Switching sessions in the desktop app could freeze the whole UI for
several seconds on heavy, tool-rich chats. Root causes and fixes:
- Cold `session.resume` built the AIAgent (MCP discovery, prompt/skill
build) *before* returning, and the desktop awaits that RPC before it
paints — so the entire switch blocked on the build. Add an opt-in
`defer_build` resume path (the contract `session.create` already uses):
return the full display transcript immediately, register an upgradable
live session, and pre-warm the agent on a short timer. The persisted
runtime identity (model/provider/base_url/api_mode/reasoning/tier) is
restored on the deferred build so it can't drop the provider.
- Nothing bounded how many in-memory agents accumulate; a user who
reconnects often piled up detached sessions for the full 6h TTL. Add a
soft LRU cap (`max_live_sessions`, default 16) that evicts the
least-recently-active DETACHED sessions (no live client) — never a
running, awaiting-input, mid-build, or live-transport one. Reopening
re-resumes from disk.
- On the prefetch-hit cold-resume path, skip rebuilding a throwaway
merged-message array (and its 1000-entry Map) when the prefetch already
painted the exact transcript; the downstream sameMessageList guard
already drops the publish, so it was pure main-thread cost.
The desktop opts into `defer_build` for every non-watch cold resume; the
eager path stays for CLI/TUI and existing callers.
c4c590e4a1