Second Opinion

/second-opinion — Cross-LLM Review via Codex CLI

Get an independent review from OpenAI via the Codex CLI. Three modes for different needs: quick pass/fail review, adversarial break-it pass, or multi-turn consultation.

Authentication

Codex CLI uses chatgpt auth mode (ChatGPT subscription), not an API key. API-key auth also works but burns tokens against your OpenAI bill; the subscription path is free at the margin and is what this skill assumes. Verify before running:

grep -q '"auth_mode": "chatgpt"' ~/.codex/auth.json && echo "OK: Using ChatGPT subscription" || echo "ERROR: Not using ChatGPT auth"

If not authenticated, tell the user to run ! codex login (interactive).

Mode Detection

Parse user input to determine mode:

Mode 1: Review (pass/fail code review)

Reviews the current diff against the base branch.

cd <project-root>
codex review --base HEAD~1 -c 'model_reasoning_effort="xhigh"' 2>/dev/null

Or against a specific base:

codex review --base main -c 'model_reasoning_effort="xhigh"' 2>/dev/null

Output handling:

• Present Codex's output verbatim inside a ### CODEX SAYS section
• Look for [P1] markers in the output — these are critical findings
• If P1 found: verdict is FAIL with list of critical issues
• If no P1: verdict is PASS
• Add token count and comparison note if Claude's /review already ran

Timeout: 300 seconds (5 minutes). If it times out, say "Diff may be too large or API is slow."

Mode 2: Challenge (adversarial)

Try to break the code. Finds edge cases, race conditions, security holes.

cd <project-root>
DIFF=$(git diff HEAD~1)
codex exec "You are an adversarial code reviewer. Your job is to BREAK this code. Find edge cases, race conditions, security vulnerabilities, error handling gaps, and failure modes. Be aggressive and thorough. Here is the diff:\n\n$DIFF" \
  -s read-only \
  -c 'model_reasoning_effort="xhigh"' \
  --json 2>/dev/null

If the user specifies a focus domain (e.g., /second-opinion challenge security):

codex exec "You are a security auditor. Find injection vectors, auth bypasses, SSRF, XSS, CSRF, and data exposure in this diff:\n\n$DIFF" \
  -s read-only \
  -c 'model_reasoning_effort="xhigh"' \
  --json 2>/dev/null

Output handling:

• Parse JSONL output: extract agent_message and reasoning fields
• Present ALL findings verbatim — never summarize or editorialize
• Group by severity if Codex provides severity markers

Mode 3: Consult (multi-turn Q&A)

Ask Codex anything with session persistence for follow-ups.

First question:

cd <project-root>
RESULT=$(codex exec "<user's question>" \
  -s read-only \
  -c 'model_reasoning_effort="xhigh"' \
  --json 2>/dev/null)
echo "$RESULT"

Save session ID for follow-ups:

SESSION_ID=$(echo "$RESULT" | python3 -c "
import sys,json
for line in sys.stdin:
    try:
        d=json.loads(line)
        if 'session_id' in d: print(d['session_id']); break
    except: pass
" 2>/dev/null)
if [[ -n "$SESSION_ID" ]]; then
  mkdir -p .context
  echo "$SESSION_ID" > .context/codex-session-id
fi

Follow-up questions:

SESSION_ID=$(cat .context/codex-session-id 2>/dev/null)
if [[ -n "$SESSION_ID" ]]; then
  codex exec resume "$SESSION_ID" "<follow-up question>" \
    -s read-only \
    -c 'model_reasoning_effort="xhigh"' \
    --json 2>/dev/null
else
  # No session — start fresh
  codex exec "<question>" -s read-only -c 'model_reasoning_effort="xhigh"' --json 2>/dev/null
fi

Golden Rules

1. Never editorialize Codex's output. Show it verbatim in a ### CODEX SAYS block.
2. Never use API key auth. Always verify chatgpt auth mode before running.
3. Always use -s read-only for challenge and consult modes (sandbox).
4. Always use model_reasoning_effort="xhigh" for maximum reasoning depth.
5. Timeout is 300s on all Bash calls to Codex.
6. Compare, don't compete. If Claude already reviewed the same code, note where Codex agrees or disagrees — both perspectives are valuable.

Error Handling

Cost

When authenticated against ChatGPT subscription, there is no per-token API cost — calls draw from the subscription quota. If you are on API-key auth, expect normal token billing (xhigh reasoning is roughly 2× the cost of medium).

Known gotchas

• Editorializing Codex's output buries the point. The whole value of a second opinion is that it disagrees with the first. Show Codex's output verbatim inside a ### CODEX SAYS block and let the human read both.
• Sandbox mode matters. Always use -s read-only for challenge and consult modes. Without it, Codex can write to the working tree.
• Session resume can fail silently. If the saved session ID is stale, codex exec resume returns nothing. Detect "no output" and fall back to a fresh session.

Anti-patterns

• Running /second-opinion and ignoring the result when it disagrees with Claude. The purpose of the skill is to surface disagreement. If both models are going to be filtered through the same hand, you might as well not run it.
• Using --model gpt-5.5 (the heavier model) on trivial questions. The slower path is worth it for hard prompts — for everything else, the default is fine.

Validated patterns

• For diff review, running codex review --base <baseline> and grepping for [P1] markers is a reliable pass/fail signal.
• For challenge mode, naming a domain (challenge security, challenge concurrency) gives much sharper output than the generic prompt.

Self-improvement

This skill ships with a lightweight feedback loop. Adopt or ignore — the skill works without it.

Trigger a review when:

• The user explicitly disagrees with the review or challenge output (strongest signal — log immediately; 2–3 corrections on the same theme → promote to body).
• A novel input category recurs across sessions.
• learnings.md crosses ~100 bullets (consolidation time).
• The skill mis-triggers or fails to trigger.
• The Codex CLI ships a breaking change.

Consolidation pass (5–10 min, weekly or threshold-driven):

• Each entry gets one fate: apply, capture, or dismiss.
• Apply = merge into Known gotchas / Anti-patterns / Validated patterns.
• Capture = leave in learnings.md for now.
• Dismiss = delete.
• Bump last-consolidated: in frontmatter.