diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4942c9c9..82d0887d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
+* Change the type of `Extensions` from `Mapping[Str, Any]` to `MutableMapping[Str, Any]`. (#2803)
 * Add `socket_options` argument to `httpx.HTTPTransport` and `httpx.AsyncHTTPTransport` classes. (#2716)
 
 ### Fixed