Fix build failure for dynamic file endpoints with trailingSlash "always"

.changeset/dark-melons-boil.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fixes a build failure for dynamic file endpoints (e.g. `[name].json.ts`) when `trailingSlash` is set to `"always"`

packages/astro/src/core/routing/params.ts

@@ -1,3 +1,4 @@
+import { hasFileExtension } from '@astrojs/internal-helpers/path';
 import type { GetStaticPathsItem } from '../../types/public/common.js';
 import type { AstroConfig } from '../../types/public/index.js';
 import type { RouteData } from '../../types/public/internal.js';
@@ -15,6 +16,12 @@ export function stringifyParams(
 	route: RouteData,
 	trailingSlash: AstroConfig['trailingSlash'],
 ) {
+	// File endpoints (e.g. [name].json.ts) should never have a trailing slash,
+	// even when trailingSlash is set to 'always'. This matches the route pattern
+	// generated by trailingSlashForPath in create-manifest.ts.
+	const effectiveTrailingSlash =
+		route.type === 'endpoint' && hasFileExtension(route.route) ? 'never' : trailingSlash;
+
 	// validate parameter values then stringify each value
 	const validatedParams: Record<string, string> = {};
 	for (const [key, value] of Object.entries(params)) {
@@ -24,5 +31,5 @@ export function stringifyParams(
 		}
 	}
 
-	return getRouteGenerator(route.segments, trailingSlash)(validatedParams);
+	return getRouteGenerator(route.segments, effectiveTrailingSlash)(validatedParams);
 }

packages/astro/test/units/routing/generator.test.ts

@@ -3,6 +3,8 @@ import { describe, it } from 'node:test';
 import type { AstroConfig } from '../../../dist/types/public/config.js';
 import type { RoutePart } from '../../../dist/types/public/internal.js';
 import { getRouteGenerator } from '../../../dist/core/routing/generator.js';
+import { stringifyParams } from '../../../dist/core/routing/params.js';
+import { dynamicPart, makeRoute, staticPart } from './test-helpers.ts';
 
 interface TestCase {
 	routeData: RoutePart[][];
@@ -160,3 +162,44 @@ describe('routing - generator', () => {
 		assert.throws(() => generator({}), TypeError);
 	});
 });
+
+describe('stringifyParams - file endpoint trailing slash', () => {
+	it('does not append trailing slash to dynamic file endpoints with trailingSlash always (issue #17241)', () => {
+		const route = makeRoute({
+			segments: [[staticPart('api')], [dynamicPart('name'), staticPart('.json')]],
+			trailingSlash: 'never',
+			route: '/api/[name].json',
+			pathname: undefined,
+			type: 'endpoint',
+		});
+
+		const result = stringifyParams({ name: 'foo' }, route, 'always');
+		assert.equal(result, '/api/foo.json', 'should not have trailing slash');
+	});
+
+	it('still appends trailing slash to dynamic page routes with trailingSlash always', () => {
+		const route = makeRoute({
+			segments: [[staticPart('blog')], [dynamicPart('slug')]],
+			trailingSlash: 'always',
+			route: '/blog/[slug]',
+			pathname: undefined,
+			type: 'page',
+		});
+
+		const result = stringifyParams({ slug: 'hello' }, route, 'always');
+		assert.equal(result, '/blog/hello/', 'should have trailing slash');
+	});
+
+	it('does not append trailing slash to dynamic endpoints without file extensions with trailingSlash never', () => {
+		const route = makeRoute({
+			segments: [[staticPart('api')], [dynamicPart('name')]],
+			trailingSlash: 'never',
+			route: '/api/[name]',
+			pathname: undefined,
+			type: 'endpoint',
+		});
+
+		const result = stringifyParams({ name: 'foo' }, route, 'never');
+		assert.equal(result, '/api/foo', 'should not have trailing slash');
+	});
+});