# Page experience diagnosis

> Use when Core Web Vitals are failing and you need a prioritised engineering brief.

## Fill in before running

- `{{URL}}`
- `{{FIELD_DATA}}`
- `{{LAB_DATA}}`
- `{{STACK}}`
- `{{THIRD_PARTIES}}`

## Prompt

```
You are diagnosing page experience problems and writing a brief engineers can act on.

Template or URL: {{URL}}
Field data (CrUX or RUM: LCP, INP, CLS, split by device): {{FIELD_DATA}}
Lab data (Lighthouse or WebPageTest summary): {{LAB_DATA}}
Stack: {{STACK}}
Known third party scripts: {{THIRD_PARTIES}}

Output:
1. A metric table: Metric | Field value | Threshold | Pass or fail | Device where it is worst.
2. For each failing metric, the likely causes ranked by probability, each with the evidence from the supplied data that supports it.
3. A fix table: Fix | Metric affected | Expected direction of change | Effort (S / M / L) | Risk of regression | Who does it.
4. "Measure this to confirm" - the exact check for each fix.

Constraints:
- Field data outranks lab data. Where they disagree, say which you are trusting and why.
- Do not predict a numeric improvement, for example "will cut LCP by 1.2s". State direction and reasoning instead.
- Call out any fix that improves lab scores without improving real user experience.
- If a third party in {{THIRD_PARTIES}} is a plausible cause, name it and say how to test by removal.
```

## Getting a better result

- Always paste field data - lab scores alone lead to optimising the wrong thing.
- Split by device before pasting, since mobile INP failures usually have a different cause to desktop.
- Ask for the measurement check to be added to the ticket so the fix can be proven.

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From the QuQi prompt library - https://www.quqi.io/prompts/page-experience-diagnosis