TPI) Agent Host Copilot session attachments from Copilot CLI trajectories

[anthonykim1]

Refs: #323773, #315193

• macOS @Yoyokrazy
• linux @TylerLeonhardt
• windows @alexr00

Complexity: 3

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Verify that Agent Host Copilot chat can attach prior Copilot agent host sessions, and extension host Copilot CLI sessions as context.

Background: Non-Copilot sessions including Claude/local/other chat sessions are not supported here.

Prerequisites:

• Use the latest VS Code Insiders
• Sign in to GitHub Copilot
• Enable chat.agentHost.enabled (this test is for Agent Host Copilot)
• Have at least one prior Copilot CLI session available to attach (create one first if needed), whether it's an agent host Copilot session or an extension host Copilot session previously

Part 1: Attach an agent host or extension host Copilot session in Agent Host chat

• Start an Agent Host Copilot chat
• Open the Sessions... picker (or type #session:) and confirm prior agent host or extension host Copilot sessions are listed
• Attach one session
• Confirm a visible Sessions chip appears on the input
• Ask a question that requires context from that prior session and confirm Copilot can use the attached trajectory to answer

Part 2: Picker only shows supported sessions

• With other session types present (for example Claude / local / other chat sessions, and any non-Copilot-CLI Agent Host sessions), open the Agent Host Sessions... picker
• Confirm only attachable Copilot CLI sessions are listed
• Confirm non-Copilot sessions including Claude/local/other chat sessions are not offered

Part 3: Reload / resume restores only the visible chip

• After attaching a session and sending a message, reload the window (or resume the chat)
• Confirm the visible Sessions chip is restored
• Confirm no extra hidden trajectory/resource attachment is surfaced in the UI after reload

Part 4: Exploratory

• Try attaching multiple sessions, re-attaching the same session, and mixing session attachments with other context
• Note any cases where:
  • a supported Copilot CLI session is missing from the picker
  • an unsupported session type appears in the Agent Host picker
  • the visible chip or the underlying trajectory context is lost after reload/resume
  • attached session context is not actually used by Copilot

[TylerLeonhardt]

Background: Non-Copilot sessions including Claude/local/other chat sessions are not supported here.

Why not? Are they not just files on disk?

[anthonykim1]

Why not? Are they not just files on disk?

In the future, we should probably have support for it.
Matrix for different harness: #323969
I don't think the non-copilot ones had event.jsonl that I used in #323773 for these attachment sessions.

[TylerLeonhardt]

Not directly, but Claude has a jsonl file format as well.