DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.
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.