diff --git a/hugr-py/src/hugr/serialization/serial_hugr.py b/hugr-py/src/hugr/serialization/serial_hugr.py
index 9be4aa69c..a8c104937 100644
--- a/hugr-py/src/hugr/serialization/serial_hugr.py
+++ b/hugr-py/src/hugr/serialization/serial_hugr.py
@@ -2,7 +2,7 @@
 
 from pydantic import BaseModel, Field, ConfigDict
 
-from .ops import NodeID, OpType, classes
+from .ops import NodeID, OpType, classes as ops_classes
 from .tys import model_rebuild
 import hugr
 
@@ -37,7 +37,9 @@ def get_version(cls) -> str:
 
     @classmethod
     def _pydantic_rebuild(cls, config: ConfigDict = ConfigDict(), **kwargs):
-        model_rebuild([(cls.__name__, cls)] + classes, config=config, **kwargs)
+        my_classes = dict(ops_classes)
+        my_classes[cls.__name__] = cls
+        model_rebuild(my_classes, config=config, **kwargs)
 
     class Config:
         title = "Hugr"
diff --git a/hugr-py/src/hugr/serialization/testing_hugr.py b/hugr-py/src/hugr/serialization/testing_hugr.py
index 5bac1f114..59db4b80d 100644
--- a/hugr-py/src/hugr/serialization/testing_hugr.py
+++ b/hugr-py/src/hugr/serialization/testing_hugr.py
@@ -1,7 +1,7 @@
 from pydantic import ConfigDict
 from typing import Literal
 from .tys import Type, SumType, PolyFuncType, ConfiguredBaseModel, model_rebuild
-from .ops import Value, OpType, classes
+from .ops import Value, OpType, classes as ops_classes
 
 
 class TestingHugr(ConfiguredBaseModel):
@@ -22,7 +22,9 @@ def get_version(cls) -> str:
 
     @classmethod
     def _pydantic_rebuild(cls, config: ConfigDict = ConfigDict(), **kwargs):
-        model_rebuild([(cls.__name__, cls)] + classes, config=config, **kwargs)
+        my_classes = dict(ops_classes)
+        my_classes[cls.__name__] = cls
+        model_rebuild(my_classes, config=config, **kwargs)
 
     class Config:
         title = "HugrTesting"
diff --git a/hugr-py/src/hugr/serialization/tys.py b/hugr-py/src/hugr/serialization/tys.py
index bc39e56ef..b75a614cd 100644
--- a/hugr-py/src/hugr/serialization/tys.py
+++ b/hugr-py/src/hugr/serialization/tys.py
@@ -1,7 +1,7 @@
 import inspect
 import sys
 from enum import Enum
-from typing import Annotated, Any, Literal, Optional, Union
+from typing import Annotated, Any, Literal, Optional, Union, Mapping
 
 from pydantic import (
     BaseModel,
@@ -354,16 +354,16 @@ class Signature(ConfiguredBaseModel):
 
 
 def model_rebuild(
-    classes: list[tuple[str, Any]],
+    classes: Mapping[str, type],
     config: ConfigDict = ConfigDict(),
     **kwargs,
 ):
     new_config = default_model_config.copy()
     new_config.update(config)
-    for c in {k: v for (k, v) in classes}.values():
+    for c in classes.values():
         if issubclass(c, ConfiguredBaseModel):
             c.set_model_config(new_config)
             c.model_rebuild(**kwargs)
 
 
-model_rebuild(classes)
+model_rebuild(dict(classes))