Function cap_organic_surbs 

fn cap_organic_surbs(
    data: &mut ApplicationDataOut,
    max_out: bool,
    surb_mgmt: &BalancerStateValues,
)

Caps how many SURBs an outgoing Session data packet may carry, per the SURB balancer.

max_out is the client’s always_max_out_surbs opt-in, which wins outright: a client that has asked for the maximum has said something about its own traffic that the balancer’s estimate cannot contradict.

Otherwise the cap comes from BalancerStateValues::organic_surbs_per_packet, which yields 0 once the counterparty is estimated to be at its target. That zero is the whole point of the function: without it, the balancer can silence its keep-alives and still have organic SURBs pouring into a full buffer, where each one evicts an older SURB.

Nothing else has to change to keep the SURB accounting straight: this runs in front of the counting sink, so estimate_surbs_with_msg reports the capped figure.