Guide · Agent skill
How to use the Cross-Origin Isolation Review agent skill
Updated · written with AI from the project's own documentation and checked against our catalogue data
The cross-origin-isolation skill gives an AI agent a checklist-style guide for reviewing a website's security headers and embedded resources so it can enable cross-origin isolation, a browser mode needed for features such as SharedArrayBuffer. This guide explains what the skill does, how to install it, how to trigger it with plain requests, and where it is not the right tool. After reading it you should be able to add the skill to Claude, ask for a review, and understand what the agent will check.
Cross-Origin Isolation Reviewthedaviddias/Front-End-Checklist · MIT Download the skill Quick start
- 1Download the ZIP. Get the cross-origin-isolation skill ZIP from the directory page.
- 2Unzip into a skills folder. Unzip into ~/.claude/skills/ for personal use, or into your project's .claude/skills/ folder for one project.
- 3Or upload it in the Claude apps. In the Claude apps, go to Settings → Capabilities → Skills and upload the ZIP.
- 4Describe the task. Ask for a review in plain language, naming the feature such as SharedArrayBuffer or workers and the resources your page loads.
- 5Review the agent's findings. Check the list of COOP, COEP and CORP issues and third-party resources the agent flags before changing anything.
- 6Verify in the browser. After any change, confirm that self.crossOriginIsolated is true in the browser before relying on isolated-only features.
What is an agent skill and what does this one do?
An agent skill is a folder of instructions that an AI agent can load when a task matches the skill's description. This one is named cross-origin-isolation and comes from the Front-End-Checklist project under the MIT licence.
When installed, the agent works from a guide for reviewing security headers and embedded resources. It checks whether the app needs the Cross-Origin-Opener-Policy (COOP) and Cross-Origin-Embedder-Policy (COEP) headers, whether third-party scripts, iframes, workers and fonts are compatible, and whether the page actually becomes crossOriginIsolated in the browser.
- Use it for security-sensitive web apps, SharedArrayBuffer usage, worker-heavy apps, editors, or measurement features that need cross-origin isolation.
- It is guidance for review, not an automated scanner. It does not scan a live site.
Install the cross-origin-isolation skill
Download the skill ZIP from the directory page. Then choose one of the two install locations below. The skill's files are references/rule.md and SKILL.md, so unzip the download so that the cross-origin-isolation folder with those files ends up inside the skills folder.
For a personal install, unzip into ~/.claude/skills/. For a single project, unzip into the project's .claude/skills/ folder. You can also upload the ZIP in the Claude apps under Settings → Capabilities → Skills.
- Personal install: ~/.claude/skills/
- Project install: .claude/skills/ inside your project
- Claude apps: Settings → Capabilities → Skills, then upload the ZIP
Ask for a review with natural requests
You do not need a special command. The skill's description tells the agent when to use it, so describe the task in plain language that mentions the relevant features or headers. Mention the app, the feature you need, and the resources it loads.
Good requests name a concrete goal. Vague requests such as 'check my site' are less likely to match the skill's description.
- Review whether my browser editor needs COOP and COEP to use SharedArrayBuffer, and list the third-party scripts that could break.
- Audit the headers on my media tool and tell me whether the page becomes crossOriginIsolated.
- Our team is planning a COOP/COEP rollout. Go through our third-party iframes and fonts and flag what would be blocked.
Understand the headers and the browser check
The guide centres on three headers and one runtime check. COOP and COEP work together to enable isolation. Cross-Origin-Resource-Policy (CORP) or CORS is used on subresources so they can be embedded. The final check is whether the browser reports the page as isolated.
The source's example uses these values. Treat them as examples to review against your own setup, not as settings to paste in.
- Cross-Origin-Opener-Policy: same-origin
- Cross-Origin-Embedder-Policy: require-corp
- Cross-Origin-Resource-Policy: same-origin
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Resource-Policy: same-origin
if (!self.crossOriginIsolated) {
console.warn('This page is not cross-origin isolated yet.')
}Safety, requirements and limits
The skill lists no requirements. It does not run scripts and does not use the network. It is a set of instructions, so the agent reads the guide and then reasons about your code and configuration.
Incorrect headers can break embeds and logins. Make changes only after the audit, and test in a staging environment before rolling them out. The skill is a focused checklist, so it does not replace a complete security audit.
- Requires: nothing listed
- Runs scripts: no
- Uses network: no
- Not a complete security audit
What people use it for
- Checking whether a browser-based code editor needs COOP and COEP before enabling SharedArrayBuffer.
- Reviewing a worker-heavy web app for resources that would break once isolation is enforced.
- Auditing third-party iframes, scripts and fonts before a careful COOP/COEP rollout.
- Looking for opener-based attack risk and XS-Leak exposure in the headers a site sends.
Frequently asked questions
How do I install the cross-origin-isolation skill?
Download the skill ZIP from the directory page and unzip it into ~/.claude/skills/ for personal use or into a project's .claude/skills/ folder. You can also upload the ZIP in the Claude apps under Settings → Capabilities → Skills.
Is the cross-origin-isolation skill free to use?
The skill is published under the MIT licence, as listed on its page. Check the licence file in the source repository for the full terms.
What does the skill need to run?
The skill lists no requirements. It does not run scripts and does not use the network, so it works as a set of instructions the agent reads.
Does this skill scan my live website?
No. The skill is a focused checklist for review and does not scan a live site. You provide the code, headers or configuration you want reviewed.
Why does my page not become crossOriginIsolated after I add the headers?
A common cause is a third-party script, iframe, worker or font that is not compatible with COEP. Check that self.crossOriginIsolated is true in the browser, and review the embedded resources for CORP or CORS support.
Who should not use this skill?
Simple static sites or apps without advanced browser features gain little and risk breakage. Non-developers without access to server configuration should also avoid it, because incorrect headers can break embeds and logins.
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Our verdict
Use this skill if you are a web developer or security-minded engineer who needs SharedArrayBuffer or heavy workers and wants a structured review of COOP, COEP and third-party embeds. Pick something else if your site is simple, you cannot change server configuration, or you need a complete security audit or a live scan.
Download the skill