fix: omit ISR data endpoints for server-only routes

.changeset/calm-bats-build.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+fix: omit ISR data endpoints for server-only routes

packages/adapter-vercel/index.js

@@ -238,6 +238,7 @@ const plugin = function (defaults = {}) {
 				if (isr) {
 					const isr_name = route.id.slice(1) || '__root__'; // should we check that __root__ isn't a route?
 					const base = `${dirs.functions}/${isr_name}`;
+					const has_page = route.page.methods.length > 0;
 					fs.mkdirSync(base, { recursive: true });
 
 					const target = `${dirs.functions}/${name}.func`;
@@ -246,7 +247,9 @@ const plugin = function (defaults = {}) {
 					// create a symlink to the actual function, but use the
 					// route name so that we can derive the correct URL
 					fs.symlinkSync(relative, `${base}.func`);
-					fs.symlinkSync(`../${relative}`, `${base}/__data.json.func`);
+					if (has_page) {
+						fs.symlinkSync(`../${relative}`, `${base}/__data.json.func`);
+					}
 
 					const pathname = get_pathname(route);
 					const json = JSON.stringify(
@@ -256,7 +259,9 @@ const plugin = function (defaults = {}) {
 					);
 
 					write(`${base}.prerender-config.json`, json);
-					write(`${base}/__data.json.prerender-config.json`, json);
+					if (has_page) {
+						write(`${base}/__data.json.prerender-config.json`, json);
+					}
 
 					const q = `?__pathname=/${pathname}`;
 
@@ -265,10 +270,12 @@ const plugin = function (defaults = {}) {
 						dest: `/${isr_name}${q}`
 					});
 
-					static_config.routes.push({
-						src: src + '/__data.json$',
-						dest: `/${isr_name}/__data.json${q}`
-					});
+					if (has_page) {
+						static_config.routes.push({
+							src: src + '/__data.json$',
+							dest: `/${isr_name}/__data.json${q}`
+						});
+					}
 				} else {
 					// Create a symlink for each route to the main function for better observability
 					// (without this, every request appears to go through `/![-]`)

packages/adapter-vercel/test/apps/basic/assert-build-output.js

@@ -0,0 +1,45 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+
+const output = '.vercel/output';
+const functions = `${output}/functions/api/json`;
+
+assert(fs.existsSync(`${functions}.func`), 'expected the API route function to be generated');
+assert(
+	fs.existsSync(`${functions}.prerender-config.json`),
+	'expected the API route ISR configuration to be generated'
+);
+
+assert(
+	!fs.existsSync(`${functions}/__data.json.func`),
+	'did not expect a data function for a server-only route'
+);
+assert(
+	!fs.existsSync(`${functions}/__data.json.prerender-config.json`),
+	'did not expect data ISR configuration for a server-only route'
+);
+
+/** @type {{ routes: Array<{ src?: string }> }} */
+const config = JSON.parse(fs.readFileSync(`${output}/config.json`, 'utf8'));
+const route_sources = config.routes.flatMap((route) =>
+	typeof route.src === 'string' ? [route.src] : []
+);
+const api_route_sources = route_sources.filter((src) => src.includes('/api/json'));
+const isr_page_route_sources = route_sources.filter((src) => src.includes('/isr'));
+assert(
+	api_route_sources.some((src) => !src.includes('__data.json')),
+	'expected the API route to be present in the Vercel routing configuration'
+);
+assert(
+	!api_route_sources.some((src) => src.includes('__data.json')),
+	'did not expect a data endpoint for a server-only route'
+);
+
+assert(
+	fs.existsSync(`${output}/functions/isr/__data.json.func`),
+	'expected a data function for an ISR page route'
+);
+assert(
+	isr_page_route_sources.some((src) => src.includes('__data.json')),
+	'expected the ISR page data endpoint to remain in the Vercel routing configuration'
+);

packages/adapter-vercel/test/apps/basic/package.json

@@ -5,7 +5,7 @@
 	"scripts": {
 		"dev": "vite dev",
 		"preview": "vite preview",
-		"build": "vite build",
+		"build": "vite build && node assert-build-output.js",
 		"test:platform": "playwright test"
 	},
 	"devDependencies": {

packages/adapter-vercel/test/apps/basic/src/routes/api/json/+server.ts

@@ -1,5 +1,11 @@
 import { json } from '@sveltejs/kit';
 
+export const config = {
+	isr: {
+		expiration: 60
+	}
+};
+
 export function GET() {
 	return json({ ok: true });
 }