ssh-agent keys not persisted

This might be my fault, but my ssh-agent keys are only persisted per-shell (session) on my laptop. My desktop has the same config and the keys are persisted even between reboots, and I don't know how to fix this. I thought fish-ssh-agent was supposed to reuse agents for all the shells, but it seems to launch one agent per shell, even though `.ssh/environment` exists and points to the last agent spawned.