The test "overlapping exchanges each report the entry that answered that call" failed once on a loaded release runner and passed on rerun. It started two exchanges back to back and relied on a 50 ms failing stall to keep the first in flight until the second arrived; nothing checked that the two calls were ever in flight together, and the failing schedule was never observed.
Two Fake behaviours, hold and hold_fail, wait on a test-owned std.Io.Semaphore and then reply or fail. The test holds both calls inside the first entry, waits on the entry's in_flight count for one and then two, releases the failing call so it fails over while the other is still held, then releases the other. The fake draws its behaviour before it raises in_flight, so a count of one also fixes which call holds which gate. Each gate has a deferred post registered after its task's deferred await, so any early return releases the held call instead of deadlocking. No sleep remains in the test; every assertion is kept. No pool bug was found. Addendum in specs/milestone-31.md.
mokhtar