6 of 10 checks differ · Claude Code 2.1.304 · Codex 0.163.0 · Checked Oct 20, 2026
Works on my agent.
Claude Code and Codex don’t treat your hooks, instruction files and skills the same way, and every release can change how. Frontmatter runs them through both harnesses on every pull request, so you see the differences before your team’s agents do.
| Claude Code | Blocked the command | guarded |
| Codex | Ran the command. Your hook returned ask, which Codex reads but doesn’t enforce. |
open |
| Claude Code | Not loaded. CLAUDE.md is used instead. | skipped |
| Codex | Loaded, then cut at 32 KB with no warning. | cut |
How it works
Add the Action
Add it to any repo where your hooks, skills and AGENTS.md live.
- uses: frontmatter/check@v1
Run on every pull request
The Action installs the latest Claude Code and Codex and runs your hooks, instruction files and skills through both. No API keys, no token spend.
See the differences
Frontmatter comments on the pull request with every difference, and fails the check when one needs your attention.
Pricing
Pre-order a year at the founding price. You keep that price for as long as you stay.
Open source
- Checks on every pull request
- Latest Claude Code and Codex
Pro
- Everything in Open source
- Reruns whenever Claude Code or Codex ships a release
- An alert when a release changes your results