From 5c6b19852fd0c7e231ba0a1993438a898d2ebc43 Mon Sep 17 00:00:00 2001
From: Nic Miller <nic@aedileconsulting.com>
Date: Mon, 3 Aug 2026 09:25:06 +0100
Subject: [PATCH] fix: don't crash the CLI when the upstream dev server dies
 mid-response

---
 src/utils/proxy.ts | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/utils/proxy.ts b/src/utils/proxy.ts
index ac65ffabae6..2806a2b46ac 100644
--- a/src/utils/proxy.ts
+++ b/src/utils/proxy.ts
@@ -556,6 +556,15 @@ const initializeProxy = async function ({
     }
 
     if (res instanceof http.ServerResponse) {
+      if (res.headersSent || res.writableEnded) {
+        // The upstream died mid-response: headers are already on the wire, so
+        // `writeHead` below would throw ERR_HTTP_HEADERS_SENT out of this event
+        // handler and terminate the whole CLI (see #5917). The response is
+        // unsalvageable — drop the socket and keep the dev server alive so
+        // requests after the upstream comes back succeed.
+        res.destroy()
+        return
+      }
       res.writeHead(500, {
         'Content-Type': 'text/plain',
       })