Atlas error: transaction numbers are only allowed on a replica set member or mongos

"Transaction numbers are only allowed on a replica set member or mongos"

This error means the driver connected directly to one mongod and bypassed replica set semantics. Transactions need the replica set (or mongos).

Confirm

1. Check the connection string for directConnection=true. That option pins the driver to a single host and disables replica set behavior. It is the usual culprit.
2. Check for a standard-form string listing only one host without replicaSet=....
3. Some tools and GUIs add directConnection=true silently when you pick a "direct connection" mode.

Fix

Remove directConnection=true and, for standard-form strings, include the full host list plus replicaSet=[name]. The SRV string handles this automatically. Keep retryWrites=true.

Verify

Run a trivial multi-document transaction (insert two docs, commit). Success proves replica set semantics are back.

Transaction prerequisites (replica set or mongos, retryWrites) per https://www.mongodb.com/docs/manual/core/transactions/, verified 2026-09-26.