Fix the other things using the model_input "singleton" also

src/ai/types/messages.py

@@ -225,7 +225,7 @@ def is_error(self) -> bool:
 
     def get_model_input(self) -> Any:
         """Return the value the LLM should see, falling back to ``result``."""
-        if self.model_input is _MODEL_INPUT_UNSET:
+        if isinstance(self.model_input, _ModelInputUnset):
             return self.result
         return self.model_input
 
@@ -236,7 +236,7 @@ def set_model_input(self, value: Any) -> None:
     @property
     def has_model_input(self) -> bool:
         """Whether ``set_model_input`` has been called on this part."""
-        return self.model_input is not _MODEL_INPUT_UNSET
+        return not isinstance(self.model_input, _ModelInputUnset)
 
 
 class ToolCallPart(pydantic.BaseModel):

tests/types/test_messages.py

@@ -288,12 +288,12 @@ def test_tool_result_file_part_base64_valid_after_round_trip() -> None:
 def test_tool_result_without_model_input_serializes_after_deep_copy() -> None:
     """A deep-copied ToolResultPart with no model_input still serializes.
 
-    ``model_input`` defaults to the ``_MODEL_INPUT_UNSET`` singleton and is
-    dropped from output via ``exclude_if=lambda v: v is _MODEL_INPUT_UNSET``,
-    an identity check.  ``model_copy(deep=True)`` rebuilds the sentinel into a
-    *new* instance, so the identity check fails, the field is no longer
-    excluded, and pydantic tries to serialize the bare sentinel.  Client apps
-    that deep-copy messages hit this on serialize.
+    ``model_input`` defaults to the ``_MODEL_INPUT_UNSET`` singleton, and
+    the sentinel checks (``exclude_if``, ``has_model_input``,
+    ``get_model_input``) test for it by type.  ``model_copy(deep=True)``
+    rebuilds the sentinel into a *new* instance: an identity (``is``) check
+    would miss it, leave the field un-excluded, and make pydantic choke on
+    the bare sentinel.  Client apps that deep-copy messages hit this.
     """
     msg = messages.Message(
         role="tool",
@@ -309,6 +309,13 @@ def test_tool_result_without_model_input_serializes_after_deep_copy() -> None:
 
     cloned = msg.model_copy(deep=True)
 
+    # The clone's sentinel is a fresh instance; the type-based checks must
+    # still treat it as unset.
+    cpart = cloned.parts[0]
+    assert isinstance(cpart, messages.ToolResultPart)
+    assert not cpart.has_model_input
+    assert cpart.get_model_input() == {"ok": 1}
+
     j = cloned.model_dump_json()
     restored = messages.Message.model_validate_json(j)
     rpart = restored.parts[0]