fix: prevent unnecessary flattening of headers during convertRes

packages/open-next/src/core/routing/util.ts

@@ -102,8 +102,9 @@ export function convertRes(res: OpenNextNodeResponse): InternalResult {
   // When using HEAD requests, it seems that flushHeaders is not called, not sure why
   // Probably some kind of race condition
   const headers = parseHeaders(res.getFixedHeaders());
+  const contentType = headers["content-type"];
   const isBase64Encoded =
-    isBinaryContentType(headers["content-type"]) ||
+    (typeof contentType === "string" && isBinaryContentType(contentType)) ||
     !!headers["content-encoding"];
   // We cannot convert the OpenNextNodeResponse to a ReadableStream directly
   // You can look in the `aws-lambda.ts` file for some context

packages/open-next/src/http/openNextResponse.ts

@@ -10,7 +10,7 @@ import { Transform } from "node:stream";
 
 import type { StreamCreator } from "types/open-next";
 import { debug } from "../adapters/logger";
-import { parseHeaders, parseSetCookieHeader } from "./util";
+import { flattenHeaders, parseSetCookieHeader } from "./util";
 
 const SET_COOKIE_HEADER = "set-cookie";
 const CANNOT_BE_USED = "This cannot be used in OpenNext";
@@ -184,7 +184,7 @@ export class OpenNextNodeResponse extends Transform implements ServerResponse {
     // We need to fix the set-cookie header here
     this.headers[SET_COOKIE_HEADER] = this._cookies;
 
-    const parsedHeaders = parseHeaders(this.headers);
+    const parsedHeaders = flattenHeaders(this.headers);
 
     // We need to remove the set-cookie header from the parsed headers because
     // it does not handle multiple set-cookie headers properly

packages/open-next/src/http/util.ts

@@ -3,7 +3,7 @@ import type http from "node:http";
 export const parseHeaders = (
   headers?: http.OutgoingHttpHeader[] | http.OutgoingHttpHeaders,
 ) => {
-  const result: Record<string, string> = {};
+  const result: Record<string, string | string[]> = {};
   if (!headers) {
     return result;
   }
@@ -12,20 +12,31 @@ export const parseHeaders = (
     if (value === undefined) {
       continue;
     }
-    result[key.toLowerCase()] = convertHeader(value);
+    result[key.toLowerCase()] =
+      typeof value === "number" ? String(value) : value;
   }
 
   return result;
 };
 
-export const convertHeader = (header: http.OutgoingHttpHeader) => {
-  if (typeof header === "string") {
-    return header;
+export const flattenHeaders = (
+  headers?: http.OutgoingHttpHeader[] | http.OutgoingHttpHeaders,
+) => {
+  const result: Record<string, string> = {};
+  if (!headers) {
+    return result;
   }
-  if (Array.isArray(header)) {
-    return header.join(",");
+
+  for (const [key, value] of Object.entries(headers)) {
+    if (value === undefined) {
+      continue;
+    }
+    result[key.toLowerCase()] = Array.isArray(value)
+      ? value.join(",")
+      : String(value);
   }
-  return String(header);
+
+  return result;
 };
 
 /**