The salvaged fix held _session_resume_lock across _make_agent (MCP discovery
+ AIAgent construction, seconds), serializing it against session.close. Since
session.close runs on the main RPC dispatch thread (not a _LONG_HANDLER), a
close racing a mid-build resume would stall all fast-path RPCs (approval.respond,
session.interrupt).
Restructure to double-checked locking: build the agent outside the lock, then
re-check _find_live_session_by_key under the lock before _init_session. A losing
concurrent resume discards its just-built agent (no worker/poller wired yet) and
reuses the winner. Updated the concurrent-resume regression test to assert the
real invariant (one surviving live session + loser agent closed) rather than the
implementation detail of a single _make_agent call.
8077e7d2fb