Fix endpoint response with immutable header (#9876)

* add test

* added runtime endpoint test for `new Response`

* added `try..catch` for reroute directive handling
Fixes #9871

* added changeset

* replaced `try..catch` with HTTP status code check
based on the suggestion of @lilnasy

* updated changeset description

* added more tests for the endpoint reroute header

* fixed grammar  in `renderEndpoint` comment

* updated endpoint tests to check for the reroute directive header in lower-case

* updated changeset description

3 files changed, 103 insertions, 1 deletions

diff --git a/.changeset/silver-peaches-pump.md b/.changeset/silver-peaches-pump.md
new file mode 100644
index 000000000..67bdfa4fd
--- /dev/null
+++ b/.changeset/silver-peaches-pump.md
@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Fixes an issue where using `Response.redirect` in an endpoint led to an error.
diff --git a/packages/astro/src/runtime/server/endpoint.ts b/packages/astro/src/runtime/server/endpoint.ts
index afafb21c7..49f00224e 100644
--- a/packages/astro/src/runtime/server/endpoint.ts
+++ b/packages/astro/src/runtime/server/endpoint.ts
@@ -42,6 +42,12 @@ export async function renderEndpoint(
 	const response = await handler.call(mod, context);
 	// Endpoints explicitly returning 404 or 500 response status should
 	// NOT be subject to rerouting to 404.astro or 500.astro.
-	response.headers.set(REROUTE_DIRECTIVE_HEADER, 'no');
+	if (response.status === 404 || response.status === 500) {
+		// Only `Response.redirect` headers are immutable, therefore a `try..catch` is not necessary.
+		// Note: `Response.redirect` can only be called with HTTP status codes: 301, 302, 303, 307, 308.
+		// Source: https://developer.mozilla.org/en-US/docs/Web/API/Response/redirect_static#parameters
+		response.headers.set(REROUTE_DIRECTIVE_HEADER, 'no');
+	}
+
 	return response;
 }
diff --git a/packages/astro/test/units/routing/endpoints.test.js b/packages/astro/test/units/routing/endpoints.test.js
new file mode 100644
index 000000000..fde7a81da
--- /dev/null
+++ b/packages/astro/test/units/routing/endpoints.test.js
@@ -0,0 +1,91 @@
+import {
+	createBasicSettings,
+	createFs,
+	createRequestAndResponse,
+	defaultLogger,
+} from '../test-utils.js';
+import { fileURLToPath } from 'node:url';
+import { expect } from 'chai';
+import { createContainer } from '../../../dist/core/dev/container.js';
+import testAdapter from '../../test-adapter.js';
+
+const root = new URL('../../fixtures/api-routes/', import.meta.url);
+const fileSystem = {
+	'/src/pages/response-redirect.ts': `export const GET = ({ url }) => Response.redirect("https://example.com/destination", 307)`,
+	'/src/pages/response.ts': `export const GET = ({ url }) => new Response(null, { headers: { Location: "https://example.com/destination" }, status: 307 })`,
+	'/src/pages/not-found.ts': `export const GET = ({ url }) => new Response('empty', { headers: { "Content-Type": "text/plain" }, status: 404 })`,
+	'/src/pages/internal-error.ts': `export const GET = ({ url }) => new Response('something went wrong', { headers: { "Content-Type": "text/plain" }, status: 500 })`,
+};
+
+describe('endpoints', () => {
+	let container;
+	let settings;
+
+	before(async () => {
+		const fs = createFs(fileSystem, root);
+		settings = await createBasicSettings({
+			root: fileURLToPath(root),
+			output: 'server',
+			adapter: testAdapter(),
+		});
+		container = await createContainer({
+			fs,
+			settings,
+			logger: defaultLogger,
+		});
+	});
+
+	after(async () => {
+		await container.close();
+	});
+
+	it('should return a redirect response with location header', async () => {
+		const { req, res, done } = createRequestAndResponse({
+			method: 'GET',
+			url: '/response-redirect',
+		});
+		container.handle(req, res);
+		await done;
+		const headers = res.getHeaders();
+		expect(headers).to.deep.include({ location: 'https://example.com/destination' });
+		expect(headers).not.to.deep.include({ 'x-astro-reroute': 'no' });
+		expect(res.statusCode).to.equal(307);
+	});
+
+	it('should return a response with location header', async () => {
+		const { req, res, done } = createRequestAndResponse({
+			method: 'GET',
+			url: '/response',
+		});
+		container.handle(req, res);
+		await done;
+		const headers = res.getHeaders();
+		expect(headers).to.deep.include({ location: 'https://example.com/destination' });
+		expect(headers).not.to.deep.include({ 'x-astro-reroute': 'no' });
+		expect(res.statusCode).to.equal(307);
+	});
+
+	it('should append reroute header for HTTP status 404', async () => {
+		const { req, res, done } = createRequestAndResponse({
+			method: 'GET',
+			url: '/not-found',
+		});
+		container.handle(req, res);
+		await done;
+		const headers = res.getHeaders();
+		expect(headers).to.deep.include({ 'x-astro-reroute': 'no' });
+		expect(res.statusCode).to.equal(404);
+	});
+
+	it('should append reroute header for HTTP status 500', async () => {
+		const { req, res, done } = createRequestAndResponse({
+			method: 'GET',
+			url: '/internal-error',
+		});
+		container.handle(req, res);
+		await done;
+		const headers = res.getHeaders();
+		expect(headers).to.deep.include({ 'x-astro-reroute': 'no' });
+		expect(res.statusCode).to.equal(500);
+	});
+});