diff --git a/hugr-py/src/hugr/tys.py b/hugr-py/src/hugr/tys.py
index 58b3fe337..a17020801 100644
--- a/hugr-py/src/hugr/tys.py
+++ b/hugr-py/src/hugr/tys.py
@@ -258,6 +258,10 @@ def __repr__(self) -> str:
     def type_bound(self) -> TypeBound:
         return TypeBound.join(*(t.type_bound() for r in self.variant_rows for t in r))
 
+    def resolve(self, registry: ext.ExtensionRegistry) -> Sum:
+        """Resolve types in the sum type using the given registry."""
+        return Sum([[ty.resolve(registry) for ty in row] for row in self.variant_rows])
+
 
 @dataclass()
 class UnitSum(Sum):
@@ -279,6 +283,9 @@ def __repr__(self) -> str:
             return "Unit"
         return f"UnitSum({self.size})"
 
+    def resolve(self, registry: ext.ExtensionRegistry) -> UnitSum:
+        return self
+
 
 @dataclass()
 class Tuple(Sum):