Codex CLI Rules

When to Use

Use this skill when you are driving Codex CLI from a script or ralph loop and need to verify it actually ran (not just exited 0). Read it BEFORE writing any wrapper that pipes Codex output into a downstream step.

Tested with Codex CLI v0.118.0+, model gpt-5.4, model_reasoning_effort: xhigh. Older CLIs (<0.118) reject xhigh as unknown variant for model_reasoning_effort — pin a fallback.

Skip if you are using Codex interactively (the TUI), or just running it once by hand.

stdin vs argv

Highest-frequency footgun. Putting a long prompt into "$(cat prompt.md)" hits ARG_MAX (~1MB on modern macOS, ~256KB on POSIX-only systems) and either truncates or fails with argument list too long.

# ❌ Bad — diff over ~500 lines silently truncates or errors
codex exec -s read-only --ephemeral -o /tmp/out.md "$(cat /tmp/prompt.md)"

# ✅ Good — pipe via stdin, trailing `-` reads from stdin
cat /tmp/prompt.md | codex exec -s read-only --ephemeral -o /tmp/out.md -

Bounded only by the model's context window, not the shell's ARG_MAX. Use this form unconditionally for any prompt that includes a git diff or file body.

Three-element contract

codex exec exits 0 even when Codex never produced a response. Exit code alone is not a success signal. Verify three elements:

cat prompt.md | codex exec -s read-only -c 'mcp_servers={}' --ephemeral \
  -o /tmp/codex.out -                                                \
  > /tmp/codex.log 2>&1 &

wait $!
[ -s /tmp/codex.out ] && [ "$(wc -c < /tmp/codex.out)" -gt 500 ] || echo "FAIL: out empty/short"
grep -q 'tokens used' /tmp/codex.log || echo "FAIL: no completion signature"

Two failure modes you'll see:

• out 5KB but log has no "tokens used" → process killed mid-run (often by | tail -N truncating final message). Re-run.
• out empty but log has "tokens used" → MCP stall or auth issue. Set -c 'mcp_servers={}' to disable MCP for the reviewer.

Three-dot diff

When feeding a PR diff to Codex for review, the diff syntax matters. Two-dot BASE..HEAD includes commits reachable from HEAD but not BASE — which on a long-running fork means BASE's own new commits get pulled in as "your changes". Three-dot BASE...HEAD walks the merge-base instead.

# ❌ Bad — if BASE moved forward since your branch diverged, you ship BASE's commits
git diff BASE..HEAD > /tmp/diff.txt

# ✅ Good — three dots walks merge-base
git diff BASE...HEAD > /tmp/diff.txt

Then embed the diff into the prompt and feed via stdin (see § stdin vs argv). Don't ask Codex to run git diff itself — its working directory under codex exec may not be what you think (see § Running from a worktree).

Running from a worktree

If you're in a git worktree (e.g., repo/.worktrees/feature-x/), codex exec defaults its cwd to the main repo's HEAD, not the worktree's HEAD. You'll get reviews of the wrong branch.

Fix: pass -C <worktree-path> explicitly.

# Wrong — Codex reads main repo's checked-out branch
codex exec -s read-only --ephemeral -o /tmp/out.md - < prompt.md

# Right — pin Codex to the worktree
codex exec -C "$(git rev-parse --show-toplevel)" -s read-only --ephemeral -o /tmp/out.md - < prompt.md

Sanity check: have Codex echo git rev-parse HEAD in its first response. If the SHA doesn't match git -C <worktree-path> rev-parse HEAD, you're reviewing the wrong tree.

Gotchas

• -m xhigh is a 400 error — xhigh is the value of model_reasoning_effort, not a model name. Use -m gpt-5.4 (or your model) and put effort in ~/.codex/config.toml or pass -c 'model_reasoning_effort="xhigh"' for a single-call override.
• Usage limit — ChatGPT-account Codex returns ERROR: You've hit your usage limit. ... try again at HH:MM AM when out of credits. Exit code is non-zero. Wall-clock reset time, not delta. Parse to schedule retry.
• | tail -N eats the final message — Codex's "final assistant message" prints last in the log. Truncating output kills the tokens used signature. Capture full log: > /tmp/log 2>&1, never | tail.

Examples

A complete invocation that satisfies all four rules:

WORKTREE=$(git rev-parse --show-toplevel)
git diff main...HEAD > /tmp/diff.txt
cat <<'EOF' > /tmp/prompt.md
Review this diff for correctness. Focus on null safety + race conditions.
---
EOF
cat /tmp/diff.txt >> /tmp/prompt.md

cat /tmp/prompt.md | codex exec \
  -C "$WORKTREE" \
  -s read-only \
  -c 'mcp_servers={}' \
  --ephemeral \
  -o /tmp/codex.out - \
  > /tmp/codex.log 2>&1

# Three-element verification
[ -s /tmp/codex.out ] || { echo "FAIL: out empty"; exit 1; }
[ "$(wc -c < /tmp/codex.out)" -gt 500 ] || { echo "FAIL: out < 500B"; exit 1; }
grep -q 'tokens used' /tmp/codex.log || { echo "FAIL: no completion"; exit 1; }
echo "OK"
cat /tmp/codex.out

Related skills

• requesting-code-review — for dispatching Claude's code-reviewer subagent. Codex driven via this skill is a complementary parallel reviewer (different model, different blind spots).
• finishing-a-development-branch — when the codex review is the last gate before push.