacedAsTextReasoningContent()
+    {
+        const string Input = """
+            {
+                "messages":[{"role":"user","content":"hello"}],
+                "model":"gpt-oss-120b"
+            }
+            """;
+
+        const string Output = """
+            {
+              "id": "c48a440c7dd64389b7fbe908e006ba3d",
+              "object": "chat.completion",
+              "created": 1770959477,
+              "model": "gpt-oss-120b",
+              "choices": [
+                {
+                  "index": 0,
+                  "message": {
+                    "role": "assistant",
+                    "content": "9.8 is larger.",
+                    "reasoning_content": "We just compare decimals: 9.11 vs 9.8. 9.8 > 9.11. Answer briefly."
+                  },
+                  "finish_reason": "stop"
+                }
+              ],
+              "usage": {
+                "prompt_tokens": 84,
+                "completion_tokens": 44,
+                "total_tokens": 128
+              }
+            }
+            """;
+
+        using VerbatimHttpHandler handler = new(Input, Output);
+        using HttpClient httpClient = new(handler);
+        using IChatClient client = CreateChatClient(httpClient, "gpt-oss-120b");
+
+        var response = await client.GetResponseAsync("hello");
+        Assert.NotNull(response);
+
+        var message = response.Messages.Single();
+        var reasoning = message.Contents.OfType<TextReasoningContent>().Single();
+        Assert.Equal("We just compare decimals: 9.11 vs 9.8. 9.8 > 9.11. Answer briefly.", reasoning.Text);
+
+        var text = message.Contents.OfType<TextContent>().Single();
+        Assert.Equal("9.8 is larger.", text.Text);
+    }
+
+    [Fact]
+    public async Task ReasoningContent_Streaming_SurfacedAsTextReasoningContent()
+    {
+        // Streaming format captured from Azure gpt-oss-120b. Reasoning chunks have reasoning_content with content:null,
+        // then content chunks have content with reasoning_content:null.
+        const string Input = """
+            {
+                "messages":[{"role":"user","content":"hello"}],
+                "model":"gpt-oss-120b",
+                "stream":true,
+                "stream_options":{"include_usage":true}
+            }
+            """;
+
+        const string Output = """
+            data: {"id":"381fb75e8a1f451f8a579c9da104b739","object":"chat.completion.chunk","created":1770959485,"model":"gpt-oss-120b","choices":[{"index":0,"delta":{"role":"assistant","content":"","reasoning_content":null},"finish_reason":null}]}
+
+            data: {"id":"381fb75e8a1f451f8a579c9da104b739","object":"chat.completion.chunk","created":1770959485,"model":"gpt-oss-120b","choices":[{"index":0,"delta":{"content":null,"reasoning_content":"User asks"},"finish_reason":null}]}
+
+            data: {"id":"381fb75e8a1f451f8a579c9da104b739","object":"chat.completion.chunk","created":1770959485,"model":"gpt-oss-120b","choices":[{"index":0,"delta":{"content":null,"reasoning_content":": which"},"finish_reason":null}]}
+
+            data: {"id":"381fb75e8a1f451f8a579c9da104b739","object":"chat.completion.chunk","created":1770959485,"model":"gpt-oss-120b","choices":[{"index":0,"delta":{"content":null,"reasoning_content":" is larger."},"finish_reason":null}]}
+
+            data: {"id":"381fb75e8a1f451f8a579c9da104b739","object":"chat.completion.chunk","created":1770959485,"model":"gpt-oss-120b","choices":[{"index":0,"delta":{"content":"9","reasoning_content":null},"finish_reason":null}]}
+
+            data: {"id":"381fb75e8a1f451f8a579c9da104b739","object":"chat.completion.chunk","created":1770959485,"model":"gpt-oss-120b","choices":[{"index":0,"delta":{"content":".8 is larger.","reasoning_content":null},"finish_reason":null}]}
+
+            data: {"id":"381fb75e8a1f451f8a579c9da104b739","object":"chat.completion.chunk","created":1770959485,"model":"gpt-oss-120b","choices":[{"index":0,"delta":{"content":null,"reasoning_content":null},"finish_reason":"stop"}]}
+
+            data: {"id":"381fb75e8a1f451f8a579c9da104b739","object":"chat.completion.chunk","created":1770959485,"model":"gpt-oss-120b","choices":[],"usage":{"completion_tokens":46,"prompt_tokens":84,"total_tokens":130}}
+
+            data: [DONE]
+
+            """;
+
+        using VerbatimHttpHandler handler = new(Input, Output);
+        using HttpClient httpClient = new(handler);
+        using IChatClient client = CreateChatClient(httpClient, "gpt-oss-120b");
+
+        List<ChatResponseUpdate> updates = [];
+        await foreach (var update in client.GetStreamingResponseAsync("hello"))
+        {
+            updates.Add(update);
+        }
+
+        // Verify reasoning content was captured from the reasoning_content deltas
+        string reasoningText = string.Concat(updates.SelectMany(u => u.Contents).OfType<TextReasoningContent>().Select(r => r.Text));
+        Assert.Equal("User asks: which is larger.", reasoningText);
+
+        // Verify regular content was also captured from the content deltas
+        Assert.Equal("9.8 is larger.", string.Concat(updates.Select(u => u.Text)));
+    }
 }