arXiv:2610.13026v1 Announce Type: new Abstract: Matter's first post-quantum change protects device attestation with ML-DSA, but its operational handshake, CASE, still derives session keys from P-256 ECDH, so recorded traffic can be decrypted once a quantum computer exists. We quantify the cost of moving CASE to ML-KEM and post-quantum signatures over Thread, where frames carry at most 127 bytes and a Matter message at most 1178 bytes. A byte-exact model of the CASE messages shows that a drop-in ML-KEM-768/ML-DSA-44 handshake grows from 1.6 kB to 22.8 kB and that no fully post-quantum configuration built from FIPS 203/204/205 algorithms fits the message limit. A Monte Carlo model of multi-hop Thread, calibrated to OpenThread and Matter source constants, shows that sending these messages as fragmented IPv6 datagrams causes spurious retransmissions whenever a message and its reply outlast Matter's retransmission timer. At three to four hops the drop-in handshake then takes 1.8 to 2.6 times longer than with a selective-repeat block transfer, with or without interference, and up to 33% of handshakes with sleepy devices fail; raising the timers does not remove the effect. Caching certificate chains and authenticating the device with a pre-distributed ML-KEM key reduce the handshake to 6.1 kB and 44 ms of device computation. As a first step, hybrid P-256+ML-KEM-512 key exchange with a cached device chain protects session keys against harvest-now, decrypt-later attacks while every message still fits the current limit.