ssh-agent is running
the key is present via ssh-add -L
ForwardAgent yes in ~/.ssh/config

this works

ssh -A -i /path/to/private_key vagrant@127.0.0.1 -p 2200 "env | grep SSH_AUTH_SOCK ; ssh -T git@github.com"
SSH_AUTH_SOCK=/tmp/ssh-9cnfbq1ocn/agent.20683
Hi sfunix! You've successfully authenticated, but GitHub does not provide shell access.

this does not

### ansible.cfg
[ssh_connection]
ssh_args = -o ControlMaster=auto -o ControlPersist=60s -o ForwardAgent=yes

### agent.yml
---
- hosts: web
  remote_user: vagrant

  tasks:
  - name: test SSH agent forwarding
    shell: echo "Client= [$SSH_CLIENT] Sock= [$SSH_AUTH_SOCK]"
    become: no
    register: myecho

  - debug: msg="{{myecho.stdout}}"

  - shell: ssh -T  git@github.com
    register: myecho

  - debug: msg="{{myecho.stdout}}"


ansible-playbook agent.yml

PLAY [web] **************************************************************************************************************************************************************

TASK [Gathering Facts] **************************************************************************************************************************************************
ok: [web]

TASK [test SSH agent forwarding] ****************************************************************************************************************************************
changed: [web]

TASK [debug] ************************************************************************************************************************************************************
ok: [web] => {
    "msg": "Client= [192.168.33.1 51925 22] Sock= []"    <<<-- no $SSH_AUTH_SOCK
}

TASK [command] **********************************************************************************************************************************************************
fatal: [web]: FAILED! => {"changed": true, "cmd": "ssh -T git@github.com", "delta": "0:00:00.317487", "end": "2017-08-23 21:20:12.841956", "failed": true, "rc": 255, "start": "2017-08-23 21:20:12.524469", "stderr": "Permission denied (publickey).", "stderr_lines": ["Permission denied (publickey)."], "stdout": "", "stdout_lines": []}
	to retry, use: --limit @/Users/pmcintire/dev/ansiblebook/ch06/playbooks/agent.retry

PLAY RECAP **************************************************************************************************************************************************************
web                        : ok=3    changed=1    unreachable=0    failed=1