From a9c111da544c976d591343db5493a7da283b72e5 Mon Sep 17 00:00:00 2001
From: cassirer-openai <cassirer@openai.com>
Date: Thu, 23 Apr 2026 19:29:45 -0700
Subject: [PATCH] [rollout_trace] Trace sessions and multi-agent edges (#18879)

## Summary

Adds the remaining session and multi-agent edge wiring needed to
reconstruct rollout relationships across spawned agents, resumed
sessions, and parent/child message delivery.

## Stack

This is PR 4/5 in the rollout trace stack.

- [#18876](https://github.com/openai/codex/pull/18876): Add rollout
trace crate
- [#18877](https://github.com/openai/codex/pull/18877): Record core
session rollout traces
- [#18878](https://github.com/openai/codex/pull/18878): Trace tool and
code-mode boundaries
- [#18879](https://github.com/openai/codex/pull/18879): Trace sessions
and multi-agent edges
- [#18880](https://github.com/openai/codex/pull/18880): Add debug trace
reduction command

## Review Notes

This is the stack layer that makes traces useful for multi-threaded
agent workflows. The main invariant is that reconstructed relationships
should come from durable rollout data rather than transient in-memory
manager state wherever possible.

The PR is intentionally small relative to the preceding layers: it uses
the recorder and reducer contracts already established by the stack and
only adds the session/agent relationship events needed by later debug
reduction.