agentHost: observe capabilities lazily in session adapters

[Alexandru Dima (alexdima)]

I generated this by scanning my logs, pls take if it's good/useful

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🤖

Found while self-hosting Insiders: scanning my recent session logs for errors and leaked-disposable warnings surfaced a listener leak originating in the Agent Host sessions provider.

The symptom

~/Library/Application Support/Code - Insiders/logs/*/window1/renderer.log:

2026-08-10 11:15:47.896 [error] potential listener LEAK detected, popular: Error
    at s.create (.../sessions.desktop.main.js)
    at ML.onFirstObserverAdded (.../sessions.desktop.main.js)
    at dS._computeFn (.../sessions.desktop.main.js)
    ...
    at dce.createAdapter (.../sessions.desktop.main.js)
    at dce._refreshSessions (.../sessions.desktop.main.js)

The minified frames deobfuscate to BaseAgentHostSessionsProvider._refreshSessions → createAdapter → new AgentHostSessionAdapter → an autorun adding an observer to a shared observable.

Root cause

AgentHostSessionAdapter's constructor registered an autorun that reads this.capabilities, which is derived from the provider-wide agentCapabilities observable. Every adapter therefore attached an observer to that one shared observable at construction time.

_refreshSessions builds an adapter for every session the host lists, so a window restoring a large session list installed one observer per session and crossed the leak-detector threshold.

The subscription was also mostly pointless. That autorun exists only to re-apply a chat catalog when capabilities hydrate late (the race #323625 addressed, where a multi-chat SessionState is processed before root state advertises supportsMultipleChats). Its body no-ops unless _lastCatalogState is set — and _lastCatalogState is only ever assigned by applyChatCatalog. Adapters that never received a catalog (the overwhelming majority in a large list) held a live observer that could never do work.

The fix

Install the capabilities observer on the first applyChatCatalog call rather than in the constructor, held in a MutableDisposable. Adapters with catalog state to reconcile subscribe; adapters without one cost nothing.

This is a natural follow-on to #328031, which collapsed N root-state event listeners into one shared lookup. This removes the remaining N per-adapter observers on that shared lookup.

Behavior preserved

Late-hydrating capabilities still re-expand a collapsed peer catalog. The existing test a peer catalog collapsed while capabilities were absent re-expands when they hydrate covers exactly that path and passes unchanged — the observer is installed by applyChatCatalog, which necessarily runs before any catalog needs reapplying.

Validation

• New test session adapters observe capabilities only after receiving a chat catalog builds 200 adapters and asserts 0 capability observers before any catalog, exactly 1 after, and correct peer-chat expansion on late hydration.
• Targeted suite (incl. the two pre-existing capability/listener tests): 3 passing.
• npm run typecheck-client and npm run valid-layers-check pass. (typecheck-client reports two pre-existing assignmentService.ts errors that also reproduce on a pristine main — unrelated to this change.)

Review note

Sandeep Somavarapu (@sandy081) flagging you since you authored applyChatCatalog, _lastCatalogState, and the "Make ISession.capabilities observable so late-hydrating capabilities reconcile" change that introduced this autorun. The one assumption worth your eyes: that an adapter which never receives a chat catalog genuinely has nothing to reconcile when capabilities hydrate late. Everything else here is listener bookkeeping.

[Copilot]

Pull request overview

Lazily observes Agent Host capabilities only when a session adapter has a chat catalog requiring reconciliation, preventing unnecessary observers.

Changes:

• Moves capability observation from construction to first catalog application.
• Adds regression coverage for lazy observation and late hydration.
• Documents the updated architecture.

Show a summary per file

File	Description
baseAgentHostSessionsProvider.ts	Lazily installs the capability observer.
localAgentHostSessionsProvider.test.ts	Tests observer counts and catalog hydration.
AGENT_HOST_SESSIONS_PROVIDER.md	Documents lazy capability observation.

Review details

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• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Balanced