Fixed periodic 500/502 eorrs [CLIENT_FETCH_ERROR] error decoding lambda response with MongoDB Adapter,

[Tasselhat]

I'm just posting this so if someone having the same issues I was finds it they will see this in search results hopefully, lost a day to trying to figure this out.
If you are using MongoDB Adapter and your auth is working but randomly loses connection to a 500/502 error and kicks the user back to the login page, but the session still exists in /api/auth/session

For example getting the error codes:
error decoding lambda response: error decoding lambda response: unexpected end of JSON input
[next-auth][error][CLIENT_FETCH_ERROR] https://next-auth.js.org/errors#client_fetch_error JSON.parse: unexpected character at line 1 column 1 of the JSON data

or in vercel logs

MongoServerSelectionError: Server selection timed out after 30000 ms at Timeout._onTimeout (/var/task/node_modules/mongodb/lib/sdam/topology.js:292:38) { reason: [TopologyDescription], code: undefined } Unhandled Rejection: MongoServerSelectionError: Server selection timed out after 30000 ms at Timeout._onTimeout (/var/task/node_modules/mongodb/lib/sdam/topology.js:292:38) { reason: [TopologyDescription], code: undefined } Node.js process exited with exit status: 128.

with correct .env variables and configurations, and properly allowing ips in mongoDB still getting these errors,

You are likely using an outdated mongodb.ts file from older documentation that sends clientPromise instead of a connected client and due to some weird issues with stale requests and caching it will occasionally send 500 erors

Refer to:
https://authjs.dev/getting-started/adapters/mongodb
vercel/vercel#11178
mongodb-developer/nextjs-with-mongodb#1