# LCP Debugging Snippets
Use these JavaScript snippets with the `evaluate_script` tool to extract deep insights from the page.
## 1. Identify LCP Element
Use this snippet to identify the LCP element and get raw timing data from the Performance API.
```javascript
async () => {
return await new Promise(resolve => {
new PerformanceObserver(list => {
const entries = list.getEntries();
const last = entries[entries.length - 1];
resolve({
element: last.element?.tagName,
id: last.element?.id,
className: last.element?.className,
url: last.url,
startTime: last.startTime,
renderTime: last.renderTime,
loadTime: last.loadTime,
size: last.size,
});
}).observe({type: 'largest-contentful-paint', buffered: true});
});
};
```
## 2. Audit Common Issues
Use this snippet to check for common DOM-based LCP issues (lazy loading, priority).
```javascript
() => {
const issues = [];
// Check for lazy-loaded images in viewport
document.querySelectorAll('img[loading="lazy"]').forEach(img => {
const rect = img.getBoundingClientRect();
if (rect.top < window.innerHeight) {
issues.push({
issue: 'lazy-loaded image in viewport',
element: img.outerHTML.substring(0, 200),
fix: 'Remove loading="lazy" from this image — it is in the initial viewport and may be the LCP element',
});
}
});
// Check for LCP-candidate images missing fetchpriority
document.querySelectorAll('img:not([fetchpriority])').forEach(img => {
const rect = img.getBoundingClientRect();
if (rect.top < window.innerHeight && rect.width * rect.height > 50000) {
issues.push({
issue: 'large viewport image without fetchpriority',
element: img.outerHTML.substring(0, 200),
fix: 'Add fetchpriority="high" to this image — it is large and visible in the initial viewport',
});
}
});
// Check for render-blocking scripts in head
document
.querySelectorAll(
'head script:not([async]):not([defer]):not([type="module"])',
)
.forEach(script => {
if (script.src) {
issues.push({
issue: 'render-blocking script in head',
element: script.outerHTML.substring(0, 200),
fix: 'Add async or defer attribute, or move to end of body',
});
}
});
return {issueCount: issues.length, issues};
};
```
