From ccbd4a96ea61401d2cdfa24e799071b4dae5abe1 Mon Sep 17 00:00:00 2001
From: Nic Polumeyv <162764842+Nic-Polumeyv@users.noreply.github.com>
Date: Sat, 18 Jul 2026 11:07:43 -0400
Subject: [PATCH] perf: build transport encoders once instead of on every
 response

---
 .changeset/plenty-camels-shave.md              |  5 +++++
 .../kit/src/runtime/server/page/actions.js     | 10 +++++-----
 .../src/runtime/server/page/data_serializer.js |  5 ++---
 packages/kit/src/runtime/server/utils.js       | 18 ++++++++++++++++++
 4 files changed, 30 insertions(+), 8 deletions(-)
 create mode 100644 .changeset/plenty-camels-shave.md

diff --git a/.changeset/plenty-camels-shave.md b/.changeset/plenty-camels-shave.md
new file mode 100644
index 000000000000..d1196aded331
--- /dev/null
+++ b/.changeset/plenty-camels-shave.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+perf: build transport encoders once instead of on every response
diff --git a/packages/kit/src/runtime/server/page/actions.js b/packages/kit/src/runtime/server/page/actions.js
index 4ec4ecaade43..a54e2eed9088 100644
--- a/packages/kit/src/runtime/server/page/actions.js
+++ b/packages/kit/src/runtime/server/page/actions.js
@@ -7,7 +7,7 @@ import { HttpError, Redirect, ActionFailure, SvelteKitError } from '@sveltejs/ki
 import { with_request_store, merge_tracing } from '@sveltejs/kit/internal/server';
 import { normalize_error } from '../../../utils/error.js';
 import { is_form_content_type, negotiate } from '../../../utils/http.js';
-import { create_replacer, handle_error_and_jsonify } from '../utils.js';
+import { create_replacer, get_encoders, handle_error_and_jsonify } from '../utils.js';
 import { record_span } from '../../telemetry/record_span.js';
 
 /** @param {RequestEvent} event */
@@ -328,11 +328,11 @@ export function uneval_action_response(data, route_id, transport) {
  * @param {ServerHooks['transport']} transport
  */
 function stringify_action_response(data, route_id, transport) {
-	const encoders = Object.fromEntries(
-		Object.entries(transport).map(([key, value]) => [key, value.encode])
+	return try_serialize(
+		data,
+		(value) => devalue.stringify(value, get_encoders(transport)),
+		route_id
 	);
-
-	return try_serialize(data, (value) => devalue.stringify(value, encoders), route_id);
 }
 
 /**
diff --git a/packages/kit/src/runtime/server/page/data_serializer.js b/packages/kit/src/runtime/server/page/data_serializer.js
index 5525fde60699..10a549d70612 100644
--- a/packages/kit/src/runtime/server/page/data_serializer.js
+++ b/packages/kit/src/runtime/server/page/data_serializer.js
@@ -3,6 +3,7 @@ import { compact } from '../../../utils/array.js';
 import { create_async_iterator } from '../../../utils/streaming.js';
 import {
 	clarify_devalue_error,
+	get_encoders,
 	get_global_name,
 	handle_error_and_jsonify,
 	serialize_uses
@@ -137,9 +138,7 @@ export function server_data_serializer_json(event, event_state, options) {
 	const iterator = create_async_iterator();
 
 	const reducers = {
-		...Object.fromEntries(
-			Object.entries(options.hooks.transport).map(([key, value]) => [key, value.encode])
-		),
+		...get_encoders(options.hooks.transport),
 		/** @param {any} thing */
 		Promise: (thing) => {
 			if (typeof thing?.then !== 'function') {
diff --git a/packages/kit/src/runtime/server/utils.js b/packages/kit/src/runtime/server/utils.js
index 1148245158a0..a7f4f2ee4141 100644
--- a/packages/kit/src/runtime/server/utils.js
+++ b/packages/kit/src/runtime/server/utils.js
@@ -285,6 +285,24 @@ export function count_non_ssi_comments(str) {
 	return (str.match(/<!--(?!#)/g) ?? []).length;
 }
 
+/** @type {WeakMap<ServerHooks['transport'], Record<string, (value: any) => any>>} */
+const encoders = new WeakMap();
+
+/**
+ * Derives the devalue encoders from the app's transport hook, once per transport object
+ * @param {ServerHooks['transport']} transport
+ */
+export function get_encoders(transport) {
+	let cached = encoders.get(transport);
+	if (cached === undefined) {
+		cached = Object.fromEntries(
+			Object.entries(transport).map(([key, value]) => [key, value.encode])
+		);
+		encoders.set(transport, cached);
+	}
+	return cached;
+}
+
 /**
  * Creates a serialiser for non-arbitrary POJOs using the app's transport hook
  * @param {ServerHooks['transport']} transport