docs: add changeset for #6162

.changesets/fix_renee_router_815_changeset.md

@@ -0,0 +1,11 @@
+### Fix cost calculation for subgraph requests with named fragments ([PR #6162](https://github.com/apollographql/router/issues/6162))
+
+In some cases where subgraph GraphQL operations contain named fragments and abstract types, demand control used the wrong type for cost calculation, and could reject valid operations.
+Now, the correct type is used.
+
+This fixes errors of the form:
+```
+Attempted to look up a field on type MyInterface, but the field does not exist
+```
+
+By [@goto-bus-stop](https://github.com/goto-bus-stop) in https://github.com/apollographql/router/pull/6162