diff --git a/python/copilot/tools.py b/python/copilot/tools.py
index 762b79c45..2de29584f 100644
--- a/python/copilot/tools.py
+++ b/python/copilot/tools.py
@@ -351,7 +351,11 @@ def _normalize_result(result: Any) -> ToolResult:
     # Everything else gets JSON-serialized (with Pydantic model support)
     def default(obj: Any) -> Any:
         if isinstance(obj, BaseModel):
-            return obj.model_dump()
+            # mode="json" coerces datetime/UUID/Decimal/Enum fields to JSON-native
+            # types. The default mode="python" leaves them as native objects, which
+            # json.dumps then passes back into this hook and we reject as
+            # unserializable -- turning an ordinary tool result into a failure.
+            return obj.model_dump(mode="json")
         raise TypeError(f"Object of type {type(obj).__name__} is not JSON serializable")
 
     try:
diff --git a/python/test_tools.py b/python/test_tools.py
index 646f17c03..fa8329cd1 100644
--- a/python/test_tools.py
+++ b/python/test_tools.py
@@ -1,6 +1,10 @@
 """Unit tests for define_tool"""
 
+import datetime
+import decimal
+import enum
 import json
+import uuid
 
 import pytest
 from pydantic import BaseModel, ConfigDict, Field, field_validator
@@ -389,6 +393,33 @@ class Item(BaseModel):
         assert parsed == [{"name": "a", "value": 1}, {"name": "b", "value": 2}]
         assert result.result_type == "success"
 
+    def test_pydantic_model_with_non_primitive_fields_is_serialized(self):
+        class IssueState(enum.StrEnum):
+            OPEN = "open"
+            CLOSED = "closed"
+
+        class Issue(BaseModel):
+            id: uuid.UUID
+            created_at: datetime.datetime
+            price: decimal.Decimal
+            state: IssueState
+
+        issue = Issue(
+            id=uuid.UUID("6b686a99-0000-4000-8000-000000000000"),
+            created_at=datetime.datetime(2026, 8, 1, 1, 49, 10),
+            price=decimal.Decimal("1.5"),
+            state=IssueState.OPEN,
+        )
+        result = _normalize_result(issue)
+        parsed = json.loads(result.text_result_for_llm)
+        assert parsed == {
+            "id": "6b686a99-0000-4000-8000-000000000000",
+            "created_at": "2026-08-01T01:49:10",
+            "price": "1.5",
+            "state": "open",
+        }
+        assert result.result_type == "success"
+
     def test_raises_for_unserializable_value(self):
         # Functions cannot be JSON serialized
         with pytest.raises(TypeError, match="Failed to serialize"):