Adapter type

@auth/fauna-adapter, @auth/mongodb-adapter

Environment

Env Info:

  System:
    OS: macOS 26.0.1
    CPU: (14) arm64 Apple M4 Max
    Memory: 382.17 MB / 36.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 23.6.0 - /Users/phoenix/.nvm/versions/node/v23.6.0/bin/node
    npm: 11.6.2 - /Users/phoenix/.nvm/versions/node/v23.6.0/bin/npm
    pnpm: 10.20.0 - /Users/phoenix/.nvm/versions/node/v23.6.0/bin/pnpm
    bun: 1.3.2 - /Users/phoenix/.bun/bin/bun
  Browsers:
    Chrome: 142.0.7444.135
    Chrome Canary: 144.0.7524.0
    Firefox: 142.0.1
    Safari: 26.0.1

Project Info:

macOs 26.0.1

[create better-t-stack](https://github.com/AmanVarshney01/create-better-t-stack)

better-auth 1.3.28
Mongodb 7.0.0
Mongoose 8.19.3
next 16.0.0
typescript 5.9.3

Reproduction URL

https://github.com/skipzero/chat-app/tree/user-list

Describe the issue

trying to sign up or login with existing account results in the bson error
on sign up...

server log...

same log on signing in if there's already an account created.

How to reproduce

I was getting warnings about mongo being an external package and conflicting version so I got mongo and mongoose to both have matching versions across the project and not throw errors/warnings. Now I get the bson issue.

Clone the repo/branch above and install regularly (npm i, pnpm..., etc). Navigate to the signup/sign in pages and enter requested info and observe the server console throw the error and nothing is added to the mongoDB.
I'm using a local copy of mongo at localhost:3001/27017. Environmental variables are...

DATABASE_URL=mongodb://localhost:27017/<DB_NAME>
BETTER_AUTH_SECRET=<AUTH_SECRET>
BETTER_AUTH_URL=http://localhost:3001
CORS_ORIGIN=http://localhost:3001

Expected behavior

I would expect a new user added if I were signing up and a new session initialised if I were signing in.