Crashes in `trace_call_llm()` on non-serializable data

[stephensmithnewfront]

Describe the bug
Calls to trace_call_llm() happen in agent flow, but trace_call_llm() crashes when it tries to serialize bytes data.

2025-05-22 19:47:11,764 - ERROR - fast_api.py:817 - Error in event_generator: Object of type bytes is not JSON serializable
Traceback (most recent call last):
 File "/Users/stephensmith/src/adk-python/src/google/adk/cli/fast_api.py", line 806, in event_generator
 async for event in runner.run_async(
 File "/Users/stephensmith/src/adk-python/src/google/adk/runners.py", line 196, in run_async
 async for event in invocation_context.agent.run_async(invocation_context):
 File "/Users/stephensmith/src/adk-python/src/google/adk/agents/base_agent.py", line 147, in run_async
 async for event in self._run_async_impl(ctx):
 File "/Users/stephensmith/src/adk-python/src/google/adk/agents/llm_agent.py", line 278, in _run_async_impl
 async for event in self._llm_flow.run_async(ctx):
 File "/Users/stephensmith/src/adk-python/src/google/adk/flows/llm_flows/base_llm_flow.py", line 279, in run_async
 async for event in self._run_one_step_async(invocation_context):
 File "/Users/stephensmith/src/adk-python/src/google/adk/flows/llm_flows/base_llm_flow.py", line 305, in _run_one_step_async
 async for llm_response in self._call_llm_async(
 File "/Users/stephensmith/src/adk-python/src/google/adk/flows/llm_flows/base_llm_flow.py", line 527, in _call_llm_async
 trace_call_llm(
 File "/Users/stephensmith/src/adk-python/src/google/adk/telemetry.py", line 121, in trace_call_llm
 json.dumps(_build_llm_request_for_trace(llm_request)),
 File "/Users/stephensmith/.pyenv/versions/3.10.13/lib/python3.10/json/__init__.py", line 231, in dumps
 return _default_encoder.encode(obj)
 File "/Users/stephensmith/.pyenv/versions/3.10.13/lib/python3.10/json/encoder.py", line 199, in encode
 chunks = self.iterencode(o, _one_shot=True)
 File "/Users/stephensmith/.pyenv/versions/3.10.13/lib/python3.10/json/encoder.py", line 257, in iterencode
 return _iterencode(o, 0)
 File "/Users/stephensmith/.pyenv/versions/3.10.13/lib/python3.10/json/encoder.py", line 179, in default
 raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type bytes is not JSON serializable

To Reproduce
Steps to reproduce the behavior:

1. Follow the quickstart in https://google.github.io/adk-docs/get-started/quickstart/
2. Use this for multi_tool_agent/agent.py:

from google.adk.agents import Agent

def get_bytes_data(city: str) -> bytes:
 """Retrieves bytes data for a specified city.

 Args:
 city (str): The name of the city for which to retrieve the bytes data.

 Returns:
 bytes: sample bytes data
 """
 if city.lower() == "new york":
 return b"sample bytes data for new york"
 else:
 return b"sample bytes data for other cities"


root_agent = Agent(
 name="sample_bytes_agent",
 model="gemini-2.0-flash",
 description=(
 "Agent to answer questions about sample bytes data for a city."
 ),
 instruction=(
 "You are a helpful agent who can answer user questions about the sample bytes data for a city."
 ),
 tools=[get_bytes_data],
)

3. Run the agent; for example, use the Dev UI via adk web
4. Ask a question like "What is the sample bytes data for New York?"

Expected behavior
Should not crash. Should skip non-serializable data in tracing functions.

Desktop (please complete the following information):

• OS: macOS Sequoia 15.4.1
• Python version(python -V): Python 3.10.13
• ADK version(pip show google-adk): 1.0.0

Additional context
Spoiler alert: a PR to fix this bug is coming momentarily.