From a811fc041a8091cbe4092f3600e3ee9ab813bbd6 Mon Sep 17 00:00:00 2001
From: Sylvester Kaczmarek
 <16242628+sylvesterkaczmarek@users.noreply.github.com>
Date: Mon, 17 Aug 2026 12:05:53 +0100
Subject: [PATCH] Accept awaitable-returning async middleware callables

---
 src/anthropic/_middleware.py                  | 10 +--
 ...est_async_middleware_awaitable_callable.py | 74 +++++++++++++++++++
 2 files changed, 79 insertions(+), 5 deletions(-)
 create mode 100644 tests/test_async_middleware_awaitable_callable.py

diff --git a/src/anthropic/_middleware.py b/src/anthropic/_middleware.py
index c4df340f5..5138d7500 100644
--- a/src/anthropic/_middleware.py
+++ b/src/anthropic/_middleware.py
@@ -119,8 +119,8 @@ def validate_async_middleware(middleware: Iterable[MiddlewareInput]) -> None:
                 )
         elif not callable(entry):
             raise TypeError(f"middleware {_middleware_name(entry)} is not callable")
-        elif not _is_async_callable(entry):
-            raise TypeError(
-                f"middleware {_middleware_name(entry)} is not an async function; "
-                "the asynchronous client requires async middleware functions"
-            )
+        # Function-style async middleware is typed as Callable[..., Awaitable],
+        # not specifically as a coroutine function. A synchronous wrapper may
+        # therefore validly return an awaitable; the async middleware chain
+        # awaits that result at invocation time. Do not reject that supported
+        # shape based only on inspect.iscoroutinefunction().
diff --git a/tests/test_async_middleware_awaitable_callable.py b/tests/test_async_middleware_awaitable_callable.py
new file mode 100644
index 000000000..26ba420f3
--- /dev/null
+++ b/tests/test_async_middleware_awaitable_callable.py
@@ -0,0 +1,74 @@
+from __future__ import annotations
+
+from typing import Any, Awaitable
+
+import anyio
+import httpx
+import pytest
+
+from anthropic import AsyncAnthropic
+from anthropic._middleware import AsyncCallNext
+from anthropic._request import APIRequest
+
+
+def test_async_client_accepts_sync_wrapper_returning_awaitable() -> None:
+    seen: list[str] = []
+
+    def middleware(request: APIRequest, call_next: AsyncCallNext) -> Awaitable[Any]:
+        seen.append(request.url)
+        return call_next(request)
+
+    async def run() -> None:
+        async def handler(request: httpx.Request) -> httpx.Response:
+            return httpx.Response(200, json={"ok": True}, request=request)
+
+        client = AsyncAnthropic(
+            api_key="test",
+            base_url="https://example.test",
+            middleware=[middleware],
+            http_client=httpx.AsyncClient(transport=httpx.MockTransport(handler)),
+        )
+        try:
+            result = await client.get("/probe", cast_to=object)
+        finally:
+            await client.close()
+
+        assert result == {"ok": True}
+
+    anyio.run(run)
+    assert seen == ["/probe"]
+
+
+def test_async_client_accepts_sync_callable_object_returning_awaitable() -> None:
+    class MiddlewareWrapper:
+        def __init__(self) -> None:
+            self.calls = 0
+
+        def __call__(self, request: APIRequest, call_next: AsyncCallNext) -> Awaitable[Any]:
+            self.calls += 1
+            return call_next(request)
+
+    wrapper = MiddlewareWrapper()
+
+    async def run() -> None:
+        async def handler(request: httpx.Request) -> httpx.Response:
+            return httpx.Response(200, json={"ok": True}, request=request)
+
+        client = AsyncAnthropic(
+            api_key="test",
+            base_url="https://example.test",
+            middleware=[wrapper],
+            http_client=httpx.AsyncClient(transport=httpx.MockTransport(handler)),
+        )
+        try:
+            assert await client.get("/probe", cast_to=object) == {"ok": True}
+        finally:
+            await client.close()
+
+    anyio.run(run)
+    assert wrapper.calls == 1
+
+
+def test_async_client_still_rejects_non_callable_middleware() -> None:
+    with pytest.raises(TypeError, match="is not callable"):
+        AsyncAnthropic(api_key="test", middleware=[object()])  # type: ignore[list-item]