Issue draft (not filed): RunState stringifies structured `ToolCallOutputItem.custom_data` during persistence

[betacatsling]

Summary

RunState persistence degrades structured values stored in ToolCallOutputItem.custom_data to repr() strings. custom_data is a public dict[str, Any] field ("SDK-only custom data attached to this tool output") that is serialized into generated_items and restored on RunState.from_json. Unlike the sibling output field, it is passed directly to _ensure_json_compatible, which falls back to json.dumps(default=str) for anything that is not natively JSON-serializable. Pydantic models and dataclasses therefore persist as strings like "value=0.9 label='high'" instead of JSON objects, and a restored run observes corrupted metadata.

Reproduction

import asyncio
from pydantic import BaseModel
from agents import Agent
from agents.items import ToolCallOutputItem
from agents.run_context import RunContextWrapper
from agents.run_state import RunState

class Score(BaseModel):
    value: float
    label: str

async def main() -> None:
    agent = Agent(name="AuditAgent")
    state = RunState(
        context=RunContextWrapper(context={}),
        original_input="input",
        starting_agent=agent,
        max_turns=1,
    )
    state._generated_items.append(
        ToolCallOutputItem(
            agent=agent,
            raw_item={"type": "function_call_output", "call_id": "c1", "output": "r"},
            output="r",
            custom_data={"score": Score(value=0.9, label="high")},
        )
    )
    json_data = state.to_json()
    print(json_data["generated_items"][0]["custom_data"])
    # {"score": "value=0.9 label='high'"}  <- repr string, not a JSON object

asyncio.run(main())

Root cause

RunState._serialize_item (src/agents/run_state.py) serializes item.output via _ensure_json_compatible(_serialize_output_value(item.output)), which recursively converts Pydantic models, dataclasses, mappings, and sequences to plain JSON values. A few lines later, custom_data is serialized via _ensure_json_compatible(custom_data) alone, skipping _serialize_output_value, so structured values hit the default=str fallback.

Proposed fix

Route custom_data through _serialize_output_value first, matching output:

result["custom_data"] = _ensure_json_compatible(_serialize_output_value(custom_data))

Note: the custom_data_extractor boundary (normalize_custom_data in src/agents/util/_custom_data.py) still enforces the documented JSON-compatible contract for extractor-produced data; this change only makes the persistence layer faithful for values set directly on the public field, consistent with how output is already handled.