We have a group of Macs that mount SMB shares over 10 GbE with jumbo frames (MTU 9000). Since late June, they have been kernel panicking several times a day with the same assertion: panic(cpu N caller ...): assertion failed: m->m_flags & M_PKTHDR, file: .../xnu/bsd/kern/uipc_mbuf.c, line: 4839 @uipc_socket.c:8260 Panicked thread: dlil_input_en0 Last started kext: com.apple.filesystems.smbfs 6.0.1 Environment Clients: Mac Studio (M1 Max and M1 Ultra) and Mac Pro (2019, Intel with T2), using the built-in 10GBASE-T at MTU 9000 macOS 26.5.1 (25F80), 26.6.2 (25G83) and 26.7 (25G229); it panics on all three Servers: Samba-based NAS, SMB 3.1.1, signing on, encryption off Filed as FB24912731 What we've found It still panics with our third-party EDR fully uninstalled. The Mac that panics needs an active SMB session. A Mac left on the network without a share mounted stayed up through several events that took down the others. Panics are often simultaneous across machines: two to six Macs, with different hardware and different macOS builds, within the same minute. It doesn't need sustained heavy throughput. Some panics came within minutes of reconnecting, during light editing. Setting kern.skywalk.flowswitch.rx_agg_tcp_host=0 did not help. The switch and server links stay up, and spanning tree doesn't change during these events. Only the Macs' ports drop. In one server-side capture, the client stopped sending within about 0.2 ms of receiving a READ response made of 8948-byte frames. That fits the panicked thread being dlil_input. Two existing threads look related Kernel panic using Vagrant synced folders via NFS beginning with macOS 15.4 (FB17853906). A DTS reply there said the issue is in kernel mbuf management and that SMB "is very likely to experience a similar panic." Incorrect packet handling in SMBClient MacOS 26, which describes a race in SMBClient under heavy load above about 10 Gbps. Questions Is this the same underlying issue as FB17853906, and is a fix planned for macOS 26? Our 2019 Mac Pros can't move to a later major release. Is there a known workaround, such as a sysctl, an nsmb.conf option, or a change to MTU or offload settings? Is there logging or a diagnostic we can leave enabled to capture more state at panic time? We can't reproduce this on demand, but between several machines we see it multiple times a day. We can provide full panic reports, sysdiagnoses, and packet captures from both client and server sides.