Python: [Bug]: agent-framework-ag-ui crashes when Handoff user-input requests emit non‑JSON‑serializable arguments

[petetakacs]

Description

When using Agent Framework AG-UI with a Handoff workflow, the server crashes with a JSON serialization error the first time a handoff agent requests user input. This happens because AG-UI’s event bridge attempts to json.dumps a HandoffAgentUserRequest dataclass emitted by HandoffAgentExecutor.

Expected:

• AG-UI should serialize the request_info arguments and emit ToolCallArgsEvent without throwing.

Actual:

• Exception: TypeError: Object of type HandoffAgentUserRequest is not JSON serializable
• The AG-UI stream ends with an unhandled exception.

Steps to reproduce:

1. Install Agent Framework Python packages (versions below).
2. Create a Handoff workflow with two agents using HandoffBuilder and expose it through AG-UI FastAPI endpoint.
3. Trigger a conversation where a specialist agent replies without calling a handoff tool (so it requests user input).
4. Observe server crash on ToolCallArgsEvent emission.

Code Sample

import asyncio
from fastapi import FastAPI
from agent_framework import HandoffBuilder
from agent_framework.azure import AzureOpenAIChatClient
from agent_framework.ag_ui import add_agent_framework_fastapi_endpoint
from azure.identity import AzureCliCredential

app = FastAPI()

async def build_agent():
    chat_client = AzureOpenAIChatClient(credential=AzureCliCredential())

    triage = chat_client.create_agent(
        name="triage",
        instructions="You are a triage agent. Handoff to specialist when needed."
    )
    specialist = chat_client.create_agent(
        name="specialist",
        instructions=(
            "You are a specialist. Respond normally and ask the user for more details."
        )
    )

    workflow = (
        HandoffBuilder(
            name="handoff_demo",
            participants=[triage, specialist],
        )
        .with_start_agent(triage)
        .add_handoff(triage, [specialist])
        .build()
    )

    return workflow.as_agent()

@app.on_event("startup")
async def startup():
    agent = await build_agent()
    add_agent_framework_fastapi_endpoint(
        app=app,
        agent=agent,
        path="/chat",
        dependencies=[],
    )

# Run with: uvicorn main:app --reload


# Client request:


curl -N -s \
  -H "Content-Type: application/json" \
  -H "Accept: text/event-stream" \
  -d '{"messages":[{"role":"user","content":"Help me with my issue"}],"state":{}}' \
  http://localhost:8000/chat

Error Messages / Stack Traces

TypeError: Object of type HandoffAgentUserRequest is not JSON serializable
  File ".../agent_framework_ag_ui/_events.py", line 181, in _handle_function_call_content
    delta_str = content.arguments if isinstance(content.arguments, str) else json.dumps(content.arguments)
  File ".../json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)

Package Versions

agent-framework: 1.0.0b260114, agent-framework-core: 1.0.0b260114, agent-framework-ag-ui: 1.0.0b260114

Python Version

Python 3.12

Additional Context

AG-UI uses raw json.dumps in _handle_function_call_content

• Handoff emits HandoffAgentUserRequest dataclass for RequestInfoEvent data
• There is already a make_json_safe helper that supports dataclasses in agent_framework_ag_ui/_utils.py

Suggested fix:

• Replace json.dumps(content.arguments) with json.dumps(make_json_safe(content.arguments))
  or serialize via prepare_function_call_results with default=str

Relevant files:

• https://github.com/microsoft/agent-framework/blob/main/python/packages/ag-ui/agent_framework_ag_ui/_events.py
• https://github.com/microsoft/agent-framework/blob/main/python/packages/ag-ui/agent_framework_ag_ui/_utils.py
• https://github.com/microsoft/agent-framework/blob/main/python/packages/core/agent_framework/_workflows/_handoff.py