> ## Documentation Index
> Fetch the complete documentation index at: https://prism-ddf93e61.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Replay Engine

> Reconstructing state and instrumenting execution.

The Replay Engine powers Tier 2 and Tier 3 diagnostics.

## Historical State

It uses the **History Archive Client** to fetch compressed ledger snapshots. These are decompressed and loaded into a local state database.

## Instrumented Sandbox

Prism uses a modified `soroban-env-host` that:

* Emits a trace event for every host function call.
* Records gas/budget consumption at each step.
* Captures event emissions and storage diffs.

## Execution Timeline

As events are emitted, the engine assembles them into a hierarchical tree representing the full invocation chain.