Development instance keys return 400 on Vercel preview deployments after upgrading to @clerk/nextjs 6.38.0

[chrisanderss0n]

Description

After upgrading @clerk/nextjs from 6.37.2 to 6.38.0 (via Dependabot), Clerk authentication on Vercel preview deployments using development instance keys (pk_test_*) stopped working. The browser console shows:

Clerk: Clerk has been loaded with development keys.
Failed to load resource: the server responded with a status of 400 ()

This was working on 6.37.2 with the same configuration and environment variables. Downgrading back to 6.37.2 resolves the issue.

Steps to reproduce

1. Create a Next.js app with @clerk/nextjs using development instance keys (pk_test_* / sk_test_*)
2. Deploy to Vercel with Preview environment using the development keys
3. Works on @clerk/nextjs@6.37.2
4. Upgrade to @clerk/nextjs@6.38.0
5. Visit the Vercel preview URL (e.g. *.vercel.app) — Clerk returns 400

Expected behavior

Development instance keys should work on Vercel preview deployments (*.vercel.app domains), as documented in Set up a preview environment:

"Configure the preview environment to use development API keys from Clerk" with "your host's provided preview domain"

Actual behavior

Clerk returns a 400 error during authentication. The app cannot load the Clerk client.

Environment

• @clerk/nextjs: 6.38.0 (broken), 6.37.2 (works)
• next: 16.1.6
• Deployment: Vercel preview (default *.vercel.app domain)
• Clerk instance: Development (using pk_test_* keys)

Suspected cause

The 6.38.0 release notes include a fix for "infinite handshake redirect loop when deploying Next.js apps with Clerk development instances to Netlify." This handshake fix may have tightened domain validation in a way that rejects *.vercel.app preview URLs for development instances.