fix: handle ValueError and RecursionError in _safe_json_serialize

src/google/adk/models/lite_llm.py

@@ -556,8 +556,11 @@ def _safe_json_serialize(obj) -> str:
   try:
     # Try direct JSON serialization first
     return json.dumps(obj, ensure_ascii=False)
-  except (TypeError, ValueError, OverflowError):
-    return str(obj)
+  except (TypeError, OverflowError, ValueError, RecursionError):
+    try:
+      return str(obj)
+    except RecursionError:
+      return "<non-serializable: recursion depth exceeded>"
 
 
 def _part_has_payload(part: types.Part) -> bool:

src/google/adk/telemetry/tracing.py

@@ -128,7 +128,7 @@ def _safe_json_serialize(obj) -> str:
     return json.dumps(
         obj, ensure_ascii=False, default=lambda o: '<not serializable>'
     )
-  except (TypeError, ValueError, OverflowError):
+  except (TypeError, OverflowError, ValueError, RecursionError):
     return '<not serializable>'
 
 

tests/unittests/models/test_litellm_safe_serialize.py

@@ -0,0 +1,67 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for _safe_json_serialize in models/lite_llm.py.
+
+Verifies that the function never raises exceptions, even for inputs that
+cause json.dumps to raise ValueError (circular references) or
+RecursionError (deeply nested structures).
+
+Fixes https://github.com/google/adk-python/issues/5412
+"""
+
+from google.adk.models.lite_llm import _safe_json_serialize
+import pytest
+
+
+def test_circular_reference_returns_str_fallback():
+  """json.dumps raises ValueError on circular references; should fall back to str()."""
+
+  class Node:
+
+    def __init__(self):
+      self.ref = self
+
+  obj = Node()
+  result = _safe_json_serialize(obj)
+  assert isinstance(result, str)
+  # Should return str(obj) fallback rather than raising ValueError
+
+
+def test_deeply_nested_structure_returns_str_fallback():
+  """json.dumps raises RecursionError on deeply nested structures."""
+  obj = current = {}
+  for _ in range(10000):
+    current["child"] = {}
+    current = current["child"]
+
+  result = _safe_json_serialize(obj)
+  assert isinstance(result, str)
+  # str(obj) itself can raise RecursionError, so expect the safe fallback
+  assert "recursion" in result.lower() or result  # non-empty string
+
+
+def test_normal_dict_serializes():
+  """Normal dicts should serialize as JSON."""
+  result = _safe_json_serialize({"key": "value", "num": 42})
+  assert '"key"' in result
+  assert '"value"' in result
+
+
+def test_non_serializable_object_falls_back_to_str():
+  """Objects without a JSON representation should fall back to str()."""
+  obj = object()
+  result = _safe_json_serialize(obj)
+  assert isinstance(result, str)
+  assert "object" in result.lower()

tests/unittests/telemetry/test_safe_json_serialize.py

@@ -0,0 +1,65 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for _safe_json_serialize in telemetry/tracing.py.
+
+Verifies that the function never raises exceptions, even for inputs that
+cause json.dumps to raise ValueError (circular references) or
+RecursionError (deeply nested structures).
+
+Fixes https://github.com/google/adk-python/issues/5411
+"""
+
+from google.adk.telemetry.tracing import _safe_json_serialize
+import pytest
+
+
+def test_circular_reference_returns_fallback():
+  """json.dumps raises ValueError on circular references; should not propagate."""
+
+  class Node:
+
+    def __init__(self):
+      self.ref = self
+
+  obj = Node()
+  result = _safe_json_serialize(obj)
+  assert isinstance(result, str)
+  # Should return the fallback rather than raising
+  assert "not serializable" in result.lower() or result  # non-empty string
+
+
+def test_deeply_nested_structure_returns_fallback():
+  """json.dumps raises RecursionError on deeply nested structures."""
+  obj = current = {}
+  for _ in range(10000):
+    current["child"] = {}
+    current = current["child"]
+
+  result = _safe_json_serialize(obj)
+  assert isinstance(result, str)
+
+
+def test_normal_dict_serializes():
+  """Normal dicts should serialize without issue."""
+  result = _safe_json_serialize({"key": "value", "num": 42})
+  assert '"key"' in result
+  assert '"value"' in result
+
+
+def test_non_serializable_object_uses_default():
+  """Objects without a JSON representation use the default callback."""
+  result = _safe_json_serialize(object())
+  assert isinstance(result, str)
+  assert "not serializable" in result.lower()