fix: preserve trailingSlash config when error boundary is above page node

.changeset/fix-trailing-slash-error-boundary.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: preserve trailingSlash config when error boundary is above page node

packages/kit/src/runtime/client/client.js

@@ -635,9 +635,9 @@ async function initialize(result, target, hydrate) {
  *   form?: Record<string, any> | null;
  * }} opts
  */
-function get_navigation_result_from_branch({ url, params, branch, status, error, route, form }) {
+function get_navigation_result_from_branch({ url, params, branch, status, error, route, form, trailing_slash }) {
 	/** @type {import('types').TrailingSlash} */
-	let slash = 'never';
+	let slash = trailing_slash ?? 'never';
 
 	// if `paths.base === '/a/b/c`, then the root route is always `/a/b/c/`, regardless of
 	// the `trailingSlash` route option, so that relative paths to JS and CSS work
@@ -1197,13 +1197,21 @@ async function load_route({ id, invalidating, url, params, route, preload }) {
 
 				const error_load = await load_nearest_error_page(i, branch, errors);
 				if (error_load) {
+					// preserve trailingSlash config from the full branch so that
+					// slicing the branch for error handling doesn't lose it (#13516)
+					let trailing_slash;
+					for (const node of branch) {
+						if (node?.slash !== undefined) trailing_slash = node.slash;
+					}
+
 					return get_navigation_result_from_branch({
 						url,
 						params,
 						branch: branch.slice(0, error_load.idx).concat(error_load.node),
 						status,
 						error,
-						route
+						route,
+						trailing_slash
 					});
 				} else {
 					return await server_fallback(url, { id: route.id }, error, status);
@@ -2401,13 +2409,21 @@ export async function set_nearest_error_page(error, status = 500) {
 
 	const error_load = await load_nearest_error_page(current.branch.length, branch, route.errors);
 	if (error_load) {
+		// preserve trailingSlash config from the full branch so that
+		// slicing the branch for error handling doesn't lose it (#13516)
+		let trailing_slash;
+		for (const node of branch) {
+			if (node?.slash !== undefined) trailing_slash = node.slash;
+		}
+
 		const navigation_result = get_navigation_result_from_branch({
 			url,
 			params: current.params,
 			branch: branch.slice(0, error_load.idx).concat(error_load.node),
 			status,
 			error,
-			route
+			route,
+			trailing_slash
 		});
 
 		current = navigation_result.state;

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/error-boundary/+page.js

@@ -0,0 +1,7 @@
+import { error } from '@sveltejs/kit';
+
+export const trailingSlash = 'always';
+
+export function load() {
+	error(500, 'deliberate error');
+}

packages/kit/test/apps/basics/src/routes/routing/trailing-slash/error-boundary/+page.svelte

@@ -0,0 +1 @@
+<p>This should not be visible</p>

packages/kit/test/apps/basics/test/cross-platform/client.test.js

@@ -1154,6 +1154,13 @@ test.describe('Routing', () => {
 		expect(new URL(page.url()).pathname).toBe('/routing/trailing-slash/never');
 		await expect(page.locator('p')).toHaveText('/routing/trailing-slash/never');
 	});
+
+	test('trailing slash is preserved when error boundary is above page config', async ({ page }) => {
+		await page.goto('/routing/trailing-slash/error-boundary/');
+		// The page throws an error, which triggers the error boundary.
+		// The URL should still have the trailing slash from the page's trailingSlash='always' config.
+		expect(new URL(page.url()).pathname).toBe('/routing/trailing-slash/error-boundary/');
+	});
 });
 
 test.describe('Shadow DOM', () => {