2 files changed, 47 insertions, 2 deletions
diff --git a/packages/integrations/node/src/createOutgoingHttpHeaders.ts b/packages/integrations/node/src/createOutgoingHttpHeaders.ts
new file mode 100644
index 000000000..5b99cfa4f
--- /dev/null
+++ b/packages/integrations/node/src/createOutgoingHttpHeaders.ts
@@ -0,0 +1,40 @@
+import type { OutgoingHttpHeaders } from 'http';
+
+/**
+ * Takes in a nullable WebAPI Headers object and produces a NodeJS OutgoingHttpHeaders object suitable for usage
+ * with ServerResponse.writeHead(..) or ServerResponse.setHeader(..)
+ * 
+ * @param webHeaders WebAPI Headers object
+ * @returns NodeJS OutgoingHttpHeaders object with multiple set-cookie handled as an array of values
+ */
+export const createOutgoingHttpHeaders = (webHeaders: Headers | undefined | null): OutgoingHttpHeaders | undefined => {
+  if (!webHeaders) {
+    return undefined;
+  }
+
+  // re-type to access Header.getSetCookie()
+  const headers = webHeaders as HeadersWithGetSetCookie;
+
+  // at this point, a multi-value'd set-cookie header is invalid (it was concatenated as a single CSV, which is not valid for set-cookie)
+  const nodeHeaders: OutgoingHttpHeaders = Object.fromEntries(headers.entries());
+
+  if (Object.keys(nodeHeaders).length === 0) {
+    return undefined;
+  }
+
+  // if there is > 1 set-cookie header, we have to fix it to be an array of values
+  if (headers.has('set-cookie')) {
+    const cookieHeaders = headers.getSetCookie();
+    if (cookieHeaders.length > 1) {
+      // the Headers.entries() API already normalized all header names to lower case so we can safely index this as 'set-cookie'
+      nodeHeaders['set-cookie'] = cookieHeaders;
+    }
+  }
+
+  return nodeHeaders;
+};
+
+interface HeadersWithGetSetCookie extends Headers {
+  // the @astrojs/webapi polyfill makes this available (as of undici@5.19.0), but tsc doesn't pick it up on the built-in Headers type from DOM lib
+  getSetCookie(): string[];
+}
diff --git a/packages/integrations/node/src/nodeMiddleware.ts b/packages/integrations/node/src/nodeMiddleware.ts
index c23cdb89c..492130daa 100644
--- a/packages/integrations/node/src/nodeMiddleware.ts
+++ b/packages/integrations/node/src/nodeMiddleware.ts
@@ -3,6 +3,7 @@ import type { IncomingMessage, ServerResponse } from 'http';
 import type { Readable } from 'stream';
 import { responseIterator } from './response-iterator';
 import type { Options } from './types';
+import { createOutgoingHttpHeaders } from './createOutgoingHttpHeaders';
 
 export default function (app: NodeApp, mode: Options['mode']) {
 	return async function (
@@ -44,12 +45,16 @@ async function writeWebResponse(app: NodeApp, res: ServerResponse, webResponse:
 
 	if (app.setCookieHeaders) {
 		const setCookieHeaders: Array<string> = Array.from(app.setCookieHeaders(webResponse));
+    
 		if (setCookieHeaders.length) {
-			res.setHeader('Set-Cookie', setCookieHeaders);
+      for (const setCookieHeader of setCookieHeaders) {
+        webResponse.headers.append('set-cookie', setCookieHeader);
+      }
 		}
 	}
 
-	res.writeHead(status, Object.fromEntries(headers.entries()));
+  const nodeHeaders = createOutgoingHttpHeaders(headers);
+  res.writeHead(status, nodeHeaders);
 	if (webResponse.body) {
 		try {
 			for await (const chunk of responseIterator(webResponse) as unknown as Readable) {