v1.0.5 regression: background agent completion notifications not delivered to model

[vbomfim]

Description

Background agent completion notifications (system_notification) are no longer delivered to the model in v1.0.5. The model never receives the notification that a background agent has completed, so it cannot react to results automatically.

Reproduction

1. Start Copilot CLI on v1.0.5
2. Launch a background agent:

   Use the task tool to explore the codebase (mode: background)
   

3. Agent completes successfully
4. Expected: Model receives system_notification and calls read_agent automatically
5. Actual: No notification delivered. Model is unaware the agent completed. User has to manually tell the model "it is done."

Version Comparison

Version	Notifications	Tested
v1.0.4-0	✅ Works — model receives notification immediately	2/2 tests
v1.0.5	❌ Broken — no notification delivered	0/3 tests

Test Methodology

• Downgraded by hiding ~/.copilot/pkg/universal/1.0.5 and using --no-auto-update
• Same binary (/opt/homebrew/bin/copilot), same session setup, same model (Claude Opus 4.6)
• Only difference: JS package version loaded from ~/.copilot/pkg/universal/
• On v1.0.4-0: notification appears as <system_notification> in model context, model reacts with read_agent
• On v1.0.5: agent completes (visible in /tasks), but model never gets a turn to process it

Impact

This breaks any workflow that relies on background agents completing and the model reacting automatically:

• SDLC pipelines (Security Guardian → Code Review Guardian → merge results)
• Background builds/tests where the model should process results
• Any multi-step workflow using mode: "background" in the task tool

Environment

• macOS (Darwin, arm64)
• Copilot CLI v1.0.5 (latest stable as of 2026-03-15)
• Model: Claude Opus 4.6
• Installed via Homebrew cask copilot-cli@prerelease

Likely Cause

v1.0.5 changelog includes:

• "Send follow-up messages to background agents with the write_agent tool for multi-turn conversations"

This new multi-turn agent feature may have changed how agent completion is signaled internally, breaking the existing notification path.