agentHost: speed up subagent session enrichment (parallel load + memoized reconstruction)

[DonJayamanne]

Why

Opening an agent session that used subagents (a parent agent that spawned helper "child" agents) was slow. To render the transcript, VS Code rebuilds the full conversation and groups each child's inner steps under its parent tool call. That rebuild was slow because:

• the client loaded each subagent one at a time (sequentially), and
• the backend rebuilt the entire parent conversation from scratch once per subagent — a session with 10 subagents re-parsed the whole event log 11 times.

What changed

• A — Parallel subagent loading (client). agentHostSessionHandler now collects all subagent insertions across the whole history, dedupes child-state loads via a Map, and loads them in parallel (Promise.all) instead of sequentially.
• B — Reconstruct once / memoization (backend). CopilotAgentSession memoizes the getEvents() + mapSessionEvents() result (_getMappedEvents), shared by getMessages, getSubagentMessages, and the new getSubagentSessions. The memo is invalidated whenever the persisted event log could change (user message, turn start/end, tool complete, subagent completed/failed, compaction, truncation, snapshot rewind) so it never serves stale turns for an active session.
• C — Eager registration up-front (backend). New optional IAgent.getSubagentSessions() + IRestoredSubagentSession. During restoreSession, the dispatcher registers every discovered child session eagerly (reusing B's single reconstruction — no extra event-log reads), so the client's later per-child lookups are instant in-memory hits.
• D — Correctness fix (client). _loadSubagentState now waits for both the child's session summary and its default-chat channel to hydrate before reading. After the multi-chat protocol split, turns live on the chat channel; awaiting only the session subscription read too early and silently produced zero inner steps.

Results

For a real 90-turn / 4-subagent session:

• enrichment: ~2161 ms → ~53 ms
• total click-to-displayed: ~5324 ms → ~2700 ms

Notes for reviewers

• The memo is intentionally scoped to the resume/restore wave and dropped on any log-mutating event and on dispose; rejected reconstructions are not cached so the next caller retries.
• getSubagentSessions is optional on IAgent; agents without subagents simply omit it.
• Tests added: eager registration during parent restore (agentService.test.ts) and memoization + invalidation (copilotAgentSession.test.ts).

For #320303

[Copilot]

Pull request overview

This PR optimizes subagent session enrichment when reopening agent-host chat sessions by parallelizing child-session hydration on the client and eliminating repeated event-log reconstruction on the backend via memoization. It also adds an optional backend API to eagerly surface subagent child sessions during parent restore so subsequent client lookups hit in-memory state.

Changes:

• Parallelize client-side subagent child-session loads during history enrichment and ensure both session + default-chat channels are hydrated before reading turns.
• Memoize getEvents() + mapSessionEvents() in CopilotAgentSession, share it across parent/subagent lookups, and invalidate it on session log mutations.
• Add optional IAgent.getSubagentSessions() and eagerly register restored subagent child sessions during parent restoreSession, with tests covering memoization and eager registration.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts	Collect subagent insertions across history, dedupe + parallelize child loads, and wait for both session and default-chat hydration before reading child turns.
src/vs/platform/agentHost/node/copilot/copilotAgentSession.ts	Add memoized reconstruction shared across message/subagent lookups, plus getSubagentSessions() for eager child discovery.
src/vs/platform/agentHost/node/copilot/copilotAgent.ts	Implement optional getSubagentSessions() for Copilot sessions (skipping peer chats, subagent URIs, and provisional sessions).
src/vs/platform/agentHost/node/agentService.ts	During restore, eagerly register subagent child sessions from agent-provided discovery to avoid per-child re-restore work.
src/vs/platform/agentHost/common/agentService.ts	Introduce IRestoredSubagentSession and optional IAgent.getSubagentSessions().
src/vs/platform/agentHost/test/node/agentService.test.ts	Add test ensuring parent restore eagerly registers subagent child state.
src/vs/platform/agentHost/test/node/copilotAgentSession.test.ts	Add test for memoization across message/subagent lookups and invalidation on log change.

Copilot's findings

• Files reviewed: 7/7 changed files
• Comments generated: 1