TPI) Agent host Copilot CLI async shell

[anthonykim1]

Refs: #318511, #318505

• macOS
• linux
• windows

Complexity: 3

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Verify that Agent Host Copilot CLI surfaces async shell/background completions as a follow-up turn when the custom terminal tool is disabled.

Prerequisites:

• Use the latest VS Code Insiders
• Sign in to GitHub Copilot
• Enable chat.agentHost.enabled
• Set chat.agentHost.customTerminalTool.enabled to false (so the SDK native shell tool is used)
• Install and authenticate Copilot CLI

Part 1: Background shell completion while idle

• Start an Agent Host Copilot CLI chat
• Ask Copilot to run a long-running command in the background (for example a build, a sleep-based task, or a script that finishes after some delay), then let the turn finish so the session is idle
• When the background shell completes, confirm a new system-initiated follow-up turn appears automatically
• Confirm the follow-up shows a compact system-initiated label rather than a fabricated user message

Part 2: Completion arriving during an active turn

• Kick off a background/detached shell, then keep Copilot busy with another request so a turn is active when the background work finishes
• Confirm the completion is shown inline within the active turn (no extra duplicate turn is spawned)

Part 3: Chained async shells

• Use a single prompt that drives two sequential async shells across turns, for example:

  Do what I say.
  Step 1: Run "sleep 6 && echo SHELL_A_DONE" as an async shell, then end your turn.
  After SHELL_A_DONE notification arrives, in your follow-up turn:
  Step 2: Run "sleep 6 && echo SHELL_B_DONE" as an async shell, then end that turn too.
  

• Confirm the first async shell launches and the turn ends
• Confirm the SHELL_A_DONE completion arrives as a system-initiated follow-up turn that then launches the second async shell
• Confirm the SHELL_B_DONE completion arrives as its own follow-up turn and the marker can be read back

Part 4: Exploratory

• Try several background/async commands and note any cases where:
  • completion is missed or never surfaces
  • duplicate or stale turns appear
  • the follow-up attaches to the wrong/previous turn
  • the system-initiated label is missing or confusing