diff --git a/packages/vinext/src/build/report.ts b/packages/vinext/src/build/report.ts
index b4d2c6135..d41e3760a 100644
--- a/packages/vinext/src/build/report.ts
+++ b/packages/vinext/src/build/report.ts
@@ -379,7 +379,7 @@ export function findDir(root: string, ...candidates: string[]): string | null {
  */
 export async function printBuildReport(options: {
   root: string;
-  pageExtensions?: string[];
+  pageExtensions: string[];
   prerenderResult?: PrerenderResult;
 }): Promise<void> {
   const { root } = options;
diff --git a/packages/vinext/src/build/run-prerender.ts b/packages/vinext/src/build/run-prerender.ts
index c84ae2b81..90745bfaa 100644
--- a/packages/vinext/src/build/run-prerender.ts
+++ b/packages/vinext/src/build/run-prerender.ts
@@ -192,7 +192,7 @@ export async function runPrerender(options: RunPrerenderOptions): Promise<Preren
 
     // ── App Router phase ──────────────────────────────────────────────────────
     if (appDir) {
-      const routes = await appRouter(appDir);
+      const routes = await appRouter(appDir, config.pageExtensions);
 
       // We don't know the exact render-queue size until prerenderApp starts, so
       // use the progress callback's `total` to update our combined total on the
@@ -224,8 +224,8 @@ export async function runPrerender(options: RunPrerenderOptions): Promise<Preren
     // ── Pages Router phase ────────────────────────────────────────────────────
     if (pagesDir) {
       const [pageRoutes, apiRoutes] = await Promise.all([
-        pagesRouter(pagesDir),
-        apiRouter(pagesDir),
+        pagesRouter(pagesDir, config.pageExtensions),
+        apiRouter(pagesDir, config.pageExtensions),
       ]);
 
       let pagesTotal = 0;
diff --git a/packages/vinext/src/cli.ts b/packages/vinext/src/cli.ts
index e801c81b9..75389d1d6 100644
--- a/packages/vinext/src/cli.ts
+++ b/packages/vinext/src/cli.ts
@@ -495,11 +495,10 @@ async function buildApp() {
     );
   }
 
+  const nextConfig = await resolveNextConfig(await loadNextConfig(process.cwd()), process.cwd());
+
   let prerenderResult;
-  const shouldPrerender =
-    parsed.prerenderAll ||
-    (await resolveNextConfig(await loadNextConfig(process.cwd()), process.cwd())).output ===
-      "export";
+  const shouldPrerender = parsed.prerenderAll || nextConfig.output === "export";
 
   if (shouldPrerender) {
     const label = parsed.prerenderAll
@@ -511,7 +510,11 @@ async function buildApp() {
   }
 
   process.stdout.write("\x1b[0m");
-  await printBuildReport({ root: process.cwd(), prerenderResult: prerenderResult ?? undefined });
+  await printBuildReport({
+    root: process.cwd(),
+    pageExtensions: nextConfig.pageExtensions,
+    prerenderResult: prerenderResult ?? undefined,
+  });
 
   console.log("\n  Build complete. Run `vinext start` to start the production server.\n");
   process.exit(0);
diff --git a/packages/vinext/src/routing/pages-router.ts b/packages/vinext/src/routing/pages-router.ts
index 43d0ec5be..91b712da1 100644
--- a/packages/vinext/src/routing/pages-router.ts
+++ b/packages/vinext/src/routing/pages-router.ts
@@ -217,7 +217,12 @@ async function scanApiRoutes(pagesDir: string, matcher: ValidFileMatcher): Promi
   let files: string[];
   try {
     files = [];
-    for await (const file of scanWithExtensions("**/*", apiDir, matcher.extensions)) {
+    for await (const file of scanWithExtensions(
+      "**/*",
+      apiDir,
+      matcher.extensions,
+      (name: string) => name.startsWith("_"),
+    )) {
       files.push(file);
     }
   } catch {
diff --git a/tests/build-report.test.ts b/tests/build-report.test.ts
index 76f79b4b7..3cd60950b 100644
--- a/tests/build-report.test.ts
+++ b/tests/build-report.test.ts
@@ -5,8 +5,10 @@
  * logic for both Pages Router and App Router routes, using real fixture files
  * where integration testing is needed.
  */
-import { describe, it, expect } from "vite-plus/test";
+import { describe, it, expect, afterEach } from "vite-plus/test";
 import path from "node:path";
+import os from "node:os";
+import fs from "node:fs/promises";
 import {
   hasNamedExport,
   extractExportConstString,
@@ -16,7 +18,10 @@ import {
   classifyAppRoute,
   buildReportRows,
   formatBuildReport,
+  printBuildReport,
 } from "../packages/vinext/src/build/report.js";
+import { invalidateAppRouteCache } from "../packages/vinext/src/routing/app-router.js";
+import { invalidateRouteCache } from "../packages/vinext/src/routing/pages-router.js";
 
 const FIXTURES_PAGES = path.resolve("tests/fixtures/pages-basic/pages");
 const FIXTURES_APP = path.resolve("tests/fixtures/app-basic/app");
@@ -453,3 +458,139 @@ describe("formatBuildReport", () => {
     expect(out).toContain("λ API  ƒ Dynamic  ◐ ISR  ○ Static");
   });
 });
+
+// ─── printBuildReport with pageExtensions ─────────────────────────────────────
+
+describe("printBuildReport respects pageExtensions", () => {
+  let tmpRoot: string;
+
+  afterEach(async () => {
+    if (tmpRoot) {
+      // Invalidate both routers' caches — pages router tests set pagesDir at
+      // tmpRoot/pages, so we invalidate that path too. This ensures a failing
+      // test that skips its own finally-block cleanup doesn't pollute later tests.
+      invalidateAppRouteCache();
+      invalidateRouteCache(path.join(tmpRoot, "pages"));
+      await fs.rm(tmpRoot, { recursive: true, force: true });
+    }
+  });
+
+  it("app router: only reports routes matching configured pageExtensions", async () => {
+    // Ported from Next.js MDX e2e pageExtensions behaviour:
+    // test/e2e/app-dir/mdx/next.config.ts
+    // https://github.com/vercel/next.js/blob/canary/test/e2e/app-dir/mdx/next.config.ts
+    tmpRoot = await fs.mkdtemp(path.join(os.tmpdir(), "vinext-report-app-"));
+    const appDir = path.join(tmpRoot, "app");
+    await fs.mkdir(path.join(appDir, "about"), { recursive: true });
+    await fs.writeFile(
+      path.join(appDir, "layout.tsx"),
+      "export default function Layout({ children }: { children: React.ReactNode }) { return <html><body>{children}</body></html>; }",
+    );
+    await fs.writeFile(
+      path.join(appDir, "page.tsx"),
+      "export default function Page() { return <div>home</div>; }",
+    );
+    // This .mdx page should be excluded when mdx is not in pageExtensions
+    await fs.writeFile(path.join(appDir, "about", "page.mdx"), "# About");
+
+    // Capture stdout output from printBuildReport
+    const lines: string[] = [];
+    const origLog = console.log;
+    console.log = (msg: string) => lines.push(msg);
+    try {
+      invalidateAppRouteCache();
+      await printBuildReport({ root: tmpRoot, pageExtensions: ["tsx", "ts", "jsx", "js"] });
+    } finally {
+      console.log = origLog;
+    }
+
+    const output = lines.join("\n");
+    // / should appear (page.tsx matches)
+    expect(output).toContain("/");
+    // /about should NOT appear (page.mdx excluded — mdx not in pageExtensions)
+    expect(output).not.toContain("/about");
+  });
+
+  it("app router: reports mdx routes when pageExtensions includes mdx", async () => {
+    tmpRoot = await fs.mkdtemp(path.join(os.tmpdir(), "vinext-report-app-mdx-"));
+    const appDir = path.join(tmpRoot, "app");
+    await fs.mkdir(path.join(appDir, "about"), { recursive: true });
+    await fs.writeFile(
+      path.join(appDir, "layout.tsx"),
+      "export default function Layout({ children }: { children: React.ReactNode }) { return <html><body>{children}</body></html>; }",
+    );
+    await fs.writeFile(
+      path.join(appDir, "page.tsx"),
+      "export default function Page() { return <div>home</div>; }",
+    );
+    await fs.writeFile(path.join(appDir, "about", "page.mdx"), "# About");
+
+    const lines: string[] = [];
+    const origLog = console.log;
+    console.log = (msg: string) => lines.push(msg);
+    try {
+      invalidateAppRouteCache();
+      await printBuildReport({ root: tmpRoot, pageExtensions: ["tsx", "ts", "jsx", "js", "mdx"] });
+    } finally {
+      console.log = origLog;
+    }
+
+    const output = lines.join("\n");
+    expect(output).toContain("/about");
+  });
+
+  it("pages router: only reports routes matching configured pageExtensions", async () => {
+    tmpRoot = await fs.mkdtemp(path.join(os.tmpdir(), "vinext-report-pages-"));
+    const pagesDir = path.join(tmpRoot, "pages");
+    await fs.mkdir(pagesDir, { recursive: true });
+    await fs.writeFile(
+      path.join(pagesDir, "index.tsx"),
+      "export default function Page() { return <div>home</div>; }",
+    );
+    // This .mdx page should be excluded when mdx is not in pageExtensions
+    await fs.writeFile(path.join(pagesDir, "about.mdx"), "# About");
+
+    const lines: string[] = [];
+    const origLog = console.log;
+    console.log = (msg: string) => lines.push(msg);
+    try {
+      invalidateRouteCache(pagesDir);
+      await printBuildReport({ root: tmpRoot, pageExtensions: ["tsx", "ts", "jsx", "js"] });
+    } finally {
+      console.log = origLog;
+      invalidateRouteCache(pagesDir);
+    }
+
+    const output = lines.join("\n");
+    expect(output).toContain("/");
+    expect(output).not.toContain("/about");
+  });
+
+  it("pages router: reports mdx routes when pageExtensions includes mdx", async () => {
+    tmpRoot = await fs.mkdtemp(path.join(os.tmpdir(), "vinext-report-pages-mdx-"));
+    const pagesDir = path.join(tmpRoot, "pages");
+    await fs.mkdir(pagesDir, { recursive: true });
+    await fs.writeFile(
+      path.join(pagesDir, "index.tsx"),
+      "export default function Page() { return <div>home</div>; }",
+    );
+    await fs.writeFile(path.join(pagesDir, "about.mdx"), "# About");
+
+    const lines: string[] = [];
+    const origLog = console.log;
+    console.log = (msg: string) => lines.push(msg);
+    try {
+      invalidateRouteCache(pagesDir);
+      await printBuildReport({
+        root: tmpRoot,
+        pageExtensions: ["tsx", "ts", "jsx", "js", "mdx"],
+      });
+    } finally {
+      console.log = origLog;
+      invalidateRouteCache(pagesDir);
+    }
+
+    const output = lines.join("\n");
+    expect(output).toContain("/about");
+  });
+});
diff --git a/tests/page-extensions-routing.test.ts b/tests/page-extensions-routing.test.ts
index 9176e3fbc..fa9d17d48 100644
--- a/tests/page-extensions-routing.test.ts
+++ b/tests/page-extensions-routing.test.ts
@@ -67,6 +67,30 @@ describe("pageExtensions route discovery", () => {
     }
   });
 
+  it("excludes _ prefixed files from api routes", async () => {
+    // Next.js ignores _-prefixed files in pages/api/ the same way it does in pages/.
+    const tmpRoot = await makeTempDir("vinext-api-underscore-");
+    const pagesDir = path.join(tmpRoot, "pages");
+    try {
+      await fs.mkdir(path.join(pagesDir, "api"), { recursive: true });
+      await fs.writeFile(
+        path.join(pagesDir, "api", "hello.ts"),
+        "export async function GET() { return Response.json({}); }",
+      );
+      await fs.writeFile(path.join(pagesDir, "api", "_helpers.ts"), "// internal helper");
+
+      invalidateRouteCache(pagesDir);
+      const apiRoutes = await apiRouter(pagesDir);
+      const patterns = apiRoutes.map((r) => r.pattern);
+
+      expect(patterns).toContain("/api/hello");
+      expect(patterns).not.toContain("/api/_helpers");
+    } finally {
+      await fs.rm(tmpRoot, { recursive: true, force: true });
+      invalidateRouteCache(pagesDir);
+    }
+  });
+
   it("discovers pages and api files using configured pageExtensions", async () => {
     const tmpRoot = await makeTempDir("vinext-pages-ext-mdx-");
     const pagesDir = path.join(tmpRoot, "pages");