diff --git a/ingestor/connection/pool.py b/ingestor/connection/pool.py
index 8e93542..73002f5 100644
--- a/ingestor/connection/pool.py
+++ b/ingestor/connection/pool.py
@@ -107,7 +107,9 @@ class OpcConnectionPool(SientiaMonitoring):
 
     async def close_all(self):
         """Disconnect every pooled client (e.g. on shutdown)."""
-        for server_id in list(self._connections):
+        # list() snapshots the keys: _evict() pops from self._connections,
+        # so iterating the dict directly would raise "changed size during iteration".
+        for server_id in list(self._connections):  # NOSONAR
             await self._evict(server_id)
 
     async def _evict(self, server_id: str):