v2: subagent: no cancellation support for background subagents

[Ploppy3]

Summary

In opencode v2, the subagent tool returns a session ID (ses_...) both at launch and in the completion notification, and sessions can be resumed via the sessionID parameter. However, there is still no way to cancel a running background subagent.

What works now (v2)

• Background subagents return a session ID at launch: ses_0721f344effe5ZAKw2R1Y9AVF1
• The ID also appears in the completion notification.
• Sessions are resumable via the sessionID parameter.

What is still missing

1. No cancellation: There is no cancel/kill/abort/stop parameter on the subagent tool and no dedicated cancel tool to stop a running background subagent.

Desired behavior

• A way to cancel a running background subagent using its session ID

[Ploppy3]

Updated the issue to reflect the latest v2 changes: background subagents now do return a session ID at launch (ses_...), which is progress. However, the core gaps remain — there's still no mechanism to resume a completed subagent or steer a running one, both of which were standard in v1's task tool. Keeping this open so it doesn't go stale while these capabilities are being worked on.

[Ploppy3]

[damjtoh]

this makes any orchestration workflow impossible, I've returned to v1 sadly 🥲

[Ploppy3]

Updated this issue: resume via sessionID now works in v2, so the report is narrowed to the missing cancellation support (no way to stop a running background subagent by its session ID).

[imarshallwidjaja]

Also hitting this. My workflow launches background workers and needs an operator-visible way to cancel one without cancelling the parent session. session.interrupt on the client works for a known child ID, but there's no tool-level surface for the model, and no way to list what's running. PRs #32425 and #34947 look like the right shape; the /tasks view from #41914 would complete it.