diff --git a/respx/models.py b/respx/models.py
index 9690324..2f12caa 100644
--- a/respx/models.py
+++ b/respx/models.py
@@ -327,14 +327,18 @@ def _next_side_effect(
         return effect
 
     def _call_side_effect(
-        self, effect: CallableSideEffect, request: httpx.Request, **kwargs: Any
+        self, effect: CallableSideEffect, request: httpx.Request, /, is_async: bool, **kwargs: Any
     ) -> RouteResultTypes:
         # Add route kwarg if the side effect wants it
         argspec = inspect.getfullargspec(effect)
         if "route" in kwargs:
             warn(f"Matched context contains reserved word `route`: {self.pattern!r}")
+        if "is_async" in kwargs:
+            warn(f"Matched context contains reserved word `is_async`: {self.pattern!r}")
         if "route" in argspec.args:
             kwargs["route"] = self
+        if "is_async" in argspec.args:
+            kwargs["is_async"] = is_async
 
         try:
             # Call side effect
@@ -357,7 +361,7 @@ def _call_side_effect(
         return result
 
     def _resolve_side_effect(
-        self, request: httpx.Request, **kwargs: Any
+            self, request: httpx.Request, /, is_async: bool, **kwargs: Any
     ) -> RouteResultTypes:
         effect = self._next_side_effect()
 
@@ -379,17 +383,17 @@ def _resolve_side_effect(
 
         # Handle `Callable` side effect
         elif callable(effect):
-            result = self._call_side_effect(effect, request, **kwargs)
+            result = self._call_side_effect(effect, request, is_async, **kwargs)
             return result
 
         # Resolved effect is a mocked response
         return effect
 
-    def resolve(self, request: httpx.Request, **kwargs: Any) -> RouteResultTypes:
+    def resolve(self, request: httpx.Request, /, is_async: bool, **kwargs: Any) -> RouteResultTypes:
         result: RouteResultTypes = None
 
         if self._side_effect:
-            result = self._resolve_side_effect(request, **kwargs)
+            result = self._resolve_side_effect(request, is_async, **kwargs)
             if result is None:
                 return None  # Side effect resolved as a non-matching route
 
@@ -406,7 +410,7 @@ def resolve(self, request: httpx.Request, **kwargs: Any) -> RouteResultTypes:
 
         return result
 
-    def match(self, request: httpx.Request) -> RouteResultTypes:
+    def match(self, request: httpx.Request, /, is_async: bool) -> RouteResultTypes:
         """
         Matches and resolves request with given patterns and optional side effect.
 
@@ -424,7 +428,7 @@ def match(self, request: httpx.Request) -> RouteResultTypes:
         if self._pass_through:
             return request
 
-        result = self.resolve(request, **context)
+        result = self.resolve(request, is_async, **context)
         return result
 
 
diff --git a/respx/router.py b/respx/router.py
index 449a5a4..8fa49d6 100644
--- a/respx/router.py
+++ b/respx/router.py
@@ -276,7 +276,7 @@ def resolver(self, request: httpx.Request) -> Generator[ResolvedRoute, None, Non
     def resolve(self, request: httpx.Request) -> ResolvedRoute:
         with self.resolver(request) as resolved:
             for route in self.routes:
-                prospect = route.match(request)
+                prospect = route.match(request, False)
                 if prospect is not None:
                     resolved.route = route
                     resolved.response = cast(ResolvedResponseTypes, prospect)
@@ -290,7 +290,7 @@ def resolve(self, request: httpx.Request) -> ResolvedRoute:
     async def aresolve(self, request: httpx.Request) -> ResolvedRoute:
         with self.resolver(request) as resolved:
             for route in self.routes:
-                prospect: RouteResultTypes = route.match(request)
+                prospect: RouteResultTypes = route.match(request, True)
 
                 # Await async side effect and wrap any exception
                 if inspect.isawaitable(prospect):