From f5d68d1a1891e8c4b7d362715bdcfd375411a53d Mon Sep 17 00:00:00 2001
From: Paul Murray <paul.murray@vercel.com>
Date: Sun, 24 May 2026 06:12:46 -0400
Subject: [PATCH 1/2] [local-proxy] fix: prevent writing response after headers
 are sent in LocalProxy

Signed-off-by: Paul Murray <paul.murray@vercel.com>
---
 packages/microfrontends/src/bin/local-proxy.ts | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/packages/microfrontends/src/bin/local-proxy.ts b/packages/microfrontends/src/bin/local-proxy.ts
index 39ed036..604764e 100644
--- a/packages/microfrontends/src/bin/local-proxy.ts
+++ b/packages/microfrontends/src/bin/local-proxy.ts
@@ -477,6 +477,13 @@ export class LocalProxy {
     this.proxy = Server.createProxyServer({ secure: true });
     this.proxy.on('error', (err, req, res) => {
       if (res instanceof http.ServerResponse) {
+        if (res.destroyed || res.writableEnded) return;
+
+        if (res.headersSent) {
+          res.destroy(err);
+          return;
+        }
+
         res.writeHead(500, {
           'Content-Type': 'text/plain',
         });
@@ -701,6 +708,14 @@ export class LocalProxy {
       req.pipe(proxyReq);
       proxyReq.on('error', (err) => {
         logger.error('Proxy request error: ', err);
+
+        if (res.destroyed || res.writableEnded) return;
+
+        if (res.headersSent) {
+          res.destroy(err);
+          return;
+        }
+
         res.writeHead(500, { 'Content-Type': 'text/plain' });
         res.end(
           `Error proxying request for ${target.application} to ${hostname}:${port}${path}`,

From d9e8413be2a97c8c93d003d0a78b8f4a3834c2da Mon Sep 17 00:00:00 2001
From: Paul Murray <paul.murray@vercel.com>
Date: Sun, 24 May 2026 06:20:15 -0400
Subject: [PATCH 2/2] add comments

Signed-off-by: Paul Murray <paul.murray@vercel.com>
---
 packages/microfrontends/src/bin/local-proxy.ts | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/packages/microfrontends/src/bin/local-proxy.ts b/packages/microfrontends/src/bin/local-proxy.ts
index 604764e..a50516d 100644
--- a/packages/microfrontends/src/bin/local-proxy.ts
+++ b/packages/microfrontends/src/bin/local-proxy.ts
@@ -477,8 +477,10 @@ export class LocalProxy {
     this.proxy = Server.createProxyServer({ secure: true });
     this.proxy.on('error', (err, req, res) => {
       if (res instanceof http.ServerResponse) {
+        /** Already closed; nothing left to send. */
         if (res.destroyed || res.writableEnded) return;
 
+        /** Headers are committed; abort instead of writing a 500. */
         if (res.headersSent) {
           res.destroy(err);
           return;
@@ -709,8 +711,10 @@ export class LocalProxy {
       proxyReq.on('error', (err) => {
         logger.error('Proxy request error: ', err);
 
+        /** Already closed; nothing left to send. */
         if (res.destroyed || res.writableEnded) return;
 
+        /** Headers are committed; abort instead of writing a 500. */
         if (res.headersSent) {
           res.destroy(err);
           return;