RunState guardrail payload serialization fails for non-JSON values

[Aphroq]

Please read this first

• Have you read the docs? Yes. This is about RunState serialization behavior rather than a documented usage option.
• Have you searched for related issues? Yes. I searched existing issues and PRs for:

Describe the bug

RunState.to_string() can fail when persisted guardrail result payloads contain non-JSON-native values. _serialize_guardrail_results() writes guardrail outputInfo and output guardrail agentOutput directly, and _serialize_tool_guardrail_results() writes tool guardrail outputInfo directly. Other RunState user payloads already use JSON-compatible normalization, so these guardrail payloads are an outlier.

Debug information

• Agents SDK version: upstream main at 683b6e79
• Python version: Python 3.12.1

Repro steps

Run this script from a checkout of upstream main at revision 683b6e79:

from datetime import datetime

from agents import Agent
from agents.guardrail import (
    GuardrailFunctionOutput,
    InputGuardrail,
    InputGuardrailResult,
    OutputGuardrail,
    OutputGuardrailResult,
)
from agents.run_context import RunContextWrapper
from agents.run_state import RunState
from agents.tool_guardrails import (
    AllowBehavior,
    ToolGuardrailFunctionOutput,
    ToolInputGuardrail,
    ToolInputGuardrailResult,
    ToolOutputGuardrail,
    ToolOutputGuardrailResult,
)


agent = Agent(name="GuardrailPayloadAgent")
state = RunState(
    _current_agent=agent,
    _starting_agent=agent,
    _original_input="input",
    _context=RunContextWrapper(context={}),
    _max_turns=1,
)
observed_at = datetime(2026, 5, 8, 12, 0, 0)

input_guardrail = InputGuardrail(
    guardrail_function=lambda ctx, ag, inp: GuardrailFunctionOutput(
        output_info={"observed_at": observed_at},
        tripwire_triggered=False,
    ),
    name="input_guardrail",
)
output_guardrail = OutputGuardrail(
    guardrail_function=lambda ctx, ag, out: GuardrailFunctionOutput(
        output_info={"observed_at": observed_at},
        tripwire_triggered=False,
    ),
    name="output_guardrail",
)
tool_input_guardrail = ToolInputGuardrail(
    guardrail_function=lambda data: ToolGuardrailFunctionOutput(
        output_info={"observed_at": observed_at},
        behavior=AllowBehavior(type="allow"),
    ),
    name="tool_input_guardrail",
)
tool_output_guardrail = ToolOutputGuardrail(
    guardrail_function=lambda data: ToolGuardrailFunctionOutput(
        output_info={"observed_at": observed_at},
        behavior=AllowBehavior(type="allow"),
    ),
    name="tool_output_guardrail",
)

state._input_guardrail_results = [
    InputGuardrailResult(
        guardrail=input_guardrail,
        output=GuardrailFunctionOutput(
            output_info={"observed_at": observed_at},
            tripwire_triggered=False,
        ),
    )
]
state._output_guardrail_results = [
    OutputGuardrailResult(
        guardrail=output_guardrail,
        agent_output={"observed_at": observed_at},
        agent=agent,
        output=GuardrailFunctionOutput(
            output_info={"observed_at": observed_at},
            tripwire_triggered=False,
        ),
    )
]
state._tool_input_guardrail_results = [
    ToolInputGuardrailResult(
        guardrail=tool_input_guardrail,
        output=ToolGuardrailFunctionOutput(
            output_info={"observed_at": observed_at},
            behavior=AllowBehavior(type="allow"),
        ),
    )
]
state._tool_output_guardrail_results = [
    ToolOutputGuardrailResult(
        guardrail=tool_output_guardrail,
        output=ToolGuardrailFunctionOutput(
            output_info={"observed_at": observed_at},
            behavior=AllowBehavior(type="allow"),
        ),
    )
]

print(state.to_string())

Actual result:

TypeError: Object of type datetime is not JSON serializable

The same failure is covered by focused regression tests:

FAILED tests/test_run_state.py::TestRunState::test_guardrail_results_to_string_normalizes_non_json_payloads
FAILED tests/test_run_state.py::TestRunState::test_tool_guardrail_results_to_string_normalizes_non_json_output_info
E   TypeError: Object of type datetime is not JSON serializable

Expected behavior

RunState serialization should remain JSON-compatible. Guardrail outputInfo, output guardrail agentOutput, and tool guardrail outputInfo should be normalized the same way as other user payloads already serialized by RunState.