Git keeps asking me for my ssh key passphrase

Once you have started the SSH agent with:

eval $(ssh-agent)

Do either:

1. To add your private key to it:

    ssh-add
   

   This will ask you your passphrase just once, and then you should be allowed to push, provided that you uploaded the public key to Github.

2. To add and save your key permanently on macOS:

    ssh-add -K  
   

   This will persist it after you close and re-open it by storing it in user's keychain.

   If you see a warning about deprecated flags, try the new variant:

    ssh-add --apple-use-keychain 
   

3. To add and save your key permanently on Ubuntu (or equivalent):

     ssh-add ~/.ssh/id_rsa
   

This has been happening to me after restarts since upgrading from OS X El Capitan (10.11) to macOS Sierra (10.12). The ssh-add solution worked temporarily but would not persist across another restart.

The permanent solution was to edit (or create) ~/.ssh/config and enable the UseKeychain option.

Host *
    UseKeychain yes

Related: macOS keeps asking my ssh passphrase since I updated to Sierra

If you've tried ssh-add and you're still prompted to enter your passphrase then try using ssh-add -K. This adds your passphrase to your keychain.

Update: if you're using macOS Sierra then you likely need to do another step as the above might no longer work. Add the following to your ~/.ssh/config:

Host *
  UseKeychain yes