Codex

Codex CLI Runner

Invokes OpenAI's Codex CLI in non-interactive mode to get an independent response from a non-Claude model. The skill shells out via the Bash tool, captures stdout, and returns the response cleanly.

When to use

• The user explicitly asks to consult Codex ("ask Codex", "what does Codex think", "second opinion from Codex")
• A crossfire-style multi-model review needs an independent OpenAI-side perspective
• The user wants an external check on a design, code change, or argument and Codex is the chosen reviewer

Do NOT trigger when:

• The user is asking about Codex CLI itself (use docs, not this skill)
• The task can be answered from your own knowledge — don't pay the latency/cost for nothing
• Codex CLI isn't installed (see "Setup check" below)

Setup the user needs

Before this skill works, the user must have:

1. Codex CLI installed and on PATH. Install with npm install -g @openai/codex (Node 20+) or via the official installer.
2. Auth configured — either:
   • codex login has been run (creates ~/.codex/auth.json), OR
   • CODEX_API_KEY env var is set (note: not OPENAI_API_KEY — the exec subcommand specifically reads CODEX_API_KEY)

If the user hasn't done this, the skill should tell them and stop, not try to proceed.

How to invoke

Step 1 — Check that Codex is available

Run a quick sanity check via Bash:

command -v codex && codex --version

If the command fails or returns nothing, tell the user:

Codex CLI doesn't appear to be installed or on PATH. Install with npm install -g @openai/codex (Node 20+ required), then run codex login to authenticate, then retry.

Stop. Do not attempt the call.

Step 2 — Construct the prompt

Decide what prompt to send Codex. For most uses this comes from:

• The user's request (if they asked for Codex's take on a specific question)
• A file path the user provided
• A prompt the parent flow assembled (e.g. a crossfire prompt + source document, in the future feature-design crossfire commands)

If the prompt is long, write it to a temp file rather than passing it inline — long shell-arg strings have quoting hazards.

Step 3 — Run Codex from the project directory

codex exec --skip-git-repo-check --sandbox read-only "$PROMPT" 2>/dev/null

Run from the project's working directory (typically the repo root) so Codex can read files referenced in the prompt. Crossfire reviewers don't just respond to inlined text — they read the project's PRD, Codex docs, code, etc. via the CLI's filesystem tools.

Flag rationale:

• exec — non-interactive subcommand; required to avoid landing in the Codex REPL
• --skip-git-repo-check — Codex refuses to run outside a git repo by default; this disables the check (handy if the project happens to not be a git repo, though most are)
• --sandbox read-only — safest default for review work. Codex can read files; cannot write, edit, or hit network. Right shape for adversarial review.
• 2>/dev/null — suppresses Codex's progress/thinking-token stream on stderr; the final response goes to stdout

For long prompts, prefer reading from a file:

codex exec --skip-git-repo-check --sandbox read-only "$(cat /path/to/prompt.md)" 2>/dev/null

Step 4 — Capture and return

Capture stdout. The full response is the model's final message. Present it back to the user with light framing — typically:

Codex says:

<response>

If the response is long, summarize first and offer the full text on request.

Optional flags worth knowing

• --ephemeral — don't persist the session rollout to disk. Use for crossfire so review runs don't pollute the user's session history.
• -m <model> — pick a specific model. Default is fine for most uses; specify when the user asks for a particular Codex model.
• -o /path/last.md — also write the final response to a file. Useful when the parent flow wants the response saved alongside other crossfire artifacts.
• --json — return newline-delimited JSON events instead of plain text. Use only if the parent flow needs structured event output.

Error handling

Symptom	Likely cause	Action
command not found	Codex CLI not installed or not on PATH	Tell user to install per Setup section
Authentication failed / 401	codex login not run, or auth expired, or CODEX_API_KEY missing	Tell user to re-auth
outside a git repo	Missing --skip-git-repo-check	Add the flag (this skill always passes it; if you see this, the invocation got mangled)
Hangs forever	Codex waiting on TTY input — happens when stdin is an empty pipe	Always pass the prompt as a positional arg, not via redirected stdin. See openai/codex#20919
Empty stdout, content on stderr	2>/dev/null was dropped or the run errored before producing output	Re-run without stderr suppression to see the actual error

Windows / Cowork sandbox note

Codex CLI installs to the host OS (typically Windows in Cowork's case). Cowork's Bash tool runs in an isolated Linux sandbox. If the sandbox can't reach the host's codex binary, this skill will fail at the Step-1 sanity check. Workarounds in priority order:

1. Install Codex CLI inside the sandbox (npm install -g @openai/codex from within Bash) — auth files won't persist between sessions, so re-auth each session via CODEX_API_KEY env var.
2. If Cowork exposes a way to invoke Windows binaries from Bash, use that path. Verify by trying codex --version from within Bash; if it works, you're set.

This is a known caveat to validate during first-time setup, not a blocker.