This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-20).

Problem

When basePath is configured in next.config.js, vinext does not properly enforce path scoping. Multiple sub-issues:

1. Routes/rewrites/redirects fire without the basePath prefix. A rewrite with default basePath: true should only match paths prefixed with the basePath (e.g., /docs). vinext matches them without the prefix, so requesting /rewrite-1 returns 200 instead of 404.

2. basePath: false external rewrites not implemented. Rewrites pointing to external URLs (https://example.com) with basePath: false should proxy the external content. vinext matches the [slug] dynamic route instead.

3. Static assets under basePath return 404. Fetching [basePath]/_next/static/... returns 404 because the static asset layer doesn't account for the basePath prefix.

4. basePath double-applied in asset URLs. HTML output contains /docs/docs/_next/static/... instead of /docs/_next/static/..., indicating the basePath is applied twice.

5. Routes outside basePath not properly rejected. Requesting /hello directly (without /docs prefix) should 404 when basePath: '/docs', but vinext serves the page.

Expected: 404
Received: 200  // for /rewrite-1 without basePath prefix

Expected: "Example Domain"
Received: "...<p id=\"slug\">slug: rewrite-no-basePath</p>..."

Expected: 200
Received: 404  // for /docs/_next/static/...

Estimated Impact

~43 test failures across the deploy suite.

Affected Test Suites

• test/e2e/basepath/query-hash.test.ts (10 failures)
• test/e2e/basepath/error-pages.test.ts (6 failures)
• test/e2e/app-dir/app-basepath/index.test.ts (5 failures)
• test/e2e/basepath/redirect-and-rewrite.test.ts (4 failures)
• test/e2e/basepath/router-events.test.ts (4 failures)
• test/e2e/invalid-static-asset-404-pages/*-base-path.test.ts (3 failures)
• test/e2e/invalid-static-asset-404-app/*-base-path.test.ts (2 failures)
• test/e2e/i18n-ignore-redirect-source-locale/redirects-with-basepath.test.ts (4 failures)
• test/e2e/next-form/basepath/next-form-basepath.test.ts (1 failure)

Recommendation

1. Reproduce first in vinext's own test suite. Add tests that configure basePath: '/docs' and verify: (a) routes without the prefix return 404, (b) rewrites/redirects only match with the prefix, (c) static assets serve correctly under the prefix. Confirm they fail.

2. Study Next.js basePath implementation. Search .nextjs-ref/packages/next/src/server/ for basePath to understand how it gates request handling.

3. Gate rewrites/redirects on basePath. When evaluating next.config.js rewrites and redirects, prepend the basePath to source patterns (unless basePath: false is set on the rule).

4. Fix static asset serving. Strip the basePath prefix before looking up assets in the _next/static directory.

5. Fix asset URL generation. Ensure the basePath is applied exactly once when generating <script> and <link> URLs in HTML output.

6. Implement external URL proxying. For rewrites with basePath: false pointing to absolute URLs, proxy the request to the external origin.