fix(api): recognize OpenRouter reasoning/reasoning_details in non-streaming OpenAI-to-Claude conversion (#6623)

changelog.d/fixes/6623-6623-mimo-502-messages.md

@@ -0,0 +1 @@
+- fix(api): recognize OpenRouter reasoning/reasoning_details in non-streaming OpenAI-to-Claude conversion (#6623)

open-sse/handlers/responseTranslator.ts

@@ -560,6 +560,31 @@ export function translateNonStreamingResponse(
   return intermediateOpenAI;
 }
 
+/**
+ * Resolve reasoning/thinking text off a non-streaming OpenAI-format message object.
+ * Checks DeepSeek-style `reasoning_content`, then the OpenRouter/StepFun aliases
+ * `reasoning` and `reasoning_details[]` (array of { text | content }), mirroring the
+ * streaming translator's fallback chain in open-sse/translator/response/openai-to-claude.ts.
+ */
+function resolveReasoningText(messageObj: JsonRecord): string {
+  if (messageObj.reasoning_content) {
+    return toString(messageObj.reasoning_content);
+  }
+  if (typeof messageObj.reasoning === "string" && messageObj.reasoning) {
+    return messageObj.reasoning;
+  }
+  if (Array.isArray(messageObj.reasoning_details)) {
+    const parts: string[] = [];
+    for (const detail of messageObj.reasoning_details) {
+      const detailObj = toRecord(detail);
+      const text = detailObj.text ?? detailObj.content;
+      if (typeof text === "string" && text) parts.push(text);
+    }
+    return parts.join("");
+  }
+  return "";
+}
+
 /**
  * Helper to convert an OpenAI chat.completion JSON object to Claude format for non-streaming.
  */
@@ -578,11 +603,12 @@ function convertOpenAINonStreamingToClaude(openaiResponse: JsonRecord): JsonReco
 
   let hasTextOrReasoning = false;
 
-  if (messageObj.reasoning_content) {
+  const reasoningText = resolveReasoningText(messageObj);
+  if (reasoningText) {
     hasTextOrReasoning = true;
     content.push({
       type: "thinking",
-      thinking: toString(messageObj.reasoning_content),
+      thinking: reasoningText,
     });
   }
 

tests/unit/issue-6623-opencode-mimo-reasoning-details-nonstream.test.ts

@@ -0,0 +1,44 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { translateNonStreamingResponse } from "../../open-sse/handlers/responseTranslator.ts";
+import { detectMalformedNonStream } from "../../open-sse/utils/diagnostics.ts";
+import { isEmptyContentResponse } from "../../open-sse/services/errorClassifier.ts";
+
+const mimoOpenRouterStyleResponse = {
+  id: "gen-1783636289-lJcRXwMde7qjgDJfHiBC",
+  object: "chat.completion",
+  created: 1783636289,
+  model: "mimo-v2.5-free",
+  choices: [
+    {
+      index: 0,
+      finish_reason: "length",
+      logprobs: null,
+      message: {
+        role: "assistant",
+        content: null,
+        refusal: null,
+        reasoning: "Hmm, the user just said hi",
+        reasoning_details: [
+          { type: "reasoning.text", text: "Hmm, the user just said hi", format: "unknown", index: 0 },
+        ],
+      },
+    },
+  ],
+  usage: { prompt_tokens: 248, completion_tokens: 10, total_tokens: 258 },
+};
+
+test("#6623 raw responseBody is not flagged empty by isEmptyContentResponse", () => {
+  assert.equal(isEmptyContentResponse(mimoOpenRouterStyleResponse), false);
+});
+
+test("#6623 /v1/messages non-stream translation of an OpenRouter reasoning-only turn is flagged malformed (502) - RED", () => {
+  const translated = translateNonStreamingResponse(mimoOpenRouterStyleResponse, "openai", "claude", null);
+  const malformedReason = detectMalformedNonStream(translated);
+  assert.equal(malformedReason, null);
+});
+
+test("#6623 /v1/chat/completions (openai->openai, no translation) is unaffected", () => {
+  const passthrough = translateNonStreamingResponse(mimoOpenRouterStyleResponse, "openai", "openai", null);
+  assert.equal(passthrough, mimoOpenRouterStyleResponse);
+});