{"name":"tengu_bg_attach_upgrade","slug":"tengu-bg-attach-upgrade","family":"flag","title":"Feature flags","noun":"feature flag","description":"Default `true`. Gates the background-session supervisor's prewarming: restarting idle background sessions whose recorded Claude Code version is stale onto the running version, in one delayed burst once the supervisor is ready and up to `tengu_bg_prewarm_per_sweep` (default `3`) per periodic sweep while memory is not low. Sessions launched in `exec` mode are skipped and a session on a newer version is not downgraded. Turning it off stops prewarming; the sweep still restarts sessions in its pinned set.","description_source":"code","described_by":{"bundle":"2.1.286","author":"claude-opus-5-5"},"presence":{"first_seen":"2.1.162","removed_in":null,"in_current_build":true,"newest_mined":"2.1.308","builds":127,"mined_builds":149,"first_cited":null},"aliases":[],"entries":[],"entries_total":0,"docs":[],"docs_total":0,"gates":[],"related":[{"name":"tengu_bg_attach_outcome","slug":"tengu-bg-attach-outcome","description":"Background attach events now record cold starts, surface and first frame details."},{"name":"tengu_bg_attach_stall_ms","slug":"tengu-bg-attach-stall-ms","description":"Default `5000` (milliseconds). How long attaching to a background session may wait for it to redraw before Claude Code restarts the session with `Session not responding`; after two such restarts it gives up, prints `Session keeps stalling at startup.` and kills the session. `0` turns the restart off; any other value is raised to at least 2000, or 12000 when a background-session launcher is configured."}],"url":"https:\/\/changelogs.core-directive.com\/reference\/flag\/tengu-bg-attach-upgrade","family_url":"https:\/\/changelogs.core-directive.com\/reference\/flag.json"}