Uncaught (in promise) TypeError: Failed to fetch - React Native | Convex | Better Auth

[damien-schneider]

Using setup with:

• React Native (with NativeWind)
• Convex
• Nextjs web app
• Better Auth

When setting up the environment variable and the convex cloud, I was able to make the nextjs web app work perfectly and make the authentication working well.

But when trying the same for the react native app, I get multiple errors, and cannot even create an account in the app. I get the following error:

Here is the .env of the react native application:

EXPO_PUBLIC_CONVEX_URL=https://posh-kangaroo-476.convex.cloud
EXPO_PUBLIC_CONVEX_SITE_URL=https://posh-kangaroo-476.convex.site

And there is ambiguity for the NATIVE_APP_URL, the SITE_URL and the backend/.env and the environment variables that we directly set in the convex dashboard UI

Other errors happening (but doesn't seem to impact directly the authentication):

Access to fetch at 'https://fantastic-cuttlefish-200.convex.site/api/auth/get-session' from origin 'http://localhost:8081' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

GET https://fantastic-cuttlefish-200.convex.site/api/auth/get-session net::ERR_FAILED 200 (OK)

Uncaught Error: Attempting to use a disconnected port object

Thanks a lot for your help by advance 🙏

[AmanVarshney01]

I believe you are using expo web 🤔

You will need to add localhost:8081 in trusted origins in auth.ts

Convex - Better Auth discord - https://discord.com/channels/1019350475847499849/1365754331873415440

Expo (React Native) | Convex + Better Auth https://share.google/xu6JkR4GZlWFwua1e

[damien-schneider]

Thanks! You were right: it was Expo Web. The Convex + Better Auth Expo guide solved it.

For anyone facing the same issue, follow this section:
https://convex-better-auth.netlify.app/framework-guides/expo#expo-web-support

Key notes:

• Update SITE_URL appropriately (for Expo Web dev, http://localhost:8081).
• Add localhost:8081 to trusted origins in convex/auth.ts.
• Enable CORS where needed (mentioned in the doc just above).
• Update src/lib/auth-client.ts to match Expo Web.

---

Proposal for better-t-stack

Add an “Expo Web” option under the Web Frontend section. When enabled:

• Force Native Frontend to React Native (the RN config).
• Scaffold a dual-ready template (Next.js + Expo Web + RN) that works out of the box by:
  • Preconfiguring convex/auth.ts with trusted origins (including http://localhost:8081) and { cors: true }.
  • Setting SITE_URL to http://localhost:8081 for Expo Web dev.
  • Wiring EXPO_PUBLIC_CONVEX_SITE_URL and EXPO_PUBLIC_CONVEX_URL consistently across envs.
  • Providing an auth-client.ts variant that handles Expo Web vs native.

This would reduce ambiguity around NATIVE_APP_URL/SITE_URL and the Convex dashboard variables, and make the RN + Expo Web + Next.js flow seamless. What do you think?