Carrier-Level DPI Blocks Can Silently Break Vercel Apps on Mobile Networks

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Vercel-hosted web apps can fail to load on certain mobile networks due to Deep Packet Inspection (DPI) firewall rules at the carrier's gateway, not developer error. When a browser initiates a TLS/HTTPS handshake, the carrier inspects the Server Name Indication (SNI) string and may throttle or drop packets tied to Vercel's shared IP blocks, causing indefinite timeouts. The issue is ISP-specific — the same app loads instantly on other networks — and even well-optimized, high-profile Vercel sites are affected. Developers can mitigate this by migrating from free *.vercel.app subdomains to custom domains and routing traffic through a reverse proxy such as Cloudflare to decouple user requests from Vercel's direct IPs. This approach helps prevent carrier-level blocks from locking out users without any changes to the underlying application code.

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