@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fix routing with base paths when trailingSlash is set to 'never'. This ensures requests to '/base' are correctly matched when the base path is set to '/base', without requiring a trailing slash.
@@ -52,28 +52,45 @@ export function findRouteToRewrite({
 	}
 
 	let pathname = newUrl.pathname;
-
 	const shouldAppendSlash = shouldAppendForwardSlash(trailingSlash, buildFormat);
+	
+	// Special handling for base path
+	if (base !== '/') {
+		// Check if this is a request to the base path
+		const isBasePathRequest = newUrl.pathname === base || 
+			newUrl.pathname === removeTrailingForwardSlash(base);
 
-	if (base !== '/' && newUrl.pathname.startsWith(base)) {
-		pathname = shouldAppendSlash
-			? appendForwardSlash(newUrl.pathname)
-			: removeTrailingForwardSlash(newUrl.pathname);
-		pathname = pathname.slice(base.length);
+		if (isBasePathRequest) {
+			// For root path requests at the base URL
+			// When trailingSlash is 'never', we should match '' (empty string pathname)
+			// When trailingSlash is 'always', we should match '/' pathname
+			pathname = shouldAppendSlash ? '/' : '';
+		} else if (newUrl.pathname.startsWith(base)) {
+			// For non-root paths under the base
+			pathname = shouldAppendSlash
+				? appendForwardSlash(newUrl.pathname)
+				: removeTrailingForwardSlash(newUrl.pathname);
+			pathname = pathname.slice(base.length);
+		}
 	}
 
+	// Ensure pathname starts with '/' when needed
 	if (!pathname.startsWith('/') && shouldAppendSlash && newUrl.pathname.endsWith('/')) {
-		// when base is in the rewrite call and trailingSlash is 'always' this is needed or it will 404.
 		pathname = prependForwardSlash(pathname);
 	}
 
-	if (pathname === '/' && base !== '/' && !shouldAppendSlash) {
-		// when rewriting to index and trailingSlash is 'never' this is needed or it will 404
-		// in this case the pattern will look for '/^$/' so '/' will never match
+	// Convert '/' to '' for trailingSlash: 'never'
+	if (pathname === '/' && !shouldAppendSlash) {
 		pathname = '';
 	}
 
-	newUrl.pathname = joinPaths(...[base, pathname].filter(Boolean));
+	// Set the final URL pathname
+	if (base !== '/' && (pathname === '' || pathname === '/') && !shouldAppendSlash) {
+		// Special case for root path at base URL with trailingSlash: 'never'
+		newUrl.pathname = removeTrailingForwardSlash(base);
+	} else {
+		newUrl.pathname = joinPaths(...[base, pathname].filter(Boolean));
+	}
 
 	const decodedPathname = decodeURI(pathname);
 	let foundRoute;
 
@@ -15,12 +15,15 @@ const fileSystem = {
 };
 
 describe('trailingSlash', () => {
+	let fixture;
 	let container;
-	let settings;
+	let baseContainer;
 
 	before(async () => {
-		const fixture = await createFixture(fileSystem);
-		settings = await createBasicSettings({
+		fixture = await createFixture(fileSystem);
+
+		// Create the first container with trailingSlash: 'always'
+		const settings = await createBasicSettings({
 			root: fixture.path,
 			trailingSlash: 'always',
 			output: 'server',
@@ -47,12 +50,44 @@ describe('trailingSlash', () => {
 			settings,
 			logger: defaultLogger,
 		});
+
+		// Create the second container with base path and trailingSlash: 'never'
+		const baseSettings = await createBasicSettings({
+			root: fixture.path,
+			trailingSlash: 'never',
+			base: 'base',
+			output: 'server',
+			adapter: testAdapter(),
+			integrations: [
+				{
+					name: 'test',
+					hooks: {
+						'astro:config:setup': ({ injectRoute }) => {
+							injectRoute({
+								pattern: '/',
+								entrypoint: './src/pages/api.ts',
+							});
+							injectRoute({
+								pattern: '/injected',
+								entrypoint: './src/pages/api.ts',
+							});
+						},
+					},
+				},
+			],
+		});
+		baseContainer = await createContainer({
+			settings: baseSettings,
+			logger: defaultLogger,
+		});
 	});
 
 	after(async () => {
 		await container.close();
+		await baseContainer.close();
 	});
 
+	// Tests for trailingSlash: 'always'
 	it('should match the API route when request has a trailing slash', async () => {
 		const { req, res, text } = createRequestAndResponse({
 			method: 'GET',
@@ -124,4 +159,26 @@ describe('trailingSlash', () => {
 		const json = await text();
 		assert.equal(json, '{"success":true}');
 	});
+
+	// Tests for trailingSlash: 'never' with base path
+	it('should not have trailing slash on root path when base is set and trailingSlash is never', async () => {
+		const { req, res, text } = createRequestAndResponse({
+			method: 'GET',
+			url: '/base',
+		});
+		baseContainer.handle(req, res);
+		const json = await text();
+		assert.equal(json, '{"success":true}');
+	});
+
+	it('should not match root path with trailing slash when base is set and trailingSlash is never', async () => {
+		const { req, res, text } = createRequestAndResponse({
+			method: 'GET',
+			url: '/base/',
+		});
+		baseContainer.handle(req, res);
+		const html = await text();
+		assert.equal(html.includes(`<span class="statusMessage">Not found</span>`), true);
+		assert.equal(res.statusCode, 404);
+	});
 });
-Original file line number
+Diff line change
@@ @@ -0,0 +1,5 @@ @@
 +---
 +'astro': patch
 +---
++
 +Fix routing with base paths when trailingSlash is set to 'never'. This ensures requests to '/base' are correctly matched when the base path is set to '/base', without requiring a trailing slash.