fix(build): strip @_@ pattern from CSS asset filenames to prevent inclusion in generated file names

packages/astro/src/core/build/static-build.ts

@@ -289,7 +289,16 @@ async function buildEnvironments(opts: StaticBuildOptions, internals: BuildInter
 						}
 						return [prefix, cleanChunkName(name), suffix].join('');
 					},
-					assetFileNames: `${settings.config.build.assets}/[name].[hash][extname]`,
+					assetFileNames(assetInfo) {
+						// Strip the @_@ extension-masking pattern from asset names, just like chunkFileNames above.
+						// The @_@ pattern is an internal mechanism for virtual module IDs and should not leak into output filenames.
+						const name = assetInfo.names?.[0] ?? '';
+						if (name.includes(ASTRO_PAGE_EXTENSION_POST_PATTERN)) {
+							const [sanitizedName] = name.split(ASTRO_PAGE_EXTENSION_POST_PATTERN);
+							return `${settings.config.build.assets}/${sanitizedName}.[hash][extname]`;
+						}
+						return `${settings.config.build.assets}/[name].[hash][extname]`;
+					},
 					...viteConfig.build?.rollupOptions?.output,
 					entryFileNames(chunkInfo) {
 						if (chunkInfo.facadeModuleId?.startsWith(VIRTUAL_PAGE_RESOLVED_MODULE_ID)) {
@@ -424,7 +433,16 @@ async function buildEnvironments(opts: StaticBuildOptions, internals: BuildInter
 							chunkFileNames(chunkInfo) {
 								return `${settings.config.build.assets}/${cleanChunkName(chunkInfo.name)}.[hash].js`;
 							},
-							assetFileNames: `${settings.config.build.assets}/[name].[hash][extname]`,
+							assetFileNames(assetInfo) {
+								// Strip the @_@ extension-masking pattern from asset names.
+								// The @_@ pattern is an internal mechanism for virtual module IDs and should not leak into output filenames.
+								const name = assetInfo.names?.[0] ?? '';
+								if (name.includes(ASTRO_PAGE_EXTENSION_POST_PATTERN)) {
+									const [sanitizedName] = name.split(ASTRO_PAGE_EXTENSION_POST_PATTERN);
+									return `${settings.config.build.assets}/${sanitizedName}.[hash][extname]`;
+								}
+								return `${settings.config.build.assets}/[name].[hash][extname]`;
+							},
 							...viteConfig.environments?.client?.build?.rollupOptions?.output,
 						},
 					},

packages/astro/test/css-deduplication.test.ts

@@ -19,8 +19,10 @@ describe('CSS deduplication for hydrated components', () => {
 		it('should not duplicate CSS for hydrated components', async () => {
 			const assets = await fixture.readdir('/_astro');
 
-			// Generated file for Counter.css (filename format changed in main-next)
-			const COUNTER_CSS_PATH = '/_astro/index@_@astro.DbgLc3FE.css';
+			// Generated file for Counter.css — find it dynamically since the hash may change
+			const counterCssFile = assets.find((f) => f.startsWith('index.') && f.endsWith('.css'));
+			assert.ok(counterCssFile, 'Expected a CSS file starting with "index."');
+			const COUNTER_CSS_PATH = `/_astro/${counterCssFile}`;
 			let file = await fixture.readFile(COUNTER_CSS_PATH);
 			file = file.replace(/\s+/g, '');