Background agents now use a pre-warmed "spare" worker mechanism (--bg-spare) for faster session startup. When a new background session is requested, the daemon can claim a pre-spawned spare worker instead of cold-starting a new process.
Evidence
Spare worker system (search for "bg claimed-spare", "[bg-spare] missing claim sock path")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.