diff --git a/httpx/_client.py b/httpx/_client.py
index 13cd933673..94e3e2e295 100644
--- a/httpx/_client.py
+++ b/httpx/_client.py
@@ -1069,7 +1069,7 @@ def options(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1098,7 +1098,7 @@ def head(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1131,7 +1131,7 @@ def post(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1168,7 +1168,7 @@ def put(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1205,7 +1205,7 @@ def patch(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1238,7 +1238,7 @@ def delete(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1784,7 +1784,7 @@ async def options(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1813,7 +1813,7 @@ async def head(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1846,7 +1846,7 @@ async def post(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1883,7 +1883,7 @@ async def put(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1920,7 +1920,7 @@ async def patch(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,
@@ -1953,7 +1953,7 @@ async def delete(
         params: QueryParamTypes | None = None,
         headers: HeaderTypes | None = None,
         cookies: CookieTypes | None = None,
-        auth: AuthTypes | UseClientDefault = USE_CLIENT_DEFAULT,
+        auth: AuthTypes | UseClientDefault | None = USE_CLIENT_DEFAULT,
         follow_redirects: bool | UseClientDefault = USE_CLIENT_DEFAULT,
         timeout: TimeoutTypes | UseClientDefault = USE_CLIENT_DEFAULT,
         extensions: RequestExtensions | None = None,