diff --git a/.changeset/great-corners-spend.md b/.changeset/great-corners-spend.md
new file mode 100644
index 000000000..0c968fa3c
--- /dev/null
+++ b/.changeset/great-corners-spend.md
@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': patch
+---
+
+fix: prevent accidental pollution of `svelteconfig.extensions`
diff --git a/packages/e2e-tests/_test_dependencies/vite-plugins/index.js b/packages/e2e-tests/_test_dependencies/vite-plugins/index.js
index 057b8d260..4044bb53b 100644
--- a/packages/e2e-tests/_test_dependencies/vite-plugins/index.js
+++ b/packages/e2e-tests/_test_dependencies/vite-plugins/index.js
@@ -57,6 +57,11 @@ export function writeResolvedConfig() {
 				if (value instanceof RegExp) return value.toString();
 				else return value;
 			}
+
+			// Handle config from svelte.config.js through vite-plugin-svelte
+			const vps = config.plugins.find((p) => p.name === 'vite-plugin-svelte');
+			const configFileOptions = vps?.api?.options;
+
 			const serializableConfig = {
 				...config,
 				plugins: config.plugins.map((p) => p.name)
@@ -66,7 +71,31 @@ export function writeResolvedConfig() {
 				fs.mkdirSync(dir);
 			}
 			const filename = path.join(dir, `vite.config.${cmd}${config.build.ssr ? '.ssr' : ''}.json`);
+			const optionsFilename = path.join(
+				dir,
+				`svelte.options.${cmd}${config.build.ssr ? '.ssr' : ''}.json`
+			);
 			fs.writeFileSync(filename, JSON.stringify(serializableConfig, replacer, '\t'), 'utf-8');
+			fs.writeFileSync(optionsFilename, JSON.stringify(configFileOptions, replacer, '\t'), 'utf-8');
+		}
+	};
+}
+
+/**
+ * write resolved config
+ * @returns {import('vite').Plugin}
+ */
+export function pushToOptimizeDepsExtensions() {
+	return {
+		name: 'push-to-optimize-deps-extensions',
+		config(config) {
+			if (!config.optimizeDeps) {
+				config.optimizeDeps = {};
+			}
+			if (!config.optimizeDeps.extensions) {
+				config.optimizeDeps.extensions = [];
+			}
+			config.optimizeDeps.extensions.push('.pushed');
 		}
 	};
 }
diff --git a/packages/e2e-tests/kit-node/__tests__/kit.spec.ts b/packages/e2e-tests/kit-node/__tests__/kit.spec.ts
index 2d36c82fb..75735c126 100644
--- a/packages/e2e-tests/kit-node/__tests__/kit.spec.ts
+++ b/packages/e2e-tests/kit-node/__tests__/kit.spec.ts
@@ -373,6 +373,30 @@ describe('kit-node', () => {
 		});
 	});
 
+	describe('resolved options', () => {
+		it('should not be polluted by other plugins pushing to extensions', async () => {
+			const configs = [];
+			if (isBuild) {
+				configs.push('serve', 'build', 'build.ssr');
+			} else {
+				configs.push('serve');
+			}
+			const expectArrayEqual = (a: string[], b: string[], message: string) => {
+				const aSorted = a.slice().sort();
+				const bSorted = b.slice().sort();
+				expect(aSorted, message).toEqual(bSorted);
+			};
+			for (const pattern of configs) {
+				const filename = `svelte.options.${pattern}.json`;
+				const config = JSON.parse(
+					await readFileContent(path.join('logs', 'resolved-configs', filename))
+				);
+				const isServe = pattern === 'serve';
+				expectArrayEqual(['.svelte'], config.extensions, `extensions in ${filename}`);
+			}
+		});
+	});
+
 	describe.runIf(isBuild)('output', () => {
 		it('should produce hermetic build', async () => {
 			const outputFiles = await glob('./build/**/*', { cwd: testDir, filesOnly: true });
diff --git a/packages/e2e-tests/kit-node/vite.config.js b/packages/e2e-tests/kit-node/vite.config.js
index 9154df4d8..44d92c348 100644
--- a/packages/e2e-tests/kit-node/vite.config.js
+++ b/packages/e2e-tests/kit-node/vite.config.js
@@ -1,5 +1,9 @@
 import { sveltekit } from '@sveltejs/kit/vite';
-import { transformValidation, writeResolvedConfig } from 'e2e-test-dep-vite-plugins';
+import {
+	transformValidation,
+	writeResolvedConfig,
+	pushToOptimizeDepsExtensions
+} from 'e2e-test-dep-vite-plugins';
 
 /** @type {import('vite').UserConfig} */
 export default {
@@ -16,6 +20,7 @@ export default {
 		sourcemap: true // must be true for hermetic build test!
 	},
 	plugins: [
+		pushToOptimizeDepsExtensions(),
 		transformValidation(),
 		sveltekit(),
 		writeResolvedConfig(),
diff --git a/packages/vite-plugin-svelte/src/utils/options.js b/packages/vite-plugin-svelte/src/utils/options.js
index 1cd0f69fd..51158428c 100644
--- a/packages/vite-plugin-svelte/src/utils/options.js
+++ b/packages/vite-plugin-svelte/src/utils/options.js
@@ -386,7 +386,7 @@ export async function buildExtraViteConfig(options, config) {
 		extraViteConfig.optimizeDeps = {
 			...extraViteConfig.optimizeDeps,
 			// Experimental Vite API to allow these extensions to be scanned and prebundled
-			extensions: options.extensions ?? ['.svelte'],
+			extensions: options.extensions ? [...options.extensions] : ['.svelte'],
 			// Add esbuild plugin to prebundle Svelte files.
 			// Currently a placeholder as more information is needed after Vite config is resolved,
 			// the real Svelte plugin is added in `patchResolvedViteConfig()`