Summary

On macOS, /codex (all three modes) fails before Codex ever runs: the mktemp
calls that create the stderr/prompt/response temp files error out, the bash block
exits 1, and the user sees no review. Two compounding bugs, both in gstack.

Environment

• gstack: 1.58.4.0
• macOS: 26.5.1 (Apple silicon)
• mktemp: /usr/bin/mktemp (BSD)
• $TMPDIR: /var/folders/.../T/ (note the trailing slash — macOS default)

Symptom

mktemp: mkstemp failed on /var/folders/.../T//codex-err-XXXXXX.txt: File exists
mktemp: mkstemp failed on /var/folders/.../T//codex-prompt-XXXXXX.txt: File exists
/opt/homebrew/bin/bash: line 13: : No such file or directory
=== CODEX_EXIT=1 ===
(The empty TMPERR/_PROMPT_FILE vars then cascade into "No such file or directory"
for every later "$TMPERR" / cat "$_PROMPT_FILE".)

Root cause

1. Suffix after the placeholder. skills/codex/SKILL.md uses templates like
   mktemp "$TMP_ROOT/codex-err-XXXXXX.txt" (also codex-prompt-XXXXXX.txt,
   codex-resp-XXXXXX.txt). GNU mktemp tolerates a suffix after XXXXXX; BSD
   mktemp (macOS) requires the placeholder at the END of the template and
   mishandles the .txt suffix.
2. Trailing slash in TMP_ROOT. bin/gstack-paths emits TMP_ROOT straight
   from $TMPDIR, which on macOS ends in /. The template then expands to a
   double-slash path (…/T//codex-err-…), compounding the failure.

Affected code

• skills/codex/SKILL.md (and SKILL.md.tmpl) — Steps 2A / 2B / 2C, every
  mktemp "$TMP_ROOT/codex-*-XXXXXX.txt".
• bin/gstack-paths — TMP_ROOT carries $TMPDIR's trailing slash.

Suggested fix

Drop the .txt suffix (X's at the end) and strip the trailing slash:

TR="${TMP_ROOT%/}"
TMPERR=$(mktemp "$TR/codex-err-XXXXXX")
_PROMPT_FILE=$(mktemp "$TR/codex-prompt-XXXXXX")
Either fix alone helps; both together is the robust portable form. Optionally have
gstack-paths normalize TMP_ROOT (_tmp_root="${_tmp_root%/}") so every skill
that consumes it benefits, not just /codex.

Repro

macOS, default $TMPDIR: run /codex review in any git repo. The first mktemp
in Step 2A fails.