fix: do not generate observability functions with fully prerendered routes

src/presets/vercel/utils.ts

@@ -2,7 +2,7 @@ import fsp from "node:fs/promises";
 import { constants } from "node:fs";
 import { defu } from "defu";
 import { writeFile } from "../_utils/fs.ts";
-import type { Nitro, NitroRouteRules, ProxyRuleOptions } from "nitro/types";
+import type { Nitro, NitroRouteRules, PrerenderRoute, ProxyRuleOptions } from "nitro/types";
 import { basename, dirname, relative, resolve } from "pathe";
 import { Router } from "../../routing.ts";
 import { joinURL, withLeadingSlash, withoutLeadingSlash } from "ufo";
@@ -37,6 +37,12 @@ const ISR_SUFFIX = "-isr"; // Avoid using . as it can conflict with routing
 
 const SAFE_FS_CHAR_RE = /[^a-zA-Z0-9_.[\]/]/g;
 
+// Vercel serves `<dir>/index.html` (and extensionless `<dir>/index`) at `<dir>`
+// using built-in directory indexes.
+const INDEX_FILE_RE = /(^|\/)index(\.html)?$/;
+
+const SURROUNDING_SLASH_RE = /^\/+|\/+$/g;
+
 function getSystemNodeVersion() {
   const systemNodeVersion = Number.parseInt(process.versions.node.split(".")[0]);
 
@@ -229,17 +235,7 @@ function generateBuildConfig(nitro: Nitro, o11Routes?: ObservabilityRoute[]) {
       name: nitro.options.framework.name,
       version: nitro.options.framework.version,
     },
-    overrides: {
-      // Nitro static prerendered route overrides
-      ...Object.fromEntries(
-        (nitro._prerenderedRoutes?.filter((r) => r.fileName !== r.route) || []).map(
-          ({ route, fileName }) => [
-            withoutLeadingSlash(fileName),
-            { path: route.replace(/^\//, "") },
-          ]
-        )
-      ),
-    },
+    overrides: getPrerenderOverrides(nitro._prerenderedRoutes),
     routes: [
       // Redirect and header rules (excluding paths handled as CDN proxy rewrites)
       ...rules
@@ -390,6 +386,38 @@ function generateBuildConfig(nitro: Nitro, o11Routes?: ObservabilityRoute[]) {
   return config;
 }
 
+/**
+ * Map prerendered files to the route they should be served from.
+ *
+ * Paths are always slash-free: Vercel strips slashes when matching, so a path
+ * that keeps a trailing slash matches nothing at all (#4392), and the root
+ * route has to map to an empty path (ufo's slash helpers cannot produce one).
+ *
+ * Files that Vercel already serves at the route using its built-in directory
+ * indexes are skipped.
+ */
+export function getPrerenderOverrides(prerenderedRoutes: PrerenderRoute[] = []) {
+  const overrides: Record<string, { path: string }> = {};
+
+  for (const { route, fileName } of prerenderedRoutes) {
+    if (!fileName) {
+      continue;
+    }
+    const file = withoutLeadingSlash(fileName);
+    const path = route.replace(SURROUNDING_SLASH_RE, "");
+    // Skip when Vercel already serves the file at `path`: either via its
+    // built-in directory index (`<dir>/index.*` at `<dir>`), or because the
+    // file already lives there. Re-keying a file onto its own path would
+    // delete it, since Vercel drops the original entry.
+    if (file === path || file.replace(INDEX_FILE_RE, "") === path) {
+      continue;
+    }
+    overrides[file] = { path };
+  }
+
+  return overrides;
+}
+
 export function deprecateSWR(nitro: Nitro) {
   if (nitro.options.future.nativeSWR) {
     return;
@@ -500,13 +528,23 @@ type ObservabilityRoute = {
   dest: string; // function name
 };
 
-function getObservabilityRoutes(nitro: Nitro): ObservabilityRoute[] {
+export function getObservabilityRoutes(nitro: Nitro): ObservabilityRoute[] {
   const compatDate =
     nitro.options.compatibilityDate.vercel || nitro.options.compatibilityDate.default;
   if (compatDate < "2025-07-15") {
     return [];
   }
 
+  // Vercel resolves functions and static files from a single path to output
+  // map that functions are added to last, so a function at the path of a
+  // prerendered file hides that file and serves the route with SSR on every
+  // request (#4242).
+  const prerenderedPaths = new Set(
+    (nitro._prerenderedRoutes || [])
+      .filter((route) => route.fileName)
+      .map((route) => route.route.replace(SURROUNDING_SLASH_RE, ""))
+  );
+
   // Sort routes by how much specific they are
   const routePatterns = [
     ...new Set([
@@ -515,7 +553,7 @@ function getObservabilityRoutes(nitro: Nitro): ObservabilityRoute[] {
         .filter((h) => !h.middleware && h.route)
         .map((h) => h.route!),
     ]),
-  ];
+  ].filter((route) => !prerenderedPaths.has(route.replace(SURROUNDING_SLASH_RE, "")));
 
   const staticRoutes: string[] = [];
   const dynamicRoutes: string[] = [];

test/presets/fixtures/slash.ts

@@ -0,0 +1,6 @@
+import { defineHandler } from "nitro/h3";
+
+export default defineHandler((event) => {
+  event.res.headers.set("content-type", "text/html");
+  return "<!DOCTYPE html><html><body>slash</body></html>";
+});

test/presets/vercel.test.ts

@@ -21,7 +21,15 @@ describe("nitro:preset:vercel:web", async () => {
           route: "/_ws",
           handler: resolve(presetFixturesDir, "websocket.ts"),
         },
+        {
+          route: "/slash",
+          handler: resolve(presetFixturesDir, "slash.ts"),
+        },
       ],
+      prerender: {
+        // trailing slash on purpose (#4392)
+        routes: ["/slash/"],
+      },
       vercel: {
         queues: {
           triggers: [
@@ -63,20 +71,7 @@ describe("nitro:preset:vercel:web", async () => {
               "name": "nitro",
               "version": "3.x",
             },
-            "overrides": {
-              "_scalar/index.html": {
-                "path": "_scalar",
-              },
-              "_swagger/index.html": {
-                "path": "_swagger",
-              },
-              "api/hey/index.html": {
-                "path": "api/hey",
-              },
-              "prerender/index.html": {
-                "path": "prerender",
-              },
-            },
+            "overrides": {},
             "routes": [
               {
                 "headers": {
@@ -260,14 +255,6 @@ describe("nitro:preset:vercel:web", async () => {
                 "dest": "/raw",
                 "src": "/raw",
               },
-              {
-                "dest": "/prerender-custom.html",
-                "src": "/prerender-custom.html",
-              },
-              {
-                "dest": "/prerender",
-                "src": "/prerender",
-              },
               {
                 "dest": "/node-compat",
                 "src": "/node-compat",
@@ -280,10 +267,6 @@ describe("nitro:preset:vercel:web", async () => {
                 "dest": "/jsx",
                 "src": "/jsx",
               },
-              {
-                "dest": "/json-string",
-                "src": "/json-string",
-              },
               {
                 "dest": "/imports",
                 "src": "/imports",
@@ -360,14 +343,6 @@ describe("nitro:preset:vercel:web", async () => {
                 "dest": "/api/kebab",
                 "src": "/api/kebab",
               },
-              {
-                "dest": "/api/hey",
-                "src": "/api/hey",
-              },
-              {
-                "dest": "/api/hello",
-                "src": "/api/hello",
-              },
               {
                 "dest": "/api/headers",
                 "src": "/api/headers",
@@ -400,18 +375,6 @@ describe("nitro:preset:vercel:web", async () => {
                 "dest": "/_vercel/cron",
                 "src": "/_vercel/cron",
               },
-              {
-                "dest": "/_swagger",
-                "src": "/_swagger",
-              },
-              {
-                "dest": "/_scalar",
-                "src": "/_scalar",
-              },
-              {
-                "dest": "/_openapi.json",
-                "src": "/_openapi.json",
-              },
               {
                 "dest": "/single-headers/[id]",
                 "src": "/single-headers/(?<id>[^/]+)",
@@ -491,17 +454,13 @@ describe("nitro:preset:vercel:web", async () => {
           [
             "functions/500.func (symlink)",
             "functions/__server.func",
-            "functions/_openapi.json.func (symlink)",
-            "functions/_scalar.func (symlink)",
-            "functions/_swagger.func (symlink)",
             "functions/_vercel",
             "functions/_ws.func (symlink)",
             "functions/api/cached.func (symlink)",
             "functions/api/db.func (symlink)",
             "functions/api/echo.func",
             "functions/api/headers.func (symlink)",
             "functions/api/hello.func",
-            "functions/api/hey.func (symlink)",
             "functions/api/kebab.func (symlink)",
             "functions/api/meta/test.func (symlink)",
             "functions/api/methods/foo.get.func (symlink)",
@@ -526,12 +485,9 @@ describe("nitro:preset:vercel:web", async () => {
             "functions/icon.png.func (symlink)",
             "functions/import-attributes.func (symlink)",
             "functions/imports.func (symlink)",
-            "functions/json-string.func (symlink)",
             "functions/jsx.func (symlink)",
             "functions/modules.func (symlink)",
             "functions/node-compat.func (symlink)",
-            "functions/prerender-custom.html.func (symlink)",
-            "functions/prerender.func (symlink)",
             "functions/raw.func (symlink)",
             "functions/replace.func (symlink)",
             "functions/route-group.func (symlink)",