From 766c9b7dad487ebacd75556b47cd380c1f729561 Mon Sep 17 00:00:00 2001
From: Sohom Chakraborty <16609933+sohom-cs@users.noreply.github.com>
Date: Fri, 30 Oct 2026 05:55:59 -0400
Subject: [PATCH] Leave computed fields out of Pydantic payloads so
 extra="forbid" models decode

Serialize with round_trip=True on both the default path and the
ToJsonOptions path, so that from_payload can validate what to_payload
wrote: computed fields are left out (validation recomputes them) and
Json[...] fields are written as JSON strings.
---
 changelog/fixed/recomputed-raccoon.md   |  1 +
 temporalio/contrib/pydantic.py          | 13 ++++--
 tests/contrib/pydantic/test_pydantic.py | 62 +++++++++++++++++++++++++
 3 files changed, 73 insertions(+), 3 deletions(-)
 create mode 100644 changelog/fixed/recomputed-raccoon.md

diff --git a/changelog/fixed/recomputed-raccoon.md b/changelog/fixed/recomputed-raccoon.md
new file mode 100644
index 000000000..cece7f41d
--- /dev/null
+++ b/changelog/fixed/recomputed-raccoon.md
@@ -0,0 +1 @@
+`temporalio.contrib.pydantic`: the Pydantic payload converter now serializes with `round_trip=True`, so models with computed fields and `extra="forbid"`, and models with `Json[...]` fields, can be decoded from its payloads; computed fields are no longer written to payloads.
diff --git a/temporalio/contrib/pydantic.py b/temporalio/contrib/pydantic.py
index bb8fe0010..2ee4d4aac 100644
--- a/temporalio/contrib/pydantic.py
+++ b/temporalio/contrib/pydantic.py
@@ -104,17 +104,24 @@ def encoding(self) -> str:
     def to_payload(self, value: Any) -> temporalio.api.common.v1.Payload | None:
         """See base class.
 
-        Uses ``pydantic_core.to_json`` to serialize ``value`` to JSON.
+        Uses ``pydantic_core.to_json`` to serialize ``value`` to JSON, with
+        ``round_trip=True`` so that ``from_payload`` can validate the output
+        back into the same value: computed fields are left out, since
+        validation recomputes them, and ``Json[...]`` fields are written as
+        JSON strings.
 
         See
         https://docs.pydantic.dev/latest/api/pydantic_core/#pydantic_core.to_json.
         """
         data = (
             self._schema_serializer.to_json(
-                value, by_alias=True, exclude_unset=self._to_json_options.exclude_unset
+                value,
+                by_alias=True,
+                exclude_unset=self._to_json_options.exclude_unset,
+                round_trip=True,
             )
             if self._to_json_options
-            else to_json(value)
+            else to_json(value, round_trip=True)
         )
         return temporalio.api.common.v1.Payload(
             metadata={"encoding": self.encoding.encode()}, data=data
diff --git a/tests/contrib/pydantic/test_pydantic.py b/tests/contrib/pydantic/test_pydantic.py
index c3ab95f18..1b565258d 100644
--- a/tests/contrib/pydantic/test_pydantic.py
+++ b/tests/contrib/pydantic/test_pydantic.py
@@ -128,6 +128,68 @@ def test_to_json_options_exclude_unset_keeps_aliases():
     assert converter.from_payload(payload, AliasModel) == AliasModel(orderId=7)
 
 
+class ForbidComputedModel(BaseModel):
+    model_config = pydantic.ConfigDict(extra="forbid")
+    text: str
+
+    @pydantic.computed_field  # type: ignore[prop-decorator]
+    @property
+    def length(self) -> int:
+        return len(self.text)
+
+
+class AllowComputedModel(BaseModel):
+    model_config = pydantic.ConfigDict(extra="allow")
+    text: str
+
+    @pydantic.computed_field  # type: ignore[prop-decorator]
+    @property
+    def length(self) -> int:
+        return len(self.text)
+
+
+class JsonFieldModel(BaseModel):
+    raw: pydantic.Json[dict[str, int]]
+
+
+@pytest.mark.parametrize(
+    "options",
+    [
+        None,
+        temporalio.contrib.pydantic.ToJsonOptions(),
+        temporalio.contrib.pydantic.ToJsonOptions(exclude_unset=True),
+    ],
+)
+@pytest.mark.parametrize(
+    ("value", "type_hint", "data"),
+    [
+        (ForbidComputedModel(text="hi"), ForbidComputedModel, b'{"text":"hi"}'),
+        (AllowComputedModel(text="hi"), AllowComputedModel, b'{"text":"hi"}'),
+        (
+            [ForbidComputedModel(text="hi")],
+            list[ForbidComputedModel],
+            b'[{"text":"hi"}]',
+        ),
+        (
+            JsonFieldModel.model_validate({"raw": '{"a":1}'}),
+            JsonFieldModel,
+            b'{"raw":"{\\"a\\":1}"}',
+        ),
+    ],
+)
+def test_payload_validates_back_to_value(
+    options: temporalio.contrib.pydantic.ToJsonOptions | None,
+    value: object,
+    type_hint: type,
+    data: bytes,
+):
+    converter = PydanticJSONPlainPayloadConverter(options)
+    payload = converter.to_payload(value)
+    assert payload is not None
+    assert converter.from_payload(payload, type_hint) == value
+    assert payload.data == data
+
+
 async def test_pydantic_data_converter_pickle():
     restored = pickle.loads(pickle.dumps(pydantic_data_converter))
     value = {"a": 1}