From 9975cd45fa3d88fca55ac3aaf6ba92e6096f8627 Mon Sep 17 00:00:00 2001
From: svelte-triage-bot <team@svelte.com>
Date: Tue, 1 Sep 2026 00:37:57 +0000
Subject: [PATCH] fix: externalize linked env dependencies

---
 .changeset/tame-loaders-resolve.md      |   5 ++
 packages/kit/src/core/env.js            |   1 +
 packages/kit/src/core/sync/sync.spec.js | 102 ++++++++++++++++++++++++
 3 files changed, 108 insertions(+)
 create mode 100644 .changeset/tame-loaders-resolve.md

diff --git a/.changeset/tame-loaders-resolve.md b/.changeset/tame-loaders-resolve.md
new file mode 100644
index 000000000000..c80dede7f083
--- /dev/null
+++ b/.changeset/tame-loaders-resolve.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: externalize linked dependencies when loading explicit environment variables
diff --git a/packages/kit/src/core/env.js b/packages/kit/src/core/env.js
index e76c1d5476d2..ec10662eefd6 100644
--- a/packages/kit/src/core/env.js
+++ b/packages/kit/src/core/env.js
@@ -47,6 +47,7 @@ export async function load_explicit_env(kit, file, root, mode) {
 
 	const server = await vite.createServer({
 		configFile: false,
+		ssr: { external: true },
 		logLevel: 'silent',
 		mode,
 		define: {
diff --git a/packages/kit/src/core/sync/sync.spec.js b/packages/kit/src/core/sync/sync.spec.js
index 74d70fb26583..6ee6d4f436bc 100644
--- a/packages/kit/src/core/sync/sync.spec.js
+++ b/packages/kit/src/core/sync/sync.spec.js
@@ -1,6 +1,9 @@
+import { execFileSync } from 'node:child_process';
 import fs from 'node:fs';
 import os from 'node:os';
 import path from 'node:path';
+import process from 'node:process';
+import { pathToFileURL } from 'node:url';
 import { expect, test } from 'vitest';
 import { process_config, validate_config } from '../config/index.js';
 import { relative_path } from '../../utils/filesystem.js';
@@ -98,3 +101,102 @@ test('requests a manifest rebuild if type generation encounters a missing route
 		fs.rmSync(root, { recursive: true, force: true });
 	}
 });
+
+test('externalizes linked dependencies when loading explicit environment variables', () => {
+	const root = fs.mkdtempSync(path.join(os.tmpdir(), 'svelte-kit-env-'));
+	const dependency = path.join(root, 'packages/linked-env-dependency');
+	const loader = path.join(root, 'loader');
+	const node_modules = path.join(root, 'node_modules');
+
+	fs.mkdirSync(path.join(root, 'src'), { recursive: true });
+	fs.mkdirSync(dependency, { recursive: true });
+	fs.mkdirSync(loader, { recursive: true });
+	fs.mkdirSync(node_modules, { recursive: true });
+
+	fs.writeFileSync(path.join(root, 'package.json'), '{"type":"module"}');
+	fs.writeFileSync(
+		path.join(root, 'src/env.js'),
+		`import { marker } from 'linked-env-dependency';
+
+export const variables = {
+	TEST: { description: marker }
+};
+`
+	);
+	fs.writeFileSync(
+		path.join(dependency, 'package.json'),
+		'{"name":"linked-env-dependency","type":"module","exports":"./index.js"}'
+	);
+	fs.writeFileSync(
+		path.join(dependency, 'index.js'),
+		`import { marker } from 'loader-target';
+
+export { marker };
+`
+	);
+	fs.writeFileSync(path.join(loader, 'target.js'), `export const marker = 'resolved by loader';\n`);
+	fs.writeFileSync(
+		path.join(loader, 'hooks.js'),
+		`export async function resolve(specifier, context, nextResolve) {
+	if (specifier === 'loader-target') {
+		return {
+			url: new URL('./target.js', import.meta.url).href,
+			shortCircuit: true
+		};
+	}
+
+	return nextResolve(specifier, context);
+}
+`
+	);
+	fs.writeFileSync(
+		path.join(loader, 'register.js'),
+		`import { register } from 'node:module';
+
+register('./hooks.js', import.meta.url);
+`
+	);
+
+	const config_url = pathToFileURL(path.resolve(import.meta.dirname, '../config/index.js')).href;
+	const env_url = pathToFileURL(path.resolve(import.meta.dirname, '../env.js')).href;
+	fs.writeFileSync(
+		path.join(root, 'run.js'),
+		`import { fileURLToPath } from 'node:url';
+import { process_config, validate_config } from ${JSON.stringify(config_url)};
+import { load_explicit_env } from ${JSON.stringify(env_url)};
+
+const config = process_config(validate_config({}), import.meta.dirname);
+const file = fileURLToPath(new URL('./src/env.js', import.meta.url));
+const { variables } = await load_explicit_env(config, file, import.meta.dirname, 'development');
+
+if (variables.TEST.description !== 'resolved by loader') {
+	throw new Error('Node loader was not used');
+}
+`
+	);
+
+	const link_type = process.platform === 'win32' ? 'junction' : 'dir';
+	fs.symlinkSync(dependency, path.join(node_modules, 'linked-env-dependency'), link_type);
+	fs.symlinkSync(
+		path.resolve(import.meta.dirname, '../../../node_modules/vite'),
+		path.join(node_modules, 'vite'),
+		link_type
+	);
+
+	try {
+		execFileSync(process.execPath, [path.join(root, 'run.js')], {
+			cwd: root,
+			env: {
+				...process.env,
+				NODE_OPTIONS: [
+					process.env.NODE_OPTIONS,
+					`--import=${pathToFileURL(path.join(loader, 'register.js')).href}`
+				]
+					.filter(Boolean)
+					.join(' ')
+			}
+		});
+	} finally {
+		fs.rmSync(root, { recursive: true, force: true });
+	}
+});