---
title: Change tracking
description: Detect added and changed components and resources.
sidebar:
  order: 7
---

`[TrackChanges]` enables added and changed stamps for a component or resource. A change filter on an untracked type is an error ([`AV006`](/reference/diagnostics)).

## What a system observes

A system observes stamps satisfying `lastSuccessfulRun < stamp <= thisRun`. A skipped or failed body does not advance its last successful tick. Conditions have separate ticks and cursors.

## What stamps a change

| Operation | Added | Changed |
| --- | --- | --- |
| Insertion | Stamped | Stamped |
| Replacement | Preserved | Stamped, including equal-value replacement |
| Moving between archetypes | Preserved | Preserved for surviving components |
| Readonly access | Unchanged | Unchanged |

A generated writable row property stamps before returning its ref. `[Each]` stamps tracked writable refs before the method call. `ResMut<T>.Value` follows the same rule.

:::note
Taking a writable ref counts as a change even when the method leaves the value unchanged. Use `Read<T>` or `scoped in` when you do not intend to write.
:::