Can Oh My Zsh plugin loader guarantee `compdef` availability before completion initialization during startup?

Answer

In the default Oh My Zsh (OMZ) distribution, compinit is executed before any .plugin.zsh files are sourced. This guarantees that a plugin can safely call compdef during its own source block, because the completion system has already been initialized.

How OMZ Arranges the Startup Sequence

1. ~/.oh-my-zsh/oh-my-zsh.sh is sourced by ~/.zshrc.
2. The file contains a source $ZSH/oh-my-zsh.sh call, which in turn
   • sets up fpath with the default plugin directories.
   • calls source $ZSH/lib/compinit.zsh (i.e., runs compinit).
   • iterates over plugins and sources each *.plugin.zsh file.
3. Any plugin that calls compdef during its sourcing registers completions directly with the already‑initialized system.

Verifying the Ordering on Your System

To confirm that this ordering holds in your environment, run:

zsh -x -i -c 'echo "START"; source $HOME/.oh-my-zsh/oh-my-zsh.sh; echo "END"'

The trace will show source $ZSH/lib/compinit.zsh before any source .../*.plugin.zsh lines. If you see compdef calls before compinit, your .zshrc or a custom script has reordered the sequence.

When the Guarantee Might Break

• Custom .zshrc changes: If you call compinit after adding fpath entries or source plugins yourself, the guarantee is lost.
• Plugins that prepend to fpath before compinit: Such plugins must manually re‑run compinit or use compdef -f to register completions.
• Zsh version differences: While the ordering remains stable, newer Zsh releases (e.g., 5.9) introduce subtle changes in how compdef parses completion files. The calling sequence still works, but behaviour may differ if you rely on edge cases.

What to Do If You Suspect a Problem

1. Check your OMZ version: omz --version (or look at ~/.oh-my-zsh/CHANGELOG.md).
2. Verify your Zsh version: zsh --version.
3. If the ordering is wrong, move the source $ZSH/lib/compinit.zsh line to the top of ~/.zshrc or remove any early compinit calls that you added.

Bottom Line

Under the stock OMZ configuration, compinit runs before plugin loading, so compdef calls inside plugins are safe. Keep your .zshrc clean and avoid re‑ordering the sequence, and the guarantee remains stable across OMZ releases.