Reproduction steps

1. Install (branch main)

$ npx create-remix
? Where would you like to create your app? ./vercel-community#1587
? What type of app do you want to create? Just the basics
? Where do you want to deploy? Choose Remix App Server if you're unsure; it's easy to change deployment targets. Vercel
? TypeScript or JavaScript? TypeScript
? Do you want me to run `npm install`? No

2. Add __foo.tsx and modify index.tsx (branch __foo)

• Works well on local
• Build fails on VERCEL_CLI_VERSION=vercel@28.15.5 and above
• Build succeeds on VERCEL_CLI_VERSION=vercel@28.15.4 but has server error due to missing serverBuildTarget

3. Add serverBuildTarget: "vercel" (branch serverBuildTarget)

• Works well on VERCEL_CLI_VERSION=vercel@28.15.4