fix(openai-responses): emit reasoning summary as delta.reasoning_content (#2159)

Integrated into release/v3.8.0 — emit reasoning summary as delta.reasoning_content for Chat Completions clients

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

@@ -823,13 +823,18 @@ export function openaiResponsesToOpenAIResponse(chunk, state) {
     };
   }
 
-  // Handle true reasoning summary ("Thought for 15s")
+  // Handle true reasoning summary ("Thought for 15s").
+  // Emit as `delta.reasoning_content` — matches the shape used by the
+  // `reasoning_content_text.delta` branch above and is what Chat clients
+  // (OpenCode, Claude Code, Cursor, etc.) actually render in their thinking
+  // panel. A nested `delta.reasoning.summary` object is swallowed by most
+  // stream mergers and never reaches the user.
   if (eventType === "response.reasoning_summary_text.delta") {
     const reasoningDelta = data.delta || "";
     if (!reasoningDelta) return null;
     const reasoningDeltaShape = state.copilotCompatibleReasoning
       ? { reasoning_text: reasoningDelta }
-      : { reasoning: { summary: reasoningDelta } };
+      : { reasoning_content: reasoningDelta };
     return {
       id: state.chatId,
       object: "chat.completion.chunk",

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

@@ -406,7 +406,7 @@ test("Responses→Chat streaming: reasoning delta emits reasoning_content in Cha
   };
   const result = openaiResponsesToOpenAIResponse(chunk, state);
   assert.ok(result, "should return a chunk");
-  assert.equal(result.choices[0].delta.reasoning.summary, "thinking step...");
+  assert.equal(result.choices[0].delta.reasoning_content, "thinking step...");
 });
 
 test("Responses→Chat streaming: Copilot mode emits reasoning_text for summary deltas", () => {

tests/unit/translator-resp-openai-responses.test.ts

@@ -289,7 +289,7 @@ test("Responses -> OpenAI: tool-call delta, reasoning delta and completed usage
 
   assert.equal(added.choices[0].delta.tool_calls[0].function.name, "weather");
   assert.equal(args.choices[0].delta.tool_calls[0].function.arguments, '{"city":"SP"}');
-  assert.equal(reasoning.choices[0].delta.reasoning.summary, "Need weather info.");
+  assert.equal(reasoning.choices[0].delta.reasoning_content, "Need weather info.");
   assert.equal(completed.choices[0].finish_reason, "tool_calls");
   assert.equal((completed as any).usage.prompt_tokens, 8);
   assert.equal((completed as any).usage.completion_tokens, 2);