[BUG] V2: subagent with pending background work (task/shell, background: true) marked completed early — results never collected

[Ploppy3]

Summary

A V2 subagent with pending background work (task/shell with background: true) ends its turn and is reported to its caller as completed while the work is still running — the parent is notified early with the subagent's premature reply.

When the work finishes, the subagent is woken and produces its real final answer, but that answer is never delivered to the parent. The result is stranded in the subagent session.

Nested variant of #45480 / #35066; contradicts the task tool's "You will be notified automatically when it finishes."

Cause

The issue is caused by OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS being set to 1. Background subagent work (background: true for task/shell) only runs when this experimental feature is enabled, and in that state the subagent is reported completed while its background work is still pending. With the variable unset, background subagents are disabled and the issue does not occur.

Environment

• opencode 2.0.11 — re-verified live on the current version (originally observed on 2.0.8)
• experimental.subagent_depth: 5
• OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=1

Reproduction

1. Subagent A starts background work (task or shell with background: true), then ends its turn.
2. Parent is notified completed with A's early reply, while the work is still running.
3. Work finishes; A is woken, collects it and produces its real final answer.
4. Parent never receives that answer.

Live re-verification (opencode 2.0.11)

• Subagent started shell work with background: true and immediately ended its turn.
• Parent was notified completed at +6.5s with the premature reply, while the shell was still running.
• Shell finished at +25.2s; the subagent was woken by the completion notification and produced REAL_FINAL.
• The parent never received REAL_FINAL; the result remains stranded in the subagent session.

Expected

Parent notified only once A is quiescent (no pending background work left), or woken again with the real result.

Actual

Parent notified early with a premature reply; A's real result is never delivered.

Related: #45480, #35066, #36349, #48361, #45560

[Ploppy3]

Fix opened in #49305 (targets v2).

[Ploppy3]

Still broken on v2.0.5 — reproduced live (Linux, background subagents enabled). Fix #49305 is unmerged (BLOCKED) and the binary lacks its symbols.

Timeline (t=0 = spawn; child starts a sleep 20 background shell, then ends its turn early):

• +3s — child idle succeeded, shell still running
• +16s — parent gets the only notification: completed + early reply, 6s before the shell finished
• +22s — shell completes; child is woken and produces its real result
• parent never receives it — nothing pending, result stranded in the child session

Nuance: the wake did reach the child here; the parent-side delivery failure is what reproduces. An isolated opencode run --standalone trial also showed the early notification, and there the pending shell never completed (likely killed at run exit) — possible extra severity variant.