fix: Fix reasoning_content for openai compatible responses

js/src/instrumentation/plugins/openai-plugin.test.ts

@@ -300,6 +300,108 @@ describe("parseMetricsFromUsage", () => {
 });
 
 describe("aggregateChatCompletionChunks", () => {
+  describe("reasoning_content", () => {
+    it("concatenates reasoning separately from content for each choice", () => {
+      const chunks = [
+        {
+          choices: [
+            {
+              index: 1,
+              delta: { role: "assistant", reasoning_content: "Let " },
+            },
+            {
+              index: 0,
+              delta: { role: "assistant", reasoning_content: "Think " },
+            },
+          ],
+        },
+        {
+          choices: [
+            { index: 0, delta: { reasoning_content: "first.", content: "An" } },
+            {
+              index: 1,
+              delta: { reasoning_content: "me think.", content: "Other" },
+            },
+          ],
+        },
+        {
+          choices: [
+            {
+              index: 1,
+              delta: { reasoning_content: null },
+              finish_reason: "length",
+            },
+            { index: 0, delta: { content: "swer" }, finish_reason: "stop" },
+          ],
+        },
+        {
+          choices: [],
+          usage: { prompt_tokens: 2, completion_tokens: 8, total_tokens: 10 },
+        },
+      ];
+      const originalChunks = structuredClone(chunks);
+
+      expect(aggregateChatCompletionChunks(chunks)).toEqual({
+        output: [
+          {
+            index: 0,
+            message: {
+              role: "assistant",
+              content: "Answer",
+              reasoning_content: "Think first.",
+            },
+            logprobs: null,
+            finish_reason: "stop",
+          },
+          {
+            index: 1,
+            message: {
+              role: "assistant",
+              content: "Other",
+              reasoning_content: "Let me think.",
+            },
+            logprobs: null,
+            finish_reason: "length",
+          },
+        ],
+        metrics: { prompt_tokens: 2, completion_tokens: 8, tokens: 10 },
+      });
+      expect(chunks).toEqual(originalChunks);
+    });
+
+    it.each([
+      { fragments: [undefined, undefined], expected: undefined },
+      { fragments: [null, undefined, null], expected: null },
+      { fragments: ["", undefined, ""], expected: "" },
+      { fragments: [null, "", null], expected: "" },
+      {
+        fragments: [null, "Think", undefined, null, "", " more", null],
+        expected: "Think more",
+      },
+    ])(
+      "preserves reasoning fragments $fragments as $expected",
+      ({ fragments, expected }) => {
+        const chunks = fragments.map((reasoning_content) => ({
+          choices: [
+            {
+              index: 0,
+              delta:
+                reasoning_content === undefined ? {} : { reasoning_content },
+            },
+          ],
+        }));
+        const { message } = aggregateChatCompletionChunks(chunks).output[0];
+
+        if (expected === undefined) {
+          expect(message).not.toHaveProperty("reasoning_content");
+        } else {
+          expect(message).toHaveProperty("reasoning_content", expected);
+        }
+        expect(message.content).toBeUndefined();
+      },
+    );
+  });
+
   describe("basic aggregation", () => {
     it("should aggregate simple text chunks", () => {
       const chunks = [

js/src/instrumentation/plugins/openai-plugin.ts

@@ -409,6 +409,7 @@ type AggregatedChatChoice = {
   index: number;
   role: string | undefined;
   content: string | undefined;
+  reasoning_content: string | null | undefined;
   refusal: string | undefined;
   audio: NonNullable<OpenAIChatChoice["message"]["audio"]> | undefined;
   toolCallsByIndex: Map<
@@ -424,6 +425,7 @@ function createAggregatedChatChoice(index: number): AggregatedChatChoice {
     index,
     role: undefined,
     content: undefined,
+    reasoning_content: undefined,
     refusal: undefined,
     audio: undefined,
     toolCallsByIndex: new Map(),
@@ -442,6 +444,9 @@ function toChatChoice(choice: AggregatedChatChoice): OpenAIChatChoice {
     message: {
       role: choice.role,
       content: choice.content,
+      ...(choice.reasoning_content !== undefined
+        ? { reasoning_content: choice.reasoning_content }
+        : {}),
       ...(choice.refusal !== undefined ? { refusal: choice.refusal } : {}),
       ...(choice.audio !== undefined ? { audio: choice.audio } : {}),
       tool_calls: toolCalls.length > 0 ? toolCalls : undefined,
@@ -453,7 +458,7 @@ function toChatChoice(choice: AggregatedChatChoice): OpenAIChatChoice {
 
 /**
  * Aggregate chat completion chunks into a single response.
- * Combines role (first), content and audio transcripts (concatenated),
+ * Combines role (first), content, reasoning_content and audio transcripts (concatenated),
  * tool_calls (by index), finish_reason (last), and usage (last chunk).
  */
 export function aggregateChatCompletionChunks(
@@ -515,6 +520,14 @@ export function aggregateChatCompletionChunks(
           (aggregatedChoice.content || "") + delta.content;
       }
 
+      if (typeof delta.reasoning_content === "string") {
+        aggregatedChoice.reasoning_content =
+          (aggregatedChoice.reasoning_content ?? "") + delta.reasoning_content;
+      } else if (delta.reasoning_content === null) {
+        // Retain explicit nulls, but never discard text from earlier chunks.
+        aggregatedChoice.reasoning_content ??= null;
+      }
+
       if (delta.refusal) {
         aggregatedChoice.refusal =
           (aggregatedChoice.refusal || "") + delta.refusal;

js/src/vendor-sdk-types/openai-common.ts

@@ -98,6 +98,7 @@ interface OpenAIChatAudio {
 export interface OpenAIChatMessage {
   role?: string;
   content?: unknown;
+  reasoning_content?: string | null;
   refusal?: string;
   audio?: OpenAIChatAudio | null;
   tool_calls?: OpenAIChatToolCall[];
@@ -135,6 +136,7 @@ export interface OpenAIChatCompletion {
 interface OpenAIChatDelta {
   role?: string;
   content?: string;
+  reasoning_content?: string | null;
   refusal?: string;
   audio?: OpenAIChatAudio | null;
   tool_calls?: OpenAIChatToolCallDelta[];