diff --git a/CHANGELOG.md b/CHANGELOG.md
index a4f49c45d1..6fc53394a7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ 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)
 * The `Response.raise_for_status()` method now returns the response instance. For example: `data = httpx.get('...').raise_for_status().json()`. (#2776)
 
diff --git a/httpx/_types.py b/httpx/_types.py
index 6b610e1408..83cf35a32a 100644
--- a/httpx/_types.py
+++ b/httpx/_types.py
@@ -16,6 +16,7 @@
     Iterator,
     List,
     Mapping,
+    MutableMapping,
     NamedTuple,
     Optional,
     Sequence,
@@ -87,7 +88,7 @@
 
 RequestContent = Union[str, bytes, Iterable[bytes], AsyncIterable[bytes]]
 ResponseContent = Union[str, bytes, Iterable[bytes], AsyncIterable[bytes]]
-ResponseExtensions = Mapping[str, Any]
+ResponseExtensions = MutableMapping[str, Any]
 
 RequestData = Mapping[str, Any]
 
@@ -104,7 +105,7 @@
 ]
 RequestFiles = Union[Mapping[str, FileTypes], Sequence[Tuple[str, FileTypes]]]
 
-RequestExtensions = Mapping[str, Any]
+RequestExtensions = MutableMapping[str, Any]
 
 
 class SyncByteStream:
diff --git a/pyproject.toml b/pyproject.toml
index acd41baf3a..753e671ebc 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -28,7 +28,7 @@ classifiers = [
 ]
 dependencies = [
     "certifi",
-    "httpcore>=0.17.2,<0.18.0",
+    "httpcore>=0.18.0,<0.19.0",
     "idna",
     "sniffio",
 ]