How to report back to Squid which peer to use?

Let’s say, I have a python script which handles ACL:

#squid.conf
# [............]

# sub-proxies/peers
cache_peer 01.01.01.99 parent 34433 0 no-query no-digest name=peer1
cache_peer 01.01.02.99 parent 34433 0 no-query no-digest name=peer2
cache_peer 01.01.03.99 parent 34433 0 no-query no-digest name=peer3
cache_peer 01.01.04.99 parent 34433 0 no-query no-digest name=peer4


external_acl_type my_acl_helper ttl=30 %LOGIN ./my_acl_helper.py
acl session_acl external my_acl_helper

# [............]

In my_acl_helper.py there’s some tricky logic of selecting a peer to be used for a request.

How should my_acl_helper.py report back to Squid which peer it’s choosen? In what format? As far as I’ve found out, it could only be OK or ERR. But the IP:port or the alias of a peer will be needed as well. How?