Feature: Emit a dedicated event when user switches to a custom agent via /agent

[Arithmomaniac]

Summary

When a user invokes /agent <name> to switch to a custom agent at the session root, no dedicated event is emitted. The only signal that an agent switch occurred is the <agent_instructions> block silently prepended to user.message.data.transformedContent. This makes it difficult for downstream tooling to reliably track which custom agent is active.

Current behavior

• /agent review-pr-comments rewrites the next user message by prepending <agent_instructions>...</agent_instructions> into transformedContent
• There is no session.agent_changed or session.agent_invoked event in events.jsonl
• subagent.selected exists but fires rarely (21 events across 920+ sessions in one user's history) and doesn't appear to be the canonical marker for root-level /agent switches
• session.mode_changed and session.model_change exist for other session-level state changes, but agents have no equivalent

Impact

• Session analytics: Tools like copilot-session-tools that enrich events.jsonl into queryable databases cannot reliably count or filter by which custom agent was used. The only workaround is parsing <agent_instructions> blocks out of transformedContent, which is fragile and undocumented.
• Skill auditing: When auditing skill/agent effectiveness, the lack of a clean event means agent usage counts are dramatically undercounted (in one audit, review-pr-comments showed 0 dispatches via task tool but actually had 390 root-level invocations hidden in transformedContent).
• Observability: Users and platform teams can't answer "which custom agents are actually being used?" without deep parsing of message content.

Proposed solution

Emit a new event when /agent <name> is invoked at the session root:

{"type": "session.agent_invoked", "agentName": "review-pr-comments", "agentSource": "user", "timestamp": "..."}

Fields:

• agentName: The agent identifier as it appears in the .agent.md filename
• agentSource: "user" for custom agents in ~/.copilot/agents/, "project" for repo-level agents, "builtin" for platform agents

This would parallel the existing session.mode_changed and session.model_change events.

Workaround (current)

Parse transformedContent for <agent_instructions> blocks and extract the agent name — but this is fragile, undocumented, and requires access to the full message payload rather than just the event stream.