Avoid Agent Host session listing starvation

src/vs/platform/agentHost/AGENTS.md

@@ -220,7 +220,7 @@ a chat URI. New provider code must consume the seams.
 
 `register` takes the resolved provenance and whether to check tombstones. Explicit `AgentService.createSession` calls skip the tombstone check and clear any tombstone for that session URI; restore and discovery calls atomically decline to register if the session is or concurrently becomes tombstoned. An explicit row is never rewritten by catalog discovery. A migration-time host-owned marker can correct a previously discovered row back to internal provenance.
 
-Providers own discovery lifecycle and push unknown chats with provider-classified provenance through `onDidDiscoverChats`. Claude, Codex, and Copilot classify their unknown native chats as external, except that Copilot keeps an unknown *legacy extension-host* chat internal because it is adoptable in place rather than someone else's session. Agent Service preserves that classification when it additively registers the event payload. Every provider starts one memoized initial attempt when the first discovery-event listener is attached; that attempt retries internally, but once it settles it is not re-armed by SDK readiness, so the only later trigger is an explicit one (for Copilot, the migrate-legacy toggle). Ordinary list refreshes never enumerate provider catalogs. External discovery has no migration marker or Copilot migrate-legacy gate; only the adoptable legacy extension-host half of Copilot's payload is withheld while migrate-legacy is off. Discovery never prunes a registry row when a provider later omits it and filters subagents and marked internal chat backings.
+Providers own discovery lifecycle and push unknown chats with provider-classified provenance through `onDidDiscoverChats`. Claude, Codex, and Copilot classify their unknown native chats as external, except that Copilot keeps an unknown *legacy extension-host* chat internal because it is adoptable in place rather than someone else's session. Agent Service preserves that classification when it additively registers the event payload. Agent Service always attaches the event listener and queues each provider's external-session discovery through `_runWhenStartupSettled`, so the request waits for both Agent Host startup and the first successful session listing. Providers registered after that barrier opens run their queued work immediately, and a later transition from `none` starts discovery directly. Adopt-in-place legacy migration remains an independent provider-initialization trigger immediately after the discovery listener is attached, and another catalog consumer may also trigger discovery after it enumerates the provider catalog. This keeps `showExternalSessions: none` from initiating native discovery while allowing independently triggered discovery to populate the hidden registry normally. Ordinary list refreshes never enumerate provider catalogs. External discovery has no migration marker or Copilot migrate-legacy gate; only the adoptable legacy extension-host half of Copilot's payload is withheld while migrate-legacy is off. Discovery never prunes a registry row when a provider later omits it and filters subagents and marked internal chat backings.
 
 Discovery is registry-first: Agent Service hands each provider an optional `setKnownSessionsFilter` seam that answers, for a whole candidate set in one registry query, which sessions the host already owns. A provider drops those candidates before any per-session database open, and Copilot additionally skips adoptable legacy classification work (project/Git resolution) while migrate-legacy is off, since those candidates would not be emitted. Agent Service in turn rejects an already-registered candidate before `_isChatBacking()` or any other per-session I/O; provenance of a registered row stays owned by the explicit create/restore paths. Tombstoned sessions are absent from the registry and therefore never reported as known, so an explicitly deleted session still reaches `register`, whose atomic tombstone check declines it.
 

src/vs/platform/agentHost/common/agent.ts

@@ -1203,6 +1203,9 @@ export interface IAgent {
 	/** Provides chats that are ready to be registered as Agent Host sessions. */
 	readonly onDidDiscoverChats: Event<readonly IAgentDiscoveredChat[]>;
 
+	/** Starts the provider's memoized native chat discovery pass. */
+	startChatDiscovery?(): Promise<void>;
+
 	/** Lets discovery drop registered candidates before per-session I/O. */
 	setKnownSessionsFilter?(filter: IAgentKnownSessionsFilter): void;
 

src/vs/platform/agentHost/common/agentHostSchema.ts

@@ -824,7 +824,7 @@ export const platformRootSchema = createSchema({
 		enum: [ChatExternalSessionsMode.None, ChatExternalSessionsMode.Recent, ChatExternalSessionsMode.Last24Hours, ChatExternalSessionsMode.Last7Days, ChatExternalSessionsMode.Last30Days],
 		enumDescriptions: [
 			localize('agentHost.config.showExternalSessions.none', "Do not show external sessions."),
-			localize('agentHost.config.showExternalSessions.recent', "Show up to the 2 most recent external sessions updated in the last 7 days. Once at least 2 local sessions exist, external sessions older than the second-newest local session are hidden."),
+			localize('agentHost.config.showExternalSessions.recent', "Show up to the 2 most recent external sessions updated in the last 7 days. At startup, external sessions older than the second-most-recently updated local session are hidden."),
 			localize('agentHost.config.showExternalSessions.last24Hours', "Show external sessions updated in the last 24 hours."),
 			localize('agentHost.config.showExternalSessions.last7Days', "Show external sessions updated in the last 7 days."),
 			localize('agentHost.config.showExternalSessions.last30Days', "Show external sessions updated in the last 30 days."),