Verify exact production traces and record rollout

apps/web/app/api/chat/route.ts

@@ -1,4 +1,5 @@
 import { NextResponse } from "next/server";
+import { randomUUID } from "node:crypto";
 import type { AnswerBlock, AnswerPlan, Citation, SendMessageResponse } from "@duoc-thu/shared-types";
 
 export const runtime = "nodejs";
@@ -211,7 +212,13 @@ export async function POST(request: Request) {
     return NextResponse.json({ error: "conversation_id_too_long" }, { status: 400 });
   }
 
-  const correlationId = `req-${Date.now()}-${Math.random().toString(36).slice(2, 7)}`;
+  const incomingCorrelationId = request.headers.get("x-correlation-id")?.trim();
+  const correlationId =
+    incomingCorrelationId && /^[A-Za-z0-9][A-Za-z0-9._:-]{0,127}$/.test(incomingCorrelationId)
+      ? incomingCorrelationId
+      : randomUUID();
+  let responseCorrelationId = correlationId;
+  let responseTraceId: string | null = null;
 
   let rag: RagResponse;
   try {
@@ -219,13 +226,19 @@ export async function POST(request: Request) {
       ? API_GATEWAY_URL
       : `${API_GATEWAY_URL}/v1/rag/query`;
 
+    const upstreamHeaders: Record<string, string> = {
+      "Content-Type": "application/json",
+      "X-Correlation-ID": correlationId,
+      "X-Client-Version": "1.0.0",
+    };
+    const traceparent = request.headers.get("traceparent");
+    const tracestate = request.headers.get("tracestate");
+    if (traceparent) upstreamHeaders.traceparent = traceparent;
+    if (tracestate) upstreamHeaders.tracestate = tracestate;
+
     const upstream = await fetch(targetUrl, {
       method: "POST",
-      headers: {
-        "Content-Type": "application/json",
-        "X-Correlation-ID": correlationId,
-        "X-Client-Version": "1.0.0",
-      },
+      headers: upstreamHeaders,
       body: JSON.stringify({
         query: content,
         subject_scope: "human",
@@ -238,6 +251,8 @@ export async function POST(request: Request) {
       // prevents the BFF itself from keeping an orphaned HTTP request open.
       signal: request.signal,
     });
+    responseCorrelationId = upstream.headers.get("x-correlation-id") ?? correlationId;
+    responseTraceId = upstream.headers.get("x-trace-id");
     if (!upstream.ok) {
       rag = {
         trace_id: `fallback-${Date.now()}`,
@@ -296,12 +311,15 @@ export async function POST(request: Request) {
     answerPlan: rag.answer_plan ? toAnswerPlan(rag.answer_plan) : undefined,
   };
 
+  const responseHeaders = new Headers({
+    "X-Correlation-ID": responseCorrelationId,
+  });
+  if (responseTraceId) responseHeaders.set("X-Trace-ID", responseTraceId);
+
   return NextResponse.json(
     { message, sessionId: conversationId ?? undefined } satisfies SendMessageResponse,
     {
-      headers: {
-        "X-Correlation-ID": correlationId,
-      },
+      headers: responseHeaders,
     }
   );
 }