[FEATURE]: parent agent should observe background subagent's real-time progress and streaming output

[smithyyang]

Feature Description

Allow the parent agent to observe a background subagent's real-time progress and streaming output while it is still running.

Current Behavior

With OPENCODE_EXPERIMENTAL_BACKGROUND_SUBAGENTS=true, a parent agent can launch a background subagent via task(background=true, ...). But once launched, the subagent is a black box — the parent can only poll task_status(task_id="...") which returns the state (running, completed, cancelled) but no intermediate output or progress information.

The only way to interact with a running subagent is task(task_id="...", prompt=...), which resumes/resumes the session but interrupts the agent's current work (it stops what it was doing and responds to the new prompt).

Proposed Feature

Add a mechanism for the parent agent to stream or poll the subagent's real-time output without interrupting it. Options to consider:

1. Stream endpoint: A way to subscribe to the subagent's tool call outputs and text generations as they happen (similar to tailing logs).
2. Progress poll: task_status could include a snapshot of the last N tool outputs or the current step description, e.g. task_status(task_id="...", include_progress=true) returning {state: "running", last_output: "Searching the web for X...", tools_called: 3}.
3. Shared scratchpad: An append-only buffer that the subagent writes to and the parent reads from, without mutual interruption.

Use Case

A parent agent launches multiple background subagents for parallel research tasks and wants to:

• See which ones are making progress vs. stuck

• Decide which results are ready early to start consolidating

• Provide progress feedback to the user without waiting for all subagents to finish

• I have searched existing issues and confirm this is not a duplicate.

Related

Builds on the background subagent delegation feature (PR #24174 / issue #21384).

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• The title should start with [FEATURE]: for feature requests (e.g. [FEATURE]: parent agent should observe background subagent's real-time progress and streaming output)
• The feature request template requires a verification checkbox confirming you've searched existing issues for duplicates

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

---

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Display subagent status in TUI prompt footer #23784: Display subagent status in TUI — similar goal of surfacing subagent progress to the parent/user
• [FEATURE]:Show progress bar for Subagent #22153: Show progress bar for subagent — overlapping intent of showing subagent progress
• [FEATURE]: Lightweight mode for async subagents — let parent agent continue working while child runs #26055: Lightweight mode for async subagents — related to parent continuing while child runs independently
• [FEATURE]: Silent result collection for background tasks to avoid consuming LLM turns #20460: Silent result collection for background tasks — related to background task output handling

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

Thanks for updating the issue! It now follows the contributing guidelines and feature request template. The needs:compliance label has been removed.

[github-actions]

Thanks for updating the issue! It now meets our contributing guidelines. We appreciate the clear description, well-explained use case, and the duplicate search. Your feature request is good to go.

[github-actions]

Thanks for updating the issue! It now meets our contributing guidelines — the title, feature description, use case, and duplicate-search checkbox all look good. The needs:compliance label has been removed.

[smithyyang]

For tasks requiring parent-child agent collaboration — especially when the parent depends on real-time feedback from its subagents to make decisions — being able to observe the subagent's live output would be essential. Without it, the parent has no way to react to intermediate results until the subagent fully terminates.