[FEATURE]: Lightweight mode for async subagents — let parent agent continue working while child runs #26055

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently, the task tool spawns a sub-agent but blocks the parent session until the child completes. The parent cannot do its own work while the child is running, which wastes both time and tokens in many workflows.

Request

Add a lightweight async sub-agent mode with two simple properties:

1. Non-blocking spawn — When a sub-agent is created, the parent session immediately gets back a session ID and continues its own work.
2. Callback on completion — When the child finishes, its result is delivered to the parent as a synthetic message (or other event), so the parent can pick up the result later.

That's all needed at the core level. Full orchestration, messaging, worktree isolation, etc. can be handled by the plugin ecosystem.

Why

A common workflow: main agent finishes editing src/auth.ts, wants to fire a "doc-agent" to update the README, and immediately continue editing src/payment.ts without waiting for the doc-agent. When the doc-agent finishes, the main agent is notified and can review the result.

This is currently impossible without heavy workarounds.

Related

• [feat] True Async/Background Sub-Agent Delegation #5887 — the primary feature request for true async/background sub-agent delegation (with extensive community discussion and existing PRs). This issue is a focused, minimal request that should fulfill [feat] True Async/Background Sub-Agent Delegation #5887's core need with the smallest possible core change.