Don't overwrite custom etag

.changeset/clever-readers-turn.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Preserve explicit ETag header

packages/kit/src/runtime/server/index.js

@@ -122,8 +122,9 @@ export async function respond(incoming, options, state = {}) {
 							: await render_page(request, route, match, options, state, ssr);
 
 					if (response) {
-						// inject ETags for 200 responses
-						if (response.status === 200) {
+						// inject ETags for 200 responses, if the endpoint
+						// doesn't have its own ETag handling
+						if (response.status === 200 && !response.headers.etag) {
 							const cache_control = get_single_valued_header(response.headers, 'cache-control');
 							if (!cache_control || !/(no-store|immutable)/.test(cache_control)) {
 								let if_none_match_value = request.headers['if-none-match'];

packages/kit/test/apps/basics/src/routes/etag/custom.js

@@ -0,0 +1,10 @@
+/** @type {import('@sveltejs/kit').RequestHandler} */
+export function get({ headers }) {
+	if (headers['if-none-match'] === '@1234@') return { status: 304 };
+	return {
+		body: `${Math.random()}`,
+		headers: {
+			etag: '@1234@'
+		}
+	};
+}

packages/kit/test/apps/basics/test/test.js

@@ -732,6 +732,20 @@ test.describe.parallel('ETags', () => {
 
 		expect(r2.status()).toBe(304);
 	});
+
+	test('custom etag', async ({ request }) => {
+		const r1 = await request.get('/etag/custom');
+		const etag = r1.headers()['etag'];
+		expect(etag).toBe('@1234@');
+
+		const r2 = await request.get('/etag/custom', {
+			headers: {
+				'if-none-match': '@1234@'
+			}
+		});
+
+		expect(r2.status()).toBe(304);
+	});
 });
 
 test.describe.parallel('Headers', () => {