Linux headless Node 24.21.0 port-selection runtime proof. No UI applies.

Process-local seccomp returns EAFNOSUPPORT for IPv6 sockets
{"label":"before","ipv4":true,"ipv6ProbeAvailable":false,"freePortAvailable":false,"preferredPortRetained":false,"occupiedPortAvailable":false}
{"label":"after","ipv4":true,"ipv6ProbeAvailable":true,"freePortAvailable":true,"preferredPortRetained":true,"occupiedPortAvailable":false}

Isolated network namespace with IPv6 addresses disabled
{"label":"before","ipv4":true,"ipv6ProbeAvailable":true,"freePortAvailable":true,"preferredPortRetained":true,"occupiedPortAvailable":false}
{"label":"after","ipv4":true,"ipv6ProbeAvailable":true,"freePortAvailable":true,"preferredPortRetained":true,"occupiedPortAvailable":false}

Syscall trace sample:
134507 socket(AF_INET6, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = -1 EAFNOSUPPORT (Address family not supported by protocol)
134507 socket(AF_INET6, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = -1 EAFNOSUPPORT (Address family not supported by protocol)
134507 socket(AF_INET6, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = -1 EAFNOSUPPORT (Address family not supported by protocol)
134507 socket(AF_INET6, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = -1 EAFNOSUPPORT (Address family not supported by protocol)

A kernel booted with ipv6.disable=1 was not available; seccomp reproduces its socket errno locally without changing host networking.