diff --git a/guppy/custom.py b/guppy/custom.py
index 8c5a4fc1..f228f46f 100644
--- a/guppy/custom.py
+++ b/guppy/custom.py
@@ -239,7 +239,9 @@ def __init__(self, op: ops.OpType) -> None:
         self.op = op
 
     def compile(self, args: list[OutPortV]) -> list[OutPortV]:
-        node = self.graph.add_node(self.op.copy(), inputs=args, parent=self.dfg.node)
+        node = self.graph.add_node(
+            self.op.model_copy(), inputs=args, parent=self.dfg.node
+        )
         return_ty = get_type(self.node)
         return [node.add_out_port(ty) for ty in type_to_row(return_ty)]
 
diff --git a/guppy/hugr/ops.py b/guppy/hugr/ops.py
index 02b393e3..f0391799 100644
--- a/guppy/hugr/ops.py
+++ b/guppy/hugr/ops.py
@@ -13,6 +13,7 @@
     FunctionType,
     PolyFuncType,
     SimpleType,
+    SumUnion,
     TypeRow,
 )
 from .val import Value
@@ -126,7 +127,7 @@ def insert_port_types(self, in_types: TypeRow, out_types: TypeRow) -> None:
     def insert_child_dfg_signature(self, inputs: TypeRow, outputs: TypeRow) -> None:
         self.inputs = inputs
         pred = outputs[0]
-        assert isinstance(pred, tys.Sum)
+        assert isinstance(pred, SumUnion)
         if isinstance(pred, tys.UnitSum):
             self.tuple_sum_rows = [[] for _ in range(pred.size)]
         else:
@@ -555,7 +556,7 @@ class TypeApplication(BaseModel):
 # --------------------------------------
 
 
-class OpDef(BaseOp, allow_population_by_field_name=True):
+class OpDef(BaseOp, populate_by_name=True):
     """Serializable definition for dynamically loaded operations."""
 
     name: str  # Unique identifier of the operation.
@@ -577,4 +578,4 @@ class OpDef(BaseOp, allow_population_by_field_name=True):
 )
 for _, c in classes:
     if issubclass(c, BaseModel):
-        c.update_forward_refs()
+        c.model_rebuild()
diff --git a/guppy/hugr/raw.py b/guppy/hugr/raw.py
index a3249a3c..bedee643 100644
--- a/guppy/hugr/raw.py
+++ b/guppy/hugr/raw.py
@@ -15,7 +15,7 @@ class RawHugr(BaseModel):
     edges: list[Edge]
 
     def packb(self) -> bytes:
-        return ormsgpack.packb(self.dict(), option=ormsgpack.OPT_NON_STR_KEYS)
+        return ormsgpack.packb(self.model_dump(), option=ormsgpack.OPT_NON_STR_KEYS)
 
     @classmethod
     def unpackb(cls, b: bytes) -> "RawHugr":
diff --git a/guppy/hugr/tys.py b/guppy/hugr/tys.py
index 9098e552..750029f1 100644
--- a/guppy/hugr/tys.py
+++ b/guppy/hugr/tys.py
@@ -1,6 +1,5 @@
 import inspect
 import sys
-from abc import ABC
 from enum import Enum
 from typing import Annotated, Literal
 
@@ -24,7 +23,7 @@ class TypeParam(BaseModel):
 
 class BoundedNatParam(BaseModel):
     tp: Literal["BoundedNat"] = "BoundedNat"
-    bound: int | None
+    bound: int | None = None
 
 
 class OpaqueParam(BaseModel):
@@ -118,26 +117,32 @@ class Tuple(BaseModel):
     inner: "TypeRow"
 
 
-class Sum(ABC, BaseModel):
-    """Sum type, variants are tagged by their position in the type row"""
+# class Sum(ABC, BaseModel):
+#     """Sum type, variants are tagged by their position in the type row"""
 
-    t: Literal["Sum"] = "Sum"
+#     t: Literal["Sum"] = "Sum"
 
 
-class UnitSum(Sum):
+class UnitSum(BaseModel):
     """Simple predicate where all variants are empty tuples"""
 
+    t: Literal["Sum"] = "Sum"
+
     s: Literal["Unit"] = "Unit"
     size: int
 
 
-class GeneralSum(Sum):
+class GeneralSum(BaseModel):
     """General sum type that explicitly stores the types of the variants"""
 
+    t: Literal["Sum"] = "Sum"
+
     s: Literal["General"] = "General"
     row: "TypeRow"
 
 
+SumUnion = UnitSum | GeneralSum
+Sum = Annotated[SumUnion, Field(discriminator="s")]
 # ----------------------------------------------
 # --------------- ClassicType ------------------
 # ----------------------------------------------
@@ -284,4 +289,4 @@ class Signature(BaseModel):
 )
 for _, c in classes:
     if issubclass(c, BaseModel):
-        c.update_forward_refs()
+        c.model_rebuild()
diff --git a/guppy/hugr/val.py b/guppy/hugr/val.py
index b8b5eabb..bb6a2cd2 100644
--- a/guppy/hugr/val.py
+++ b/guppy/hugr/val.py
@@ -18,7 +18,7 @@ class FunctionVal(BaseModel):
     """A higher-order function value."""
 
     v: Literal["Function"] = "Function"
-    hugr: Any  # TODO
+    hugr: Any = None  # TODO
 
 
 class Tuple(BaseModel):
@@ -50,4 +50,4 @@ class Sum(BaseModel):
 )
 for _, c in classes:
     if issubclass(c, BaseModel):
-        c.update_forward_refs()
+        c.model_rebuild()
diff --git a/pyproject.toml b/pyproject.toml
index 6d7333c8..88c8d8d3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -11,7 +11,7 @@ dependencies = [
     "graphviz",
     "networkx",
     "ormsgpack",
-    "pydantic==1.10.8",
+    "pydantic==2.5.3",
 ]
 
 [project.optional-dependencies]