Codex

Codex is an independent agent on PATH (codex — invoke as command codex if a shell alias shadows it), sharing this working tree and already authenticated. It is a second opinion, not ground truth: verify what it reports, own what it changes. It reads the same skills your repo carries.

Leave Codex's model at its CLI default unless the user asks for a specific one. The default follows the current Codex release; do not pin a dated model ID in this skill. If an explicit override is needed, verify it is current and pass the requested model with -m <model>.

If codex is not installed

When codex is missing from PATH, offer to install it — ask the user for approval first, never install on your own initiative. On yes, follow the current instructions at https://developers.openai.com/codex/cli. First-run authentication is interactive — hand that step to the user. Verify with codex --version before proceeding.

Prompting Codex

Prompt Codex like an operator, not a collaborator: compact, block-structured with XML tags. State the task, what "done" looks like, and the few constraints that matter. A tighter prompt beats a bigger run — improve the contract before raising --effort.

• One task per run. Split unrelated asks (review, then fix, then docs) into separate runs; a mixed prompt gets a mixed result.
• Name skills instead of restating them. Codex reads the same skills your repo carries — say "follow write-docs for the doc", "obey refactor-clean: no compatibility wrappers." Don't re-explain what a skill already carries.
• Blocks, added only where the task needs them:
  • <task> — the concrete job, the repo/failure context, the expected end state. Nearly always present.
  • <output_contract> — exact shape, highest-value first, compact.
  • <default_follow_through> — take the low-risk interpretation and keep going; stop only when a missing detail changes correctness, safety, or an irreversible action.
  • <verification_loop> — before finalizing, check the result against the requirements and the changed files; revise rather than ship the first draft. Any risky fix.
  • <grounding> — ground every claim in code or tool output; label inferences as inferences. Review and research.
  • <action_safety> — keep the diff tightly scoped; no drive-by refactors. Write tasks.
• Anti-patterns: vague framing ("take a look"), no output contract ("report back"), "think harder" in place of a contract, mixing jobs in one run, and demanding certainty the evidence can't support.

Review — proactive

Run a Codex review whenever you have a substantive diff you'd want a second set of eyes on — a refactor, a tricky algorithm, renderer work, a security-sensitive change — before declaring it done or committing. Skip it for trivial edits (typos, comments, doc-only).

1. Fix the diff scope before launching: --uncommitted for working-tree changes, --base <branch> for a branch diff, --commit <sha> for a landed commit. Resolve a landed commit to its full SHA. Use the non-interactive review entry point with an explicit read-only sandbox and separate progress and verdict files in a fresh temporary directory:

   review_dir=$(mktemp -d "${TMPDIR:-/tmp}/codex-review.XXXXXX")
   codex exec --sandbox read-only -C "$(git rev-parse --show-toplevel)" review \
     --commit "$(git rev-parse HEAD)" --json -o "$review_dir/verdict.md" \
     < /dev/null > "$review_dir/events.jsonl" 2> "$review_dir/stderr.log"
   

   Background long runs and retain their process/session handle. Scope flags and custom instructions are mutually exclusive: a scope flag takes no prompt. For custom framing, specify the exact diff to inspect in the prompt; don't rely on default scope or traverse all refs/checkpoint history. Never state the answer you expect (unprimed, same discipline as screenshot-critique).

2. Monitor progress in events.jsonl and failures in stderr.log. An active reviewer reading relevant code is not hung merely because five minutes elapsed. If it stalls or drifts outside the diff and its dependencies, inspect the last command/error before interrupting; correct the cause or narrow the task before retrying. Completion requires exit status zero, turn.completed, and a nonempty final verdict. A timeout, turn.failed, or missing verdict is incomplete, never a clean review.

3. Triage every finding: confirm it against the code before acting. Preserve Codex's evidence boundaries — an inference it labelled is not a fact. Overlap with your own doubts is high-priority evidence; a finding you dismiss needs a stated reason, not silence.

4. Report the outcome to the user — what Codex flagged, what you fixed, what you dismissed and why. Done when every finding is either fixed or explicitly dismissed.

Implementation — explicit ask only

Delegate implementation to Codex only when the user names Codex for the task. Never hand it work on your own initiative, and never re-delegate follow-up work without a fresh ask.

1. Slice the task sharp before delegating — goal, constraints, and how to verify — using the prompt discipline above. An underspecified task stays with you until a fresh agent couldn't misread it.
2. Start from a clean tree (or record the baseline commit) so Codex's diff is separable from yours.
3. Pick the sandbox by what the task must RUN:
   • Pure code + typecheck/unit: codex exec --sandbox workspace-write "<task>". Network is off; add -c sandbox_workspace_write.network_access=true only when the task must fetch (e.g. new deps).
   • Browser verification, dev servers, or full test runs: use codex exec --dangerously-bypass-approvals-and-sandbox "<task>". The sandbox blocks localhost binds (listen EPERM on vite/playwright), so a sandboxed codex ships code it never saw run. Bypass trades that blindness for zero OS control: only in a dedicated git worktree, only with a prompt you authored end-to-end (never relaying third-party text), and the diff review you owe afterwards is the control. Use -o <file> to capture the final message and background long calls; suppress codex's stderr thinking-noise with 2>/dev/null so it doesn't bloat your context (drop it only to debug a failing run), and add --skip-git-repo-check to run outside a git repo. Non-interactive runs never ask for approval either way.
4. Follow up with codex exec resume <session-id> "<follow-up>", taking the id from the run header. resume --last means the most recent session globally — a review or any other codex run in between will hijack it. If two resume rounds don't converge, stop delegating and finish it yourself — iterating a confused agent costs more than taking over.
5. You own the result: read the full diff, run the tests, and only then report it. "Codex says it's done" is not done.

Exec liveness — a hang looks like work

A backgrounded codex exec can wedge at startup: process alive at ~0% CPU, but no session file under ~/.codex/sessions/<Y/M/D>/, no network socket, no tree changes. "Process running" is NOT "working."

• Stdin is always a file or /dev/null, never an inherited terminal. Left on an interactive/piped stdin with no prompt source, exec prints Reading additional input from stdin... and blocks forever — the most common hang. Either pass the prompt as a positional arg with < /dev/null, or feed a prompt file with codex exec [flags] - < prompt.txt (the - makes codex read the task from stdin, and a missing file fails the redirect loudly — unlike "$(cat prompt.txt)", which silently sends the fallback string as the task). Add nohup/& as needed.
• Watchdog: put a unique marker in the prompt, then kill the exec if grep -rl "<marker>" ~/.codex/sessions/<Y/M/D>/ finds no session within ~3 minutes. Relaunching after a kill reliably works.
• Trust the worktree. Headless exec in a directory Codex doesn't trust can block forever on an invisible prompt. Git worktrees are separate paths from the trusted repo root — add [projects."<worktree-path>"]\ntrust_level = "trusted" to ~/.codex/config.toml before exec'ing in one. This is the safe fix; the bypass flags stay forbidden here.
• Don't launch two execs in the same instant, and kill stale hung execs before starting a new one.
• Launch from the repo/worktree ROOT. The writable sandbox root is the CWD at launch: exec'd from a subdirectory (e.g. web/), every edit outside it is rejected as "writing outside of the project" and a never approval policy can't recover — the run burns with zero files changed. Pass -C <root> to set the working root explicitly instead of cd-ing into it.

Rules

• Don't touch the working tree while a Codex exec is running on it.
• Explicit --sandbox read-only for reviews, consultation and questions; workspace-write only for delegated implementation.
• --dangerously-bypass-approvals-and-sandbox is reserved for tasks that must run browsers/servers/full suites (above) — dedicated worktree, self-authored prompt, mandatory diff review after. --full-auto has been removed (it aliased workspace-write) — don't reach for it.
• Reasoning effort and model are config/flag overrides — -c model_reasoning_effort=high, -m <model> (the old --effort flag is gone in current Codex). Leave both at their defaults unless the user asks; tighten the prompt first and keep model IDs unversioned. If a version is required, verify that it is current before using it.