fix: handle Pydantic models in _safe_json_serialize for tool tracing

Tools returning Pydantic BaseModel instances have their trace output
silently replaced with '<not serializable>' because json.dumps cannot
serialize Pydantic models natively.

Update the default handler to call model_dump(mode='json') on
BaseModel instances before falling back, consistent with how other
functions in the same module already handle Pydantic objects.

Fixes #4629

Author: aperepel

src/google/adk/telemetry/tracing.py

@@ -67,18 +67,25 @@
 def _safe_json_serialize(obj) -> str:
   """Convert any Python object to a JSON-serializable type or string.
 
+  Handles Pydantic BaseModel instances (common as tool return types)
+  by calling model_dump() before JSON encoding.
+
   Args:
     obj: The object to serialize.
 
   Returns:
-    The JSON-serialized object string or <non-serializable> if the object cannot be serialized.
+    The JSON-serialized object string or <not serializable> if the object
+    cannot be serialized.
   """
+  from pydantic import BaseModel
+
+  def _default(o: Any) -> Any:
+    if isinstance(o, BaseModel):
+      return o.model_dump(mode='json')
+    return '<not serializable>'
 
   try:
-    # Try direct JSON serialization first
-    return json.dumps(
-        obj, ensure_ascii=False, default=lambda o: '<not serializable>'
-    )
+    return json.dumps(obj, ensure_ascii=False, default=_default)
   except (TypeError, OverflowError):
     return '<not serializable>'
 

tests/unittests/telemetry/test_spans.py

@@ -517,3 +517,58 @@ def test_trace_merged_tool_disabling_request_response_content(
       "Attribute 'gcp.vertex.agent.tool_response' was incorrectly set on the"
       ' span.'
   )
+
+
+# ---------------------------------------------------------------------------
+# _safe_json_serialize tests
+# ---------------------------------------------------------------------------
+
+from google.adk.telemetry.tracing import _safe_json_serialize
+from pydantic import BaseModel as PydanticBaseModel
+
+
+class _SampleToolResult(PydanticBaseModel):
+  query: str
+  total: int
+  items: list[str] = []
+
+
+class _NestedModel(PydanticBaseModel):
+  inner: _SampleToolResult
+
+
+def test_safe_json_serialize_plain_dict():
+  """Plain dicts serialize normally."""
+  result = _safe_json_serialize({'key': 'value', 'num': 42})
+  assert json.loads(result) == {'key': 'value', 'num': 42}
+
+
+def test_safe_json_serialize_pydantic_model_in_dict():
+  """Pydantic models nested in a dict are serialized via model_dump."""
+  model = _SampleToolResult(query='test', total=2, items=['a', 'b'])
+  result = _safe_json_serialize({'result': model})
+  parsed = json.loads(result)
+  assert parsed == {'result': {'query': 'test', 'total': 2, 'items': ['a', 'b']}}
+
+
+def test_safe_json_serialize_nested_pydantic_model():
+  """Nested Pydantic models are fully serialized."""
+  inner = _SampleToolResult(query='q', total=0, items=[])
+  outer = _NestedModel(inner=inner)
+  result = _safe_json_serialize({'result': outer})
+  parsed = json.loads(result)
+  assert parsed['result']['inner'] == {'query': 'q', 'total': 0, 'items': []}
+
+
+def test_safe_json_serialize_top_level_pydantic_model():
+  """A top-level Pydantic model (not wrapped in a dict) is serialized."""
+  model = _SampleToolResult(query='direct', total=1, items=['x'])
+  result = _safe_json_serialize(model)
+  parsed = json.loads(result)
+  assert parsed == {'query': 'direct', 'total': 1, 'items': ['x']}
+
+
+def test_safe_json_serialize_non_serializable_fallback():
+  """Objects that are neither JSON-native nor Pydantic fall back gracefully."""
+  result = _safe_json_serialize({'value': object()})
+  assert '<not serializable>' in result