diff --git a/src/mcp/shared/session.py b/src/mcp/shared/session.py
index 243eef5ae..c0c23dc9b 100644
--- a/src/mcp/shared/session.py
+++ b/src/mcp/shared/session.py
@@ -130,7 +130,9 @@ async def respond(self, response: SendResultT | ErrorData) -> None:
         """
         if not self._entered:  # pragma: no cover
             raise RuntimeError("RequestResponder must be used as a context manager")
-        assert not self._completed, "Request already responded to"
+        # Guard against race: if cancel() already set _completed, skip silently.
+        if self._completed:
+            return
 
         if not self.cancelled:  # pragma: no branch
             self._completed = True