server.ServerOptions.maxInFlight

Every invocation is a tmux client process with its own pipes, so a caller that fans out over a whole server turns its own concurrency into process and descriptor pressure. tmux runs commands on one thread: bench-modes.ts measures twelve concurrent creations costing the same twenty-five invocations as twelve sequential ones — twenty-four either way, plus the query that reads the result back — and arriving out of order, so the ceiling bounds the cost without bounding the work. Waiting for a slot spends the request's own deadline, and a request that never gets one fails not_started.