tmux was running as x86 on my M4

I set up a new M4 Mac and lived in tmux as usual. Then I noticed that inside tmux, arch printed i386. Outside tmux it printed arm64. Every shell that tmux started was running under Rosetta.

arch -arm64 brew install tmux didn’t change it, and neither did starting zsh with arch -arm64. The terminal app wasn’t set to open with Rosetta either.

The answer was in which -a:

which -a brew tmux
file "$(which tmux)"

I had two Homebrew installs. One was in /usr/local, the Intel prefix, and one was in /opt/homebrew, the Apple Silicon prefix. /usr/local/bin came first on my PATH, so the tmux I ran was the Intel build, and an x86 tmux starts x86 shells.

I removed the Intel Homebrew with its uninstall script. After that, the shell setup only needs one brew shellenv line, for the Apple Silicon install, in ~/.zprofile:

eval "$(/opt/homebrew/bin/brew shellenv)"

If you do the same, save the output of brew list first, because the uninstall removes every package installed with the Intel Homebrew. The uninstall script also takes --path=/usr/local to target one prefix, which is safer than trusting PATH order. And the tmux server keeps running after you fix the binary, so run tmux kill-server, or new sessions attach to the old x86 server.