git push / git clone fails with 'git@github.com: Permission denied (publickey). fatal: Could not read from remote repository.' How do I fix SSH key authentication?

Cause: the server (GitHub, GitLab or another git host) didn't accept any SSH key your client offered. Usual reasons: no key exists, the key isn't loaded in ssh-agent, the public key was never added to your account (or was added to a different account), the wrong key is offered for this host, file permissions on ~/.ssh are too open, or the command ran as a different user (for example with sudo, in a container or a CI job) that has no keys. How to check: - ssh -T git@github.com (or git@gitlab.com). Success prints a greeting with your username; add -v to see which key files are tried and which one is rejected. - ssh-add -l lists the keys loaded in the agent. - git remote -v shows whether the remote uses SSH (git@...) or HTTPS. Fix: 1. Create a key if you have none: ssh-keygen -t ed25519 -C "you@example.com". 2. Load it: eval "$(ssh-agent -s)" then ssh-add ~/.ssh/id_ed25519. 3. Add the PUBLIC key (~/.ssh/id_ed25519.pub) to your account's SSH keys on the host, or as a deploy key on the repo (enable write access if you need to push). Never paste the private key anywhere. 4. If you have several keys, pin the right one in ~/.ssh/config: Host github.com IdentityFile ~/.ssh/id_ed25519 IdentitiesOnly yes (For a one-off: GIT_SSH_COMMAND="ssh -i ~/.ssh/key -o IdentitiesOnly=yes" git push.) 5. Fix permissions: chmod 700 ~/.ssh and chmod 600 on the private key. 6. Don't run git with sudo; root has its own ~/.ssh. In CI, load the key through the CI's secret store or use the platform's token-based checkout. 7. If SSH port 22 is blocked on your network, GitHub accepts SSH on port 443 at ssh.github.com, or switch the remote to HTTPS with a token. Verify: ssh -T git@github.com shows your username, then retry the push or clone. Sources: https://docs.github.com/en/authentication/troubleshooting-ssh/error-permission-denied-publickey https://docs.gitlab.com/user/ssh_troubleshooting/ https://git-scm.com/book/en/v2/Git-on-the-Server-Generating-Your-SSH-Public-Key