Fix: Show helpful error when using new App() syntax in Svelte 5

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -603,7 +603,8 @@ export function client_component(analysis, options) {
 				)
 			)
 		);
-	} else if (dev) {
+	} else {
+		// Always check for invalid constructor usage, not just in dev mode
 		component_block.body.unshift(b.stmt(b.call('$.check_target', b.id('new.target'))));
 	}
 

packages/svelte/tests/runtime-runes/samples/new-component-error/_config.js

@@ -0,0 +1,28 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	compileOptions: {
+		dev: false  // Test the fix works in production mode (where the issue was most severe)
+	},
+	async test({ mod, assert }) {
+		// Try to instantiate the component using the old `new Component()` syntax
+		// This should now throw a helpful error message instead of the cryptic "nodes_start" error
+		try {
+			const ComponentClass = mod.default;
+			const app = new ComponentClass();
+			assert.fail('Expected error when calling new ComponentClass()');
+		} catch (error) {
+			// The main fix: should NOT get the cryptic nodes_start error anymore
+			assert.ok(!error.message.includes('nodes_start'), 'Should not get cryptic nodes_start error');
+
+			// Should get a helpful error message instead
+			const isHelpfulError = error.message.includes('no longer valid in Svelte 5') ||
+								  error.message.includes('https://svelte.dev/e/component_api_invalid_new');
+			assert.ok(isHelpfulError, 'Should get a helpful error message or URL');
+
+			// Should be a proper Svelte error
+			assert.equal(error.name, 'Svelte error', 'Should be a Svelte error');
+		}
+	}
+});

packages/svelte/tests/runtime-runes/samples/new-component-error/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	let count = 0;
+</script>
+
+<h1>Hello {count}</h1>
+<button on:click={() => count++}>Click me</button>