Correctly identify readable node streams

.changeset/big-houses-bow.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Correctly identify readable node streams

packages/kit/src/runtime/server/utils.js

@@ -32,16 +32,28 @@ export function decode_params(params) {
 	return params;
 }
 
+/** @param {any} obj **/
+function is_readable_node_stream(obj) {
+	// Copied from nodejs sources
+	return !!(
+		(
+			obj &&
+			typeof obj.pipe === 'function' &&
+			typeof obj.on === 'function' &&
+			(!obj._writableState || obj._readableState?.readable !== false) && // Duplex
+			(!obj._writableState || obj._readableState)
+		) // Writable has .pipe.
+	);
+}
+
 /** @param {any} body */
 export function is_pojo(body) {
 	if (typeof body !== 'object') return false;
 
 	if (body) {
 		if (body instanceof Uint8Array) return false;
 
-		// body could be a node Readable, but we don't want to import
-		// node built-ins, so we use duck typing
-		if (body._readableState && body._writableState && body._events) return false;
+		if (is_readable_node_stream(body)) return false;
 
 		// similarly, it could be a web ReadableStream
 		if (typeof ReadableStream !== 'undefined' && body instanceof ReadableStream) return false;