diff --git a/src/aioweb_tls/backends.py b/src/aioweb_tls/backends.py
index a1ba284..f5b6e51 100644
--- a/src/aioweb_tls/backends.py
+++ b/src/aioweb_tls/backends.py
@@ -105,8 +105,17 @@ class CurlCffi:
         )
 
     def is_closed(self, session) -> bool:
-        """whether the curl_cffi session is closed"""
-        return bool(getattr(session, "closed", False))
+        """whether the curl_cffi session is closed
+
+        curl_cffi tracks closed state in the private `_closed` (no public `closed`
+        property), so read that; fall back to a public `closed` if a future version
+        adds one. TLSSession's own `_closed` flag is the primary signal — this is a
+        best-effort backend check for out-of-band closes.
+        """
+        closed = getattr(session, "_closed", None)
+        if closed is None:
+            closed = getattr(session, "closed", False)
+        return bool(closed)
 
     def cookies_for_url(self, session, url) -> dict:
         """cookies curl_cffi would send for url (best-effort)"""