Sub-agent messages are queued instead of streamed in real-time

[darthmolen]

Problem

When a session dispatches sub-agents (e.g., using the task tool for specialized agents), message_modified events are queued and delivered in batches instead of streaming in real-time. The user sees a long pause while the sub-agent works, then a burst of content all at once.

Expected Behavior

Sub-agent output should stream smoothly in real-time, similar to how Claude Desktop handles sub-agent streaming. Messages should merge seamlessly into the conversation with continuous progress updates.

Actual Behavior

• Long pause (9-47 seconds) while sub-agent works
• Then burst of chunks delivered at <50ms intervals
• Content arrives batched, not streamed
• UX feels frozen/laggy during sub-agent execution

Evidence

Automated streaming analysis on SDK 0.1.22 (2026-02-15). All 6 test prompts assessed as BATCHED:

Prompt	Max Delta	Pauses >5s	Bursts <50ms	Assessment
code-review (code-review-agent)	34,304ms	1	74	BATCHED
explore-authentication (explore-agent)	34,535ms	1	77	BATCHED
explore-test-files (explore-agent)	36,437ms	1	16	BATCHED
general-purpose-analysis (general-purpose)	12,816ms	2	91	BATCHED
parallel-exploration (multiple-agents)	47,344ms	1	165	BATCHED
task-run-tests (task-agent)	9,498ms	1	152	BATCHED

Pattern: Max delta shows 9-47s pauses. "Bursts <50ms" column shows content arriving in rapid bursts after the pause, confirming queueing rather than real-time delivery.

Reproduction

1. Create session with @github/copilot-sdk@0.1.22
2. Send message that triggers sub-agent dispatch (e.g., "Run the test suite using the task agent")
3. Observe message_modified event timing — long gap followed by rapid burst

Environment

• SDK: @github/copilot-sdk@0.1.22
• Node.js: v20.20.0
• OS: Ubuntu (WSL2)

Suggested Fix

Emit message_modified events for sub-agents immediately as content is generated, rather than queuing them until the sub-agent completes. This would match the smooth streaming behavior seen in Claude Desktop's sub-agent handling.

[github-actions]

Thank you for opening this issue and providing such detailed documentation! The automated streaming analysis with clear evidence makes this very actionable.

I've labeled this as:

• bug — The sub-agent messages are being queued instead of streamed in real-time
• sdk/nodejs — Issue affects the Node.js SDK (@github/copilot-sdk@0.1.22)

The evidence clearly shows the batching behavior with 9-47 second pauses followed by rapid bursts. This will be reviewed by the team to improve the sub-agent streaming UX.

AI generated by Issue Triage Agent

[cmart12]

Closing — this was addressed in v0.3.0 (release notes). Sub-agent streaming is now supported: assistant.message_delta and assistant.reasoning_delta events are delivered for sub-agents, with an agentId field identifying which sub-agent produced them. See the v0.3.0 changelog for details and the includeSubAgentStreamingEvents option.

[cmart12]

Reopening — closed prematurely during analysis. Will re-evaluate before any further action.

[stephentoub]

Generated by Copilot.

I believe this has already been fixed/addressed.

Fix summary: Sub-agent streaming events are now supported and the SDK path opts into receiving those streaming events.

Closing this as completed.