> ## Documentation Index
> Fetch the complete documentation index at: https://www.testmuai.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Performance Assertions

> Verify Core Web Vitals and other performance metrics (LCP, CLS, INP, FCP, TTFB) captured during test execution.

***

<Note>
  **Using Claude Code, Cursor, or another coding agent?** Paste this into your prompt to run cross-browser and real-device tests, debug sessions, and wire up CI on the TestMu AI cloud:

  ```
  Read https://www.testmuai.com/support/docs/SKILL.md to set up TestMu AI (formerly LambdaTest) cloud testing.
  ```
</Note>

Performance assertions let you verify [Core Web Vitals](https://web.dev/articles/vitals) and other key performance metrics for the current page.

## How Capture Works

Performance data is **navigation-based**. Metrics are measured for the most recent page navigation:

* **Automatic measurement**: KaneAI uses the [web-vitals](https://github.com/GoogleChrome/web-vitals) library to capture metrics
* **Per-navigation scope**: Metrics reflect the last full page load. If you navigate to a new page, the metrics reset for that navigation
* **Point-in-time snapshot**: When a performance checkpoint triggers, KaneAI captures the current metrics at that moment

### What This Means for Your Tests

* Performance metrics describe the **last navigation**. If you navigate to Page A then Page B, the metrics reflect Page B
* Place performance assertions **after** the page you want to measure has fully loaded
* Use a wait step if the page needs time to settle before measuring

## Available Metrics

| Metric   | What It Measures                                                              | Good Threshold | Learn More                                    |
| -------- | ----------------------------------------------------------------------------- | -------------- | --------------------------------------------- |
| **LCP**  | Largest Contentful Paint: when the largest visible element finishes rendering | \< 2,500ms     | [web.dev/lcp](https://web.dev/articles/lcp)   |
| **CLS**  | Cumulative Layout Shift: visual stability, how much the page layout shifts    | \< 0.1         | [web.dev/cls](https://web.dev/articles/cls)   |
| **INP**  | Interaction to Next Paint: responsiveness to user input                       | \< 200ms       | [web.dev/inp](https://web.dev/articles/inp)   |
| **FCP**  | First Contentful Paint: when the first content appears on screen              | \< 1,800ms     | [web.dev/fcp](https://web.dev/articles/fcp)   |
| **TTFB** | Time to First Byte: server response time                                      | \< 800ms       | [web.dev/ttfb](https://web.dev/articles/ttfb) |

> **Note**: Not all metrics are available for every page. INP requires user interaction to trigger. Some metrics may be `null` if the browser hasn't measured them yet.

## Example Assertions

```
Assert: page LCP is under 2500ms
Assert: CLS is below 0.1
Assert: TTFB is under 800ms
Assert: FCP is less than 1800ms
Assert: page performance meets Core Web Vitals thresholds
```

## Example Extractions

```
Store the page LCP value
Extract all web vitals metrics
Store the TTFB for this page
```

## Example If/Else

```
If LCP is under 2500ms then continue, else report performance issue
```

## Tips

* **Wait for load**: Place a wait step before performance assertions to ensure the page has fully loaded and metrics are available
* **Navigate first**: Metrics are per-navigation. Make sure you've navigated to the target page before asserting
* **Not all metrics are instant**: CLS accumulates over time, INP requires interaction. LCP and FCP are typically available after the page visually completes loading