Description

PR microsoft/agent-framework#3275 fixed the JSON serialization problem in the Agent Framework AG-UI integration, closing microsoft/agent-framework#3239, by serializing tool-call arguments with make_json_safe() before json.dumps().

However, the same unsafe serialization pattern still appears to exist in agent-framework-core observability. In latest agent-framework-core==1.3.0, agent_framework/observability.py::_capture_messages() still serializes OpenTelemetry message payloads with raw json.dumps(...), and the function-call branch keeps raw content.arguments.

I confirmed this is still an issue in the core workflow/request-info path: a workflow request_info event can be converted into function-call arguments containing a dataclass payload, and core observability then fails when _capture_messages() tries to serialize it with raw json.dumps(...).

Evidence from latest release

Package checked:

agent-framework-core==1.3.0
agent-framework-foundry==1.3.0
agent-framework-foundry-hosting==1.0.0a260507

In agent_framework/observability.py:

span.set_attribute(
    OtelAttr.OUTPUT_MESSAGES if output else OtelAttr.INPUT_MESSAGES,
    json.dumps(otel_messages, ensure_ascii=False),
)

The function_call branch preserves raw arguments:

case "function_call":
    return {
        "type": "tool_call",
        "id": content.call_id,
        "name": content.name,
        "arguments": content.arguments,
    }

There is no make_json_safe(...) and no default=... encoder in this path.

Why this matters

Workflow request_info events can contain framework or dataclass payloads. When those are surfaced as function-call arguments and telemetry captures messages, core observability can raise:

TypeError: Object of type HandoffAgentUserRequest is not JSON serializable

This affects Handoff/workflow scenarios outside the AG-UI adapter path, including Foundry-hosted agent scenarios where telemetry captures messages.

Expected behavior

Core observability should safely serialize message/tool-call payloads that contain dataclasses or framework objects emitted by workflow/handoff request-info flows.

Actual behavior

Core observability still passes raw function-call arguments into json.dumps(...), which can fail for dataclass/framework payloads.

Related issues / PRs

• microsoft/agent-framework#3239 reported the AG-UI serialization failure.
• microsoft/agent-framework#3275 fixed the AG-UI path by serializing arguments with make_json_safe(content.arguments).
• This issue is about the remaining core observability path in agent-framework-core.