Claude Code writes bash syntax under zsh: 51 failures per 1,000 Bash calls, 76% silent

stagefright5 · ghdev · 2026-10-17

Issue #103027 in anthropics/claude-code (a follow-up to #67146) quantifies Claude Code writing bash-style commands while the Bash tool runs under zsh on macOS, using roughly 12 weeks of the author's own transcripts.

Measurements (Jul 28 – Oct 17, 2026, Claude Code 2.1.x, mostly Opus 5.5)

287 sessions, 61,040 Bash calls, 2,570 calls containing a zsh error line ((eval):N: …), subagent transcripts included
Failure rate rose 3.5×: 14.7 per 1,000 calls from Jul 28 to Sep 20, then 51.3 per 1,000 from Sep 21 to Oct 17
76% were silent: the call still exited 0, leaving the error buried mid-output
87% were repeats of a slip type the same session had already hit
Claude blamed zsh in only 82 replies, about 3% of slips
Direct cost was small ($53 at API list prices, 0.7% of spend, 71 minutes of model time); the bigger risk is Claude drawing conclusions from silently incomplete output

Dominant failure patterns

Unquoted glob in a flag value (1,296): e.g. grep -rn foo src --include=.ts expands against the cwd and aborts with no matches found
Unmatched path glob (656): e.g. ls libs//README.md aborts instead of passing the pattern through
= word expansion (549): echo ==== as a separator triggers =cmd expansion and === not found
Unsplit variable (28, usually silent): G="git -C repo"; $G log passes one argument in zsh
Colon modifiers (21): git show $B:shared/package.json gets :s/:t/:h/:r treated as history modifiers, mangling ref:path
Array subscript (7): grep -oE "$pat[^};]" is read as a subscript on $pat
Bash-only expansion (2): ${!v} yields bad substitution

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Claude Code writes bash syntax under zsh: 51 failures per 1,000 Bash calls, 76% silent

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