Terminal input lags behind when using mux with OpenSSH 10.1p1 + macOS 26.1b2

[Jan Schermer]

Hello,
I just upgraded openssh on my laptop using homebrew to 10.1p1 and macOS to developer beta 2 and it no longer works right.

At first I thought it was lag, but it is actually ssh waiting for input of several (more) characters before it both displays output or executes remote commands (input).

It onlly happens if 10.1p1 is the version running the master mux.
When macOS default client (OpenSSH_10.0p2 in this version) is the one hosting the master, then everything works fine.
Vice versa, when OpenSSH_10.0p2 is the one attaching to 10.1p1 socket running the master then it lags.

I have narrowed the problem down to simply using or not using ControlMaster (ControlPath=socket vs ControlPath=none).
Tested with several versions of remote sshd on Debian, Ubuntu and FreeBSD and ssh client in iTerm2 and Terminal.app without change.
Recompiling from source with newest Xcode on 26.1b2 didn’t fix it. Unfortunately I don’t have a non-beta OS to test.

Let me know how I can help debug this or if someone can replicate it.

Thanks
Jan