Task tool can drop a completed subagent result after persistence

Description

When the Task tool invokes a subagent, the child can complete with finish: stop and persist a valid final JSON result, while the parent receives an empty or absent task result.

This differs from #38866 because there is no provider stream error, and from #37852 because the finish reason is not unknown. The child output exists and is complete; it is lost between child completion and parent delivery.

Expected: the parent receives the persisted subagent result.

Actual: the parent receives no usable result and treats the completed task as empty or malformed.

Plugins

A custom Task-based orchestration integration.

OpenCode version

1.18.30

Steps to reproduce

1. Configure a subagent that returns a small valid JSON object.
2. Invoke it through the Task tool from a parent session.
3. Observe that the child session completes with finish: stop.
4. Confirm that the valid JSON result is persisted in the child session.
5. Observe that the parent receives an empty or absent task result.

The failure is intermittent, but the distinguishing evidence is that child completion and persistence succeed without a stream error.

Screenshot and/or share link

Not available.

Operating System

Windows 11

Terminal

OpenCode Desktop