---
title: FASTLY_TRACE_ID
summary: null
url: https://www.fastly.com/documentation/reference/compute/ecp-env/fastly-trace-id
---

A UUID generated by Fastly for each sandbox. This is often a useful value to include in log messages, and also to send to upstream servers as an additional custom HTTP header, allowing for straightforward correlation of which WebAssembly sandbox processed a request to requests later processed by an origin server. If a sandbox is used to process multiple downstream requests, then you may wish to use the per-request UUID associated with each individual request handle instead of this environment variable.