From 6f92832bd35e294d2c77c3efbdbc95afa0c64a1e Mon Sep 17 00:00:00 2001
From: astrobot-houston <fred+astrobot@astro.build>
Date: Tue, 30 Jun 2026 18:14:13 +0000
Subject: [PATCH] fix: prevent trailing slash on dynamic file endpoints during
 build (#17241)\n\nOverride trailingSlash to 'never' in stringifyParams for
 endpoint routes\nwith file extensions, matching the route pattern generated
 by\ntrailingSlashForPath in create-manifest.ts. This fixes a regression
 from\n#17224 where dynamic file endpoints (e.g. [name].json.ts)
 with\ntrailingSlash: 'always' would fail to build with TypeError:
 Missing\nparameter."

---
 .changeset/dark-melons-boil.md                |  5 +++
 packages/astro/src/core/routing/params.ts     |  9 +++-
 .../test/units/routing/generator.test.ts      | 43 +++++++++++++++++++
 3 files changed, 56 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/dark-melons-boil.md

diff --git a/.changeset/dark-melons-boil.md b/.changeset/dark-melons-boil.md
new file mode 100644
index 000000000000..2a6f032c46cc
--- /dev/null
+++ b/.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"`
diff --git a/packages/astro/src/core/routing/params.ts b/packages/astro/src/core/routing/params.ts
index 0f9bae073cfb..1595b6cba1fc 100644
--- a/packages/astro/src/core/routing/params.ts
+++ b/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);
 }
diff --git a/packages/astro/test/units/routing/generator.test.ts b/packages/astro/test/units/routing/generator.test.ts
index 32d0fe72a16d..3e3c7c929c4b 100644
--- a/packages/astro/test/units/routing/generator.test.ts
+++ b/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');
+	});
+});