Built-in Browser pane blocks Vercel-hosted subresources (net::ERR_BLOCKED_BY_CLIENT)

Description

The built-in Browser pane tool (mcp__Claude_Browser__*) fails to load nearly all subresources (images, CSS, JS chunks) on Vercel-hosted sites, while the top-level HTML document loads fine (HTTP 200). This happens even when navigating directly to the affected asset URL as a new top-level navigation, which succeeds — the blocking only occurs when the resource is requested as a subresource of a page.

Steps to reproduce

1. Open the Browser pane and navigate to any Vercel-hosted Next.js site, e.g. https://nextjs.org (Vercel's own official site) or any custom *.vercel.app / custom-domain Vercel deployment.
2. Check console messages — nearly every asset request fails with net::ERR_BLOCKED_BY_CLIENT.
3. Navigate directly to one of the blocked asset URLs (e.g. an image under the same domain) as its own top-level navigation — it loads successfully.
4. Navigate to a non-Vercel-hosted site (e.g. https://en.wikipedia.org) — no blocking occurs, page renders normally with full styling.

Expected behavior

Subresources on Vercel-hosted sites should load normally, same as on any other host.

Actual behavior

Console fills with repeated net::ERR_BLOCKED_BY_CLIENT errors for _next/static/css/*, _next/static/chunks/*, and other same-origin image assets. Pages render as unstyled plain HTML (no CSS, no images, no JS hydration).

Impact

This makes it impossible to visually verify Vercel-hosted deployments (a very common hosting target for Next.js apps) using the Browser pane's screenshot/rendering tools. Text-based tools (get_page_text, read_page) still work since the HTML document itself loads.

Environment

Observed via the Claude desktop app's built-in Browser pane (Code tab), testing a Next.js 15 App Router site deployed on Vercel with a custom domain, as well as nextjs.org itself as a control test.

Start in the built-in Browser pane using https://nextjs.org and compare its console errors with a direct top-level navigation to an affected asset. Reproduce the subresource failures for _next/static CSS, chunks, and images, then verify that Vercel-hosted pages render with styling, images, and JavaScript while non-Vercel sites remain unaffected.