[BUG] Expo Authorization Proxy returns 400 instead of redirect when deployed on Cloudflare via opennextjs-cloudflare

[imalec-huang]

Describe the bug

Hi, thanks for this project!

I’m deploying a Next.js 15 + better-auth + expo project using @opennextjs/cloudflare. Everything works fine when accessing /api/auth/... from a web client, including login/logout/check.

However, when using Expo together with better-auth/expo-plugin, Expo tries to request:

/api/auth/expo-authorization-proxy?authorizationURL=...

and Cloudflare always returns:

{
"code": "UNEXPECTED_ERROR",
"message": "Unexpected error"
}

instead of the expected 302 redirect.

What I tested

❌ production (Cloudflare)
fails with 400

❌localhost run command： opennextjs-cloudflare preview
same behavior as production (400)

✅ next dev
works perfectly (302 redirect)

This means the issue only happens on the Cloudflare adapter runtime, not in Next.js dev or better-auth itself.

Expected behavior

/api/auth/expo-authorization-proxy should perform a redirect (302), same as in development.

Actual behavior

400 Unexpected Error inside Cloudflare runtime

Versions

"@opennextjs/cloudflare": "^1.14.4"
"next": "15.5.7"
"better-auth": "^1.4.5"

Extra note

This only happens with Expo authorization plugin integration. Other authentication routes behave normally.

Thanks! (The redirect ghosts seem to vanish only in dev mode 😅)

Steps to reproduce

Expected behavior

@opennextjs/cloudflare version

1.14.4

Wrangler version

4.53.0

next info output

pnpm next info
npm warn Unknown env config "verify-deps-before-run". This will stop working in the next major version of npm.
npm warn Unknown env config "verify-deps-before-run". This will stop working in the next major version of npm.
This project is configured to use pnpm because /Users/imalec/workspaces/resolvai/resolvai-auth/package.json has a "packageManager" field
This project is configured to use pnpm because /Users/imalec/workspaces/resolvai/resolvai-auth/package.json has a "packageManager" field

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.1.0: Mon Oct 20 19:34:05 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T6041
  Available memory (MB): 65536
  Available CPU cores: 14
Binaries:
  Node: 22.17.0
  npm: 11.6.0
  Yarn: N/A
  pnpm: 10.25.0
Relevant Packages:
  next: 15.5.7 // An outdated version detected (latest is 16.0.8), upgrade is highly recommended!
  eslint-config-next: 15.5.4
  react: 19.2.1
  react-dom: 19.2.1
  typescript: 5.9.3
Next.js Config:
  output: standalone
 ⚠ An outdated version detected (latest is 16.0.8), upgrade is highly recommended!
   Please try the latest canary version (`npm install next@canary`) to confirm the issue still exists before creating a new issue.
   Read more - https://nextjs.org/docs/messages/opening-an-issue

Additional context

No response

[vicb]

Could you please attach a repro (link to a GH repository) with a minimal repro?