how to debug oh-my-zsh auto-completion not working?

The title largely says it. Most of my auto-completion is working, but git is not. It does not complete commands, e.g. git check, nor does it complete branches, e.g. git checkout ge. It used to until at most a few weeks ago (no idea what changed). Other completions, such as filenames and directories and commands, work fine. macOS, git installed via homebrew, zsh with oh-my-zsh, lots of common (i.e. bash and zsh) commands in ~/.common_profile, which is sourced by both ~/.zshrc and ~/.bashrc. It is not that disabling it, as I tried commenting it out. I have both git and gitfast plugins enabled. The git plugin aliases seem to be working fine. I can run ^X? to get a dump, but no idea how to interpret the dump. UPDATE 1: I switched from the default macOS zsh (5.8) to the one installed via homebrew (5.9) and now it works. But I still want to understand why it failed. UPDATE 2: As suggested by Gilles in the comments, I posted the debug output. See this gist .