diff --git a/HISTORY.md b/HISTORY.md
index e7c8d61021..d536440ddc 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,6 +1,12 @@
 Release History
 ===============
 
+3.7.1 (2024-06-??)
+------------------
+
+**Fixed**
+- auth argument not accepting a function according to static type checkers. (#133)
+
 3.7.0 (2024-06-24)
 ------------------
 
diff --git a/src/niquests/_typing.py b/src/niquests/_typing.py
index 6a880f07a3..b301e74f82 100644
--- a/src/niquests/_typing.py
+++ b/src/niquests/_typing.py
@@ -26,10 +26,11 @@
 from .auth import AuthBase
 from .structures import CaseInsensitiveDict
 
+if typing.TYPE_CHECKING:
+    from .models import PreparedRequest
+
 #: (Restricted) list of http verb that we natively support and understand.
-HttpMethodType: typing.TypeAlias = (
-    str  # todo: have typing.Literal when ready to drop Python 3.7
-)
+HttpMethodType: typing.TypeAlias = str
 #: List of formats accepted for URL queries parameters. (e.g. /?param1=a&param2=b)
 QueryParameterType: typing.TypeAlias = typing.Union[
     typing.List[typing.Tuple[str, typing.Union[str, typing.List[str]]]],
@@ -89,6 +90,7 @@
     typing.Tuple[typing.Union[str, bytes], typing.Union[str, bytes]],
     str,
     AuthBase,
+    typing.Callable[["PreparedRequest"], "PreparedRequest"],
 ]
 #: Map for each protocol (http, https) associated proxy to be used.
 ProxyType: typing.TypeAlias = typing.Dict[str, str]
diff --git a/src/niquests/models.py b/src/niquests/models.py
index 513e4b1bd4..9f2b0ff40e 100644
--- a/src/niquests/models.py
+++ b/src/niquests/models.py
@@ -323,6 +323,8 @@ def __init__(self) -> None:
         self.ocsp_verified: bool | None = None
         #: upload progress if any.
         self.upload_progress: TransferProgress | None = None
+        #: internal usage only. warn us that we should re-compute content-length and await auth() outside of PreparedRequest.
+        self._asynchronous_auth: bool = False
 
     @property
     def oheaders(self) -> Headers:
@@ -636,7 +638,11 @@ def prepare_auth(self, auth: HttpAuthenticationType | None, url: str = "") -> No
                     "Unexpected non-callable authentication. Did you pass unsupported tuple to auth argument?"
                 )
 
-            if not asyncio.iscoroutinefunction(auth.__call__):
+            self._asynchronous_auth = hasattr(
+                auth, "__call__"
+            ) and asyncio.iscoroutinefunction(auth.__call__)
+
+            if not self._asynchronous_auth:
                 # Allow auth to make its changes.
                 r = auth(self)