How to fix Agent admitted failure to sign using the key in SSH

First, verify your SSH agent is running and load your key:

eval "$(ssh-agent -s)"
ssh-add

This starts the SSH agent in the background and adds your default SSH key (~/.ssh/id_rsa). Test your connection:

ssh -vT [email protected]

If you see "Hi username! You've successfully authenticated," the issue is resolved.

If your key isn't in the default location, explicitly add it:

eval "$(ssh-agent -s)"
ssh-add ~/.ssh/my_custom_key

Replace my_custom_key with your actual key filename. You can add multiple keys:

ssh-add ~/.ssh/id_rsa
ssh-add ~/.ssh/id_ed25519
ssh-add ~/.ssh/github_personal_key

Verify all loaded keys with:

ssh-add -l

On Ubuntu, GNOME Keyring sometimes intercepts SSH requests but fails to handle certain key formats. Test if it's the problem:

SSH_AUTH_SOCK=0 ssh -vT [email protected]

If this works, GNOME Keyring is likely the culprit. To permanently fix this on Ubuntu:

1. Reboot your system
2. Open Unity's Dash (or Activities menu)
3. Search for and open "Startup Applications"
4. Uncheck "SSH Key Agent"
5. Reboot again

After reboot, run eval "$(ssh-agent -s)" and ssh-add to use the standard SSH agent instead.

To quickly test if the agent is the problem without making changes:

ssh -o IdentityAgent=none -vT [email protected]

This bypasses the SSH agent entirely. If this works, the issue is definitely with your agent configuration. You can make this permanent by adding it to ~/.ssh/config:

Host github.com
  IdentityAgent none

However, this is a workaround—investigate and fix the actual agent issue for better security and flexibility.

To ensure the SSH agent runs every time you open a terminal, add this to your ~/.bashrc or ~/.zshrc:

if [ -z "$SSH_AUTH_SOCK" ]; then
  eval "$(ssh-agent -s)"
  ssh-add
fi

This checks if an agent is already running before starting a new one, preventing multiple agent processes. Reload your shell:

source ~/.bashrc  # or source ~/.zshrc

Now your SSH keys will be automatically loaded when you open a new terminal.