fix: Use URL parsing instead of substring matching in isExternal

.changeset/pink-years-teach.md

@@ -0,0 +1,8 @@
+---
+"@opennextjs/aws": patch
+---
+
+fix: Correct external URL detection in isExternal using proper URL parsing
+
+Replaces substring-based host matching with URL parsing to correctly determine whether a rewritten URL is external.
+This fixes an issue where NextResponse.rewrite() would treat certain external URLs as internal when their pathname contained the host as a substring, causing unexpected 404s during middleware rewrites.

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

@@ -30,10 +30,18 @@ import { generateMessageGroupId } from "./queue.js";
 export function isExternal(url?: string, host?: string) {
   if (!url) return false;
   const pattern = /^https?:\/\//;
+  if (!pattern.test(url)) return false;
+
   if (host) {
-    return pattern.test(url) && !url.includes(host);
+    try {
+      const parsedUrl = new URL(url);
+      return parsedUrl.host !== host;
+    } catch {
+      // If URL parsing fails, fall back to substring check
+      return !url.includes(host);
+    }
   }
-  return pattern.test(url);
+  return true;
 }
 
 export function convertFromQueryString(query: string) {

packages/tests-unit/tests/core/routing/util.test.ts

@@ -87,6 +87,24 @@ describe("isExternal", () => {
   it("returns false for absolute https url same host", () => {
     expect(isExternal("https://absolute.com/path", "absolute.com")).toBe(false);
   });
+
+  it("returns true for absolute https url different host but same host in path", () => {
+    expect(isExternal("https://absolute.com/local.com", "local.com")).toBe(
+      true,
+    );
+  });
+
+  it("returns false for same host with port", () => {
+    expect(isExternal("https://localhost:3000/path", "localhost:3000")).toBe(
+      false,
+    );
+  });
+
+  it("returns true for different port on same hostname", () => {
+    expect(isExternal("https://localhost:3001/path", "localhost:3000")).toBe(
+      true,
+    );
+  });
 });
 
 describe("convertFromQueryString", () => {