[troubleshooting notes]: Specify the key file explicitly on the connect command: ssh -i ~/.ssh/lambda_key.pem ubuntu@[instance IP]. If it still fails, confirm the key you passed at instance launch matches the local private key (the public half must be on the instance). Permission denied (publickey) on Lambda almost always means the wrong local key, not a server-side problem.

Context: SSH to a Lambda (lambda.ai) GPU Cloud instance fails with Permission denied (publickey) right after launch, even though an SSH key was added when the instance was created. The default login user on Lambda instances is ubuntu, and the ssh client often picks the wrong local key.