fix(frontend): Sort sidebar agents by last used

[ashrafchowdury]

Context

Opening the Agents group in the sidebar showed five agents in what looked like a random order. The group rendered the first five records from the shared workflow list, whose order follows creation, so the agents on screen had no relation to the ones you actually work with.

Changes

The Agents group now sorts by last used before the five-item cut, so the top of the list is the agents you touched most recently.

"Last used" is the agent's newest session activity. It comes from the session rows the sidebar already fetches for the Sessions group (same query keys, so no extra request), overlaid with this browser's local playground store so a session you just started reorders the list immediately, ahead of the server window's stale time.

An agent with no session history falls back to updated_at ?? created_at. That is one ranking key, not two tiers, so a freshly created agent carries the newest timestamp in the list and opens at the top.

Sorting lives in a new list atom the registry points at, so the order lands before resolveChildren slices the visible five, and the full list stays intact for the "Show all" overflow count.

Before: first five workflow records, creation order.
After: five most recently used agents, newest first.

Tests / notes

• agentsSource.test.ts covers ordering, the created/updated fallback, a brand-new agent ranking on top, ties keeping source order, and the sort-before-limit guarantee.
• sessions.localActivity.test.ts pins the local overlay: a seeded blank tab does not count as usage (opening a playground seeds an empty tab, so "visited" must not read as "used"), a settled turn does, and non-app scopes are skipped.
• pnpm test:unit green on all touched suites, tsc and pnpm lint-fix clean.
• Automation-only agents (scheduled runs, never a human session) fall back to their workflow timestamp. The sidebar's session policy is exclude-trigger, and counting trigger sessions would need a separate query key (a second request per sidebar open), so it was left out on purpose.
• Stacked on feat/agent-icon-picker ([feat] Let each agent carry its own icon and colour #6062), which also edits registry.ts. Base this PR on that branch; it should merge first.

What to QA

• Use a few agents in a known order, then open the Agents group. The list is newest-used first.
• Start a new session on an agent low in the list. It jumps to the top without a reload.
• Create a new agent. It appears at the top of the group.
• Regression: with the Sessions group also open, check the Network tab shows no second session-list request.

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Project	Deployment	Actions	Updated (UTC)
agenta-documentation	Ready	Preview	Aug 18, 2026 4:03pm

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[github-actions]

Railway Preview Environment

Preview URL	https://gateway-pr-6104.up.railway.app/w
Project	agenta-oss-clone-spike
Image tag	pr-6104-1e43de5
Status	Deployed
Railway logs	Open logs
Workflow logs	View workflow run
Updated at 2026-08-18T16:14:01.170Z

[mmabrouk]

Review summary

Focused on performance. No blocking issue: the stamps driving the new ordering are written once per settled turn, not per streamed token, so the sidebar does not re-render on every frame while chatting. Core logic reads correctly and the traps are well covered by tests. One change requested below, plus two low-severity cleanups.

Requested change

• Avoid re-sorting + re-rendering the whole Agents group on every completed chat turn. sidebarAgentsListAtom depends on localAgentActivityAtom, which reads the entire sessionMessagesAtom map. That map is rewritten each time a turn settles (bumpSessionActivityAtom), so every turn completion re-sorts all agents (not just the visible five) and returns a new array + new object reference ({...query, data: [...]}), re-rendering the group even when the order is unchanged. Please memoize the emitted list so an unchanged order returns the previous array reference (reference-check the sorted ids), and/or narrow localAgentActivityAtom's dependency on sessionMessagesAtom — it subscribes to the full messages map only to feed isSessionHusk's messages[id]?.length lookup, so it invalidates even for sessions that already have lastMessageAt and skip the husk check. Fine at today's scale; this keeps it from becoming a hotspot in a heavy workspace.

Non-blocking cleanups

• Sort key computed inside the comparator. sortAgentsByLastUsed calls rank() (which does Date.parse via at() for agents lacking usage) inside .sort(), i.e. O(n log n) parses instead of O(n). Precompute rank once per workflow into a map, then sort.
• Math.max(0, ...used.map(sessionActivity)) spread in localAgentActivityAtom throws on a pathologically large per-app session list (arg-count ceiling). Unlikely from real localStorage, but a reduce avoids the ceiling.

Confirmed good

• No second network request: sidebarSessionRowsAtom reuses the existing recent + pinned session query data rather than issuing a new window query.
• Local-stamp-wins-when-newer, husk exclusion, non-agent scope exclusion, brand-new-agent fallback, and stable-tie ordering all read correctly and are pinned by the two test files.

Note: stacked on feat/agent-icon-picker (#6062); both touch registry.ts, so #6062 should merge first. This is an advisory review.