A parent that fanned out 1,320 subagents over 13h reached 2.6 GB RSS
(1.9 GB anonymous heap). Every closed child AIAgent stayed reachable and
still owned a copy of its full message history. gc.get_referrers on a
finished child (30-child fan-out bench, evals/fanout_resource_bench.py)
showed two retainers:
1. bind_subagent_parent() stored the agent strongly in the
`hermes_subagent_lifecycle_parent` ContextVar. Each child binds ITSELF
for its own turn, and every asyncio Handle/Future scheduled during
that turn (LSP reader loops, kernel pipe transports) snapshots the
Context — 56 live Contexts held 14 finished children after the bench.
The ContextVar now holds a weakref (non-weakrefable doubles fall back
to a closure); get_active_subagent_parent() dereferences it.
2. AIAgent.close() cleared _session_messages but not the
_db_flush_scan_prefix snapshot (a `messages[:]` shallow copy taken on
every successful DB flush) nor _streamed_assistant_text_parts, so the
agent — kept alive by (1) — retained every message dict. close() now
drops both.
The delegate_task result entry never carried `messages`; a pin test
confirms the per-child result JSON is unchanged.
Bench (30 children / 10 worktrees, ~100 KB final replies so retention is
visible): post-fan-out live child AIAgents 14 -> 0; RSS after fan-out
636 MB -> 556 MB. With the harness' tiny default replies both runs sit at
~192-194 MB (the children's transcripts were never the dominant cost
there; the leaked objects were).
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