Permission denied (publickey)

Common causes

• Your public key is not added to the account (GitHub settings, or ~/.ssh/authorized_keys on the server).
• The client is offering the wrong key, or the key is not loaded in ssh-agent.
• Wrong username: e.g. ubuntu vs ec2-user on cloud images, or anything other than git@ for Git hosts.
• Server-side permissions on ~/.ssh or authorized_keys are too open, so sshd ignores them.

How to fix it

1. Run ssh -v user@host to see which keys are offered and whether one is accepted.
2. Add your key to the agent (ssh-add ~/.ssh/id_ed25519) or set IdentityFile for the host in ~/.ssh/config.
3. For GitHub, test with ssh -T git@github.com after adding the public key to your account.
4. On the server: chmod 700 ~/.ssh and chmod 600 ~/.ssh/authorized_keys.