fix(sitemap): add .html extension to URLs when build.format is file

When `build.format` is set to `'file'`, output files have .html
extensions (e.g., about.html) but the sitemap was generating URLs
without extensions (e.g., /about instead of /about.html), resulting
in 404s when crawlers follow those links.

Fixes #15526

.changeset/sitemap-file-format.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/sitemap': patch
+---
+
+Fix sitemap URLs missing `.html` extension when `build.format` is set to `file`

packages/integrations/sitemap/src/index.ts

@@ -146,7 +146,11 @@ const createPlugin = (options?: SitemapOptions): AstroIntegration => {
 
 							const newUrl = new URL(fullPath, finalSiteUrl).href;
 
-							if (config.trailingSlash === 'never') {
+							if (config.build.format === 'file' && !newUrl.endsWith('/')) {
+								// When build.format is 'file', output files have .html extension
+								// (e.g., /about → /about.html), so sitemap URLs should match
+								urls.push(newUrl + '.html');
+							} else if (config.trailingSlash === 'never') {
 								urls.push(newUrl);
 							} else if (config.build.format === 'directory' && !newUrl.endsWith('/')) {
 								urls.push(newUrl + '/');

packages/integrations/sitemap/test/trailing-slash.test.js

@@ -41,13 +41,13 @@ describe('Trailing slash', () => {
 				await fixture.build();
 			});
 
-			it('URLs do not end with trailing slash', async () => {
+			it('URLs have .html extension', async () => {
 				const data = await readXML(fixture.readFile('/sitemap-0.xml'));
 				const urls = data.urlset.url;
 
 				assert.equal(urls[0].loc[0], 'http://example.com');
-				assert.equal(urls[1].loc[0], 'http://example.com/one');
-				assert.equal(urls[2].loc[0], 'http://example.com/two');
+				assert.equal(urls[1].loc[0], 'http://example.com/one.html');
+				assert.equal(urls[2].loc[0], 'http://example.com/two.html');
 			});
 		});
 	});