From 7f19790da0a5bc86e81a364d45436b4b69264b6e Mon Sep 17 00:00:00 2001
From: Phil Pluckthun <phil@kitten.sh>
Date: Mon, 16 Feb 2026 02:48:41 +0000
Subject: [PATCH] fix: Drop `setHeaders` call for consistent `setHeader` calls
 (#35)

---
 .changeset/loud-ideas-type.md |  5 +++++
 src/fetch.ts                  | 19 +++++++++++++++----
 2 files changed, 20 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/loud-ideas-type.md

diff --git a/.changeset/loud-ideas-type.md b/.changeset/loud-ideas-type.md
new file mode 100644
index 0000000..42460a2
--- /dev/null
+++ b/.changeset/loud-ideas-type.md
@@ -0,0 +1,5 @@
+---
+'fetch-nodeshim': patch
+---
+
+Avoid `setHeaders` to increase consistency, fix `Set-Cookie` case for older Node versions, and work around bug in Bun <=1.3.9
diff --git a/src/fetch.ts b/src/fetch.ts
index 3d5c323..a352eac 100644
--- a/src/fetch.ts
+++ b/src/fetch.ts
@@ -33,10 +33,21 @@ const assignOutgoingMessageHeaders = (
   outgoing: http.OutgoingMessage,
   headers: Headers
 ) => {
-  if (typeof outgoing.setHeaders === 'function') {
-    outgoing.setHeaders(headers);
-  } else {
-    for (const [key, value] of headers) outgoing.setHeader(key, value);
+  // Preassemble array headers, mostly only for Set-Cookie
+  // We're avoiding `getSetCookie` since support is unclear in Node 18
+  const collection: Record<string, string | string[]> = {};
+  for (const [key, value] of headers) {
+    if (Array.isArray(collection[key])) {
+      collection[key].push(value);
+    } else if (collection[key] != undefined) {
+      collection[key] = [collection[key], value];
+    } else {
+      collection[key] = value;
+    }
+  }
+  // We don't use `setHeaders` due to a Bun bug (Fix: https://github.com/oven-sh/bun/pull/27050)
+  for (const key in collection) {
+    outgoing.setHeader(key, collection[key]);
   }
 };
 
-- 
2.51.2