Atlas error: ServerSelectionTimeoutError, no suitable servers

ServerSelectionTimeoutError

Full text: ServerSelectionTimeoutError: [host]:[port] timed out or No suitable servers found. The driver found the cluster topology but could not establish a working connection to any node.

Confirm in order

1. IP access list: Security, Network Access. Is the client's egress IP listed? This is the cause in the large majority of Atlas cases. Note the client IP is the egress IP, which for CI, VPNs, and containers is rarely the IP you think.
2. Cluster state: is the cluster paused? A paused cluster refuses connections with timeouts, not a clear error.
3. DNS: can the machine resolve the SRV record? nslookup -type=SRV _mongodb._tcp.[cluster-host]. If not, use the standard connection string form.
4. Local firewall or proxy blocking outbound 27017.

Fix

Add the egress IP to the access list (with an expiry for temporary access), resume the cluster, or switch to the standard string. Then retry.

Verify

The startup ping succeeds from the failing network. If it works from your laptop but not the deploy target, the difference is the network path, and the IP access list is still suspect number one.

Server selection failure causes and the troubleshooting order (IP access list first, then cluster state, DNS, firewall) per https://www.mongodb.com/docs/atlas/troubleshoot-connection/ and https://www.mongodb.com/docs/atlas/security/ip-access-list/, verified 2026-09-26.