diff --git a/src/google/adk/models/lite_llm.py b/src/google/adk/models/lite_llm.py
index ad7bc63b81e..ae1e375e00e 100644
--- a/src/google/adk/models/lite_llm.py
+++ b/src/google/adk/models/lite_llm.py
@@ -50,6 +50,7 @@
 from pydantic import BaseModel
 from pydantic import Field
 from pydantic import PrivateAttr
+from pydantic_core import to_jsonable_python
 from typing_extensions import NotRequired
 from typing_extensions import override
 from typing_extensions import Required
@@ -1292,6 +1293,14 @@ def completion(
 def _safe_json_serialize(obj: object) -> str:
   """Convert any Python object to a JSON-serializable type or string.
 
+  A tool result can hold values the json module cannot encode, such as
+  datetimes, Decimals, UUIDs, enums, sets, bytes or Pydantic models (an
+  ``output_schema`` agent run through ``AgentTool`` returns dates and Decimals
+  this way). Those are converted with ``pydantic_core.to_jsonable_python``, as
+  the Gemini and Gemma paths do, so the model still gets JSON rather than the
+  Python repr of the whole payload. Only a payload Pydantic cannot serialize
+  either falls back to ``str()``.
+
   Args:
     obj: The object to serialize.
 
@@ -1302,6 +1311,13 @@ def _safe_json_serialize(obj: object) -> str:
   try:
     # Try direct JSON serialization first
     return json.dumps(obj, ensure_ascii=False)
+  except (TypeError, ValueError, OverflowError, RecursionError):
+    pass
+  try:
+    jsonable = to_jsonable_python(
+        obj, by_alias=False, exclude_none=True, bytes_mode="base64"
+    )
+    return json.dumps(jsonable, ensure_ascii=False)
   except (TypeError, ValueError, OverflowError, RecursionError):
     return str(obj)
 
diff --git a/tests/unittests/models/test_litellm.py b/tests/unittests/models/test_litellm.py
index 18176f313e4..f63b750d2e6 100644
--- a/tests/unittests/models/test_litellm.py
+++ b/tests/unittests/models/test_litellm.py
@@ -14,6 +14,9 @@
 
 import base64
 import contextlib
+import datetime
+import decimal
+import enum
 import json
 import logging
 import os
@@ -25,6 +28,7 @@
 from unittest.mock import MagicMock
 from unittest.mock import Mock
 from unittest.mock import patch
+import uuid
 import warnings
 
 from google.adk.agents.context_cache_config import ContextCacheConfig
@@ -1224,6 +1228,54 @@ def test_safe_json_serialize_circular_dict_falls_back_to_str():
   assert isinstance(_safe_json_serialize(obj), str)
 
 
+def test_safe_json_serialize_converts_values_json_cannot_encode():
+  class _Status(enum.Enum):
+    PAID = "paid"
+
+  class _Line(BaseModel):
+    sku: str
+    note: str | None = None
+
+  payload = {
+      "due_date": datetime.date(2026, 12, 1),
+      "total": decimal.Decimal("12.50"),
+      "status": _Status.PAID,
+      "id": uuid.UUID("12345678-1234-5678-1234-567812345678"),
+      "lines": [_Line(sku="A-1")],
+      "missing": None,
+  }
+
+  assert json.loads(_safe_json_serialize(payload)) == {
+      "due_date": "2026-12-01",
+      "total": "12.50",
+      "status": "paid",
+      "id": "12345678-1234-5678-1234-567812345678",
+      "lines": [{"sku": "A-1"}],
+      "missing": None,
+  }
+
+
+@pytest.mark.asyncio
+async def test_content_to_message_param_function_response_with_date_is_json():
+  part = types.Part.from_function_response(
+      name="extract_invoice",
+      response={
+          "due_date": datetime.date(2026, 12, 1),
+          "total": decimal.Decimal("12.50"),
+      },
+  )
+  part.function_response.id = "tool_call_1"
+  content = types.Content(role="user", parts=[part])
+
+  message = await _content_to_message_param(content)
+
+  assert message["role"] == "tool"
+  assert json.loads(message["content"]) == {
+      "due_date": "2026-12-01",
+      "total": "12.50",
+  }
+
+
 def test_safe_json_serialize_circular_list_falls_back_to_str():
   obj = []
   obj.append(obj)