---
title: Fastly-Stats
summary: null
url: https://www.fastly.com/documentation/reference/http/http-headers/Fastly-Stats
---

This header is written by Fastly when a response is transformed by [Image Optimizer](https://docs.fastly.com/products/image-optimizer) (`io`) or the [On-the-Fly Packaging service](https://docs.fastly.com/products/fastlys-onthefly-packaging-service) (`otfp`).

## Format

```
io=1; otfp=1
```