fix: emit reasoning_content in Responses→Chat streaming translation

open-sse/translator/response/openai-responses.ts

@@ -602,10 +602,21 @@ export function openaiResponsesToOpenAIResponse(chunk, state) {
     return null;
   }
 
-  // Reasoning events (convert to content or skip)
+  // Reasoning events — emit as reasoning_content in Chat format
   if (eventType === "response.reasoning_summary_text.delta") {
-    // Optionally include reasoning as content, or skip
-    return null;
+    const reasoningDelta = data.delta || "";
+    if (!reasoningDelta) return null;
+    return {
+      id: state.chatId,
+      object: "chat.completion.chunk",
+      created: state.created,
+      model: state.model || "gpt-4",
+      choices: [{
+        index: 0,
+        delta: { reasoning_content: reasoningDelta },
+        finish_reason: null,
+      }],
+    };
   }
 
   // Ignore other events

tests/unit/responses-translation-fixes.test.mjs

@@ -342,7 +342,7 @@ test("Chat→Responses: deprecated function role message converted to function_c
   assert.equal(fcOutput.call_id, fcItem.call_id);
 });
 
-const { openaiToOpenAIResponsesResponse } = await import(
+const { openaiToOpenAIResponsesResponse, openaiResponsesToOpenAIResponse } = await import(
   "../../open-sse/translator/response/openai-responses.ts"
 );
 const { initState } = await import("../../open-sse/translator/index.ts");
@@ -388,3 +388,18 @@ test("Chat→Responses streaming: completed event includes accumulated output",
   const msgOutput = completedEvent.data.response.output.find((o) => o.type === "message");
   assert.ok(msgOutput, "should have message output item");
 });
+
+test("Responses→Chat streaming: reasoning delta emits reasoning_content in Chat chunk", () => {
+  const state = { started: false, chatId: null, created: null, toolCallIndex: 0, finishReasonSent: false };
+
+  const chunk = {
+    type: "response.reasoning_summary_text.delta",
+    delta: "thinking step...",
+    item_id: "rs_1",
+    output_index: 0,
+    summary_index: 0,
+  };
+  const result = openaiResponsesToOpenAIResponse(chunk, state);
+  assert.ok(result, "should return a chunk");
+  assert.equal(result.choices[0].delta.reasoning_content, "thinking step...");
+});