When you install a server, your client (Claude Code, Cursor, etc.) ends up with the upstream's endpoint written into its own config and talks to it directly. No proxy, no status rewriting, x402 is whatever the upstream sends, unmodified, and your client-side payment handler runs natively.
So paid third-party MCP servers can absolutely negotiate through mcp.hosting-installed configs, and the hosted-proxy path doesn't break x402.