Claude Code runs each Bash tool command through my login shell. On my Mac that’s zsh, the macOS default. Claude writes bash one-liners anyway, and five zsh defaults kept breaking them across my sessions and their subagents. The errors all start with (eval):1:. If you use Claude Code on a Mac and see that prefix, this is probably why. My cd hook was the first sign that the agent’s shell is my shell.
Unquoted variables aren’t split into words. zsh leaves SH_WORD_SPLIT off, so a command kept in a variable stays one word:
$ AB="agent-browser --session demo"; $AB open http://localhost:4000/sign-up
(eval):1: no such file or directory: /Users/me/.local/bin/agent-browser --session demo
The same thing passed 19 file names to a script as one argument, and broke a curl call written as R="--resolve ..."; curl $R .... A function or a tiny wrapper script works:
oc() { env -i HOME="$T/home" PATH="$P" opencode "$@"; }
So does an array, args+=(--attach "$f") and then "${args[@]}". Command substitution does get split, so for f in $(cat files.txt) worked, which made the difference easy to miss.
A glob that matches nothing is an error. This one came up 29 times:
$ grep -rn "trigger" lib --include=*.ex
(eval):1: no matches found: --include=*.ex
zsh tries to expand --include=*.ex as a file pattern, finds nothing, and stops. Quoting it, --include='*.ex', is the fix. Claude usually dropped the flag and grepped everything instead.
=word expands to a command path.
$ tail -6 a.sql; echo ===; tail -8 b.sql
(eval):1: == not found
$var:x is a history modifier, even inside double quotes.
$ git push -q origin "$commit:refs/heads/pr-assets/screens"
error: src refspec 296baa5...efs/heads/pr-assets/screens does not match any
zsh read :r as “remove the extension” and used up the r of refs. "${commit}:refs/heads/..." works.
$var[...] is a subscript. In an ffmpeg filter string, d=$D[bg] became an empty duration:
[Parsed_color_15 @ 0x...] Unable to parse option value "" as duration
Moving long ffmpeg commands into a #!/bin/bash script got rid of that whole class of problem.
Claude found the cause almost every time, usually right after the first failure. It didn’t carry it forward. The word-splitting error came back five more times over three weeks, and the glob error kept coming through November. It happened twice while I gathered the material for this post. A line in my global CLAUDE.md about quoting globs and not keeping commands in variables is the next thing I’ll try.