Using different Git identities for different projects on the same machine

It is useful to manage different identities depending on your Git repository, especially when working with both personal and professional repositories on the same machine.
You can achieve this by setting SSH host entries explicitly.

The SSH Alias Strategy

The fix for identity confusion is defining explicit aliases in ~/.ssh/config.
Instead of connecting to the default host, create a custom Host entry.

Host github.com-work
    HostName github.com
    User git
    IdentityFile ~/.ssh/id_rsa_work
    IdentitiesOnly yes

Don't forget to include IdentitiesOnly yes.
It forces SSH to use only the specified key, which prevents the agent from offering other identities to the server.

Transparent Rewriting with insteadOf

Aliases are useful, but typing git clone git@github.com-work:repo every time would make it pointless.
Git’s insteadOf feature solves this by rewriting URLs.

git config --global url."git@github.com-work:".insteadOf "git@github.com:"

With this configuration, you clone using the standard URL and the routing happens automatically.

Automating with Ansible

I use Ansible to ensure these configurations are consistent across every workstation.
But you certainly can automate this in any other manner.

Managing the SSH Config

- name: SSH - Define work identity
  blockinfile:
    path: "~/.ssh/config"
    block: |
      Host github.com-work
        HostName github.com
        User git
        IdentityFile ~/.ssh/id_rsa_work
        IdentitiesOnly yes
    marker: "# {mark} ANSIBLE MANAGED BLOCK: WORK IDENTITY"

Configuring Git Rewrites

- name: Git - Configure URL rewriting for work
  git_config:
    name: url."git@github.com-work:".insteadOf
    value: "git@github.com:"
    scope: global

Secure Key Deployment

- name: SSH - Deploy private key from Vault
  copy:
    content: "{{ vault_work_ssh_key }}"
    dest: "~/.ssh/id_rsa_work"
    mode: '0600'

Conclusion

This setup removes the cognitive load of switching contexts.
The system routes the correct key based on the host.