diff --git a/.changeset/gold-fireants-talk.md b/.changeset/gold-fireants-talk.md
new file mode 100644
index 000000000000..8a5f0b4269ed
--- /dev/null
+++ b/.changeset/gold-fireants-talk.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: always add `@sveltejs/kit` to `noExternal` for ssr build
diff --git a/packages/kit/src/exports/vite/index.js b/packages/kit/src/exports/vite/index.js
index ca0693a3a6a3..b7ef5324aa35 100644
--- a/packages/kit/src/exports/vite/index.js
+++ b/packages/kit/src/exports/vite/index.js
@@ -218,21 +218,6 @@ function kit({ svelte_config }) {
 
 			const generated = path.posix.join(kit.outDir, 'generated');
 
-			// This ensures that esm-env is inlined into the server output with the
-			// export conditions resolved correctly through Vite. This prevents adapters
-			// that bundle later on from resolving the export conditions incorrectly
-			// and for example include browser-only code in the server output
-			// because they for example use esbuild.build with `platform: 'browser'`
-			const noExternal = ['esm-env'];
-
-			// Vitest bypasses Vite when loading external modules, so we bundle
-			// when it is detected to keep our virtual modules working.
-			// See https://github.com/sveltejs/kit/pull/9172
-			// and https://vitest.dev/config/#deps-registernodeloader
-			if (process.env.TEST) {
-				noExternal.push('@sveltejs/kit');
-			}
-
 			// dev and preview config can be shared
 			/** @type {import('vite').UserConfig} */
 			const new_config = {
@@ -269,7 +254,23 @@ function kit({ svelte_config }) {
 					]
 				},
 				ssr: {
-					noExternal
+					noExternal: [
+						// This ensures that esm-env is inlined into the server output with the
+						// export conditions resolved correctly through Vite. This prevents adapters
+						// that bundle later on from resolving the export conditions incorrectly
+						// and for example include browser-only code in the server output
+						// because they for example use esbuild.build with `platform: 'browser'`
+						'esm-env',
+						// We need this for two reasons:
+						// 1. Without this, `@sveltejs/kit` imports are kept as-is in the server output,
+						//    and that causes modules and therefore classes like `Redirect` to be imported twice
+						//    under different IDs, which breaks a bunch of stuff because of failing instanceof checks.
+						// 2. Vitest bypasses Vite when loading external modules, so we bundle
+						//    when it is detected to keep our virtual modules working.
+						//    See https://github.com/sveltejs/kit/pull/9172
+						//    and https://vitest.dev/config/#deps-registernodeloader
+						'@sveltejs/kit'
+					]
 				}
 			};