Avoid invalidation for idle provisional sessions

Keep expensive session-list computations coalesced when invisible provisional composers are created or disposed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: benibenj

src/vs/platform/agentHost/node/agentService.ts

@@ -2765,6 +2765,7 @@ export class AgentService extends Disposable implements IAgentService {
 			this._createProviderSession(provider, config, deferWorktreeCreation),
 		]);
 		const session = created.session;
+		const isIdleProvisional = created.provisional === true && !config?.importConversation;
 		this._logService.trace(`[AgentService] createSession: initialization complete`);
 		const creationReference = readSessionCreationReference(config?._meta);
 		if (creationReference && !isEphemeral) {
@@ -2783,7 +2784,9 @@ export class AgentService extends Disposable implements IAgentService {
 					() => this._sessionRegistry.tombstone(session),
 					`tombstoning ephemeral session ${session.toString()}`,
 				);
-				this._invalidateSessionList();
+				if (!isIdleProvisional) {
+					this._invalidateSessionList();
+				}
 			} catch (err) {
 				await this._rollbackProviderSession(provider, session);
 				throw err;
@@ -2795,7 +2798,9 @@ export class AgentService extends Disposable implements IAgentService {
 					() => this._sessionRegistry.register(session, { provider: provider.id, startTime: registeredAt, modifiedTime: registeredAt, source: 'explicit' }, { checkTombstone: false }),
 					`registration for ${session.toString()}`,
 				);
-				this._invalidateSessionList();
+				if (!isIdleProvisional) {
+					this._invalidateSessionList();
+				}
 			} catch (err) {
 				await this._rollbackProviderSession(provider, session);
 				throw err;
@@ -2835,7 +2840,7 @@ export class AgentService extends Disposable implements IAgentService {
 		// updates while resolving that snapshot; without a state entry those
 		// actions are rejected as targeting an unknown session and custom agents
 		// can disappear from the picker permanently.
-		const provisionalState = created.provisional && !config?.importConversation
+		const provisionalState = isIdleProvisional
 			? (() => {
 				const summary = this._buildInitialSummary(provider, session, config, created, '');
 				const state = this._stateManager.createSession(summary, { emitNotification: false });
@@ -3931,6 +3936,7 @@ export class AgentService extends Disposable implements IAgentService {
 		const sessionKey = session.toString();
 		this._cancelPendingSessionGc(session);
 		const isEphemeral = this._stateManager.isEphemeralSession(sessionKey);
+		const isIdleProvisional = this._stateManager.isIdleProvisionalSession(sessionKey);
 		this._stateManager.invalidateSessionChatResolutions(session.toString());
 		const sessionChats = this._stateManager.getSessionState(session.toString())?.chats ?? [];
 		for (const chat of sessionChats) {
@@ -3955,7 +3961,9 @@ export class AgentService extends Disposable implements IAgentService {
 				`unregistration for ${session.toString()}`,
 			);
 		}
-		this._invalidateSessionList();
+		if (!isIdleProvisional) {
+			this._invalidateSessionList();
+		}
 		if (provider) {
 			this._providerService.releaseSession(session.toString());
 			this._clearDownloadProgressInterest(session.toString());

src/vs/platform/agentHost/test/node/agentService.test.ts

@@ -6073,6 +6073,48 @@ suite('AgentService (node dispatcher)', () => {
 			);
 		});
 
+		test('idle provisional create and dispose do not invalidate the session list', async () => {
+			class ConfigurableProvisionalAgent extends MockAgent {
+				provisional = true;
+				override readonly chats: IAgentChats = withChatOverrides(getChatSurface(this), base => ({
+					createChat: async (chat, context, options) => {
+						const created = await base.createChat(chat, context, options);
+						return created && this.provisional ? { ...created, provisional: true } : created;
+					},
+				}));
+			}
+
+			const localService = disposables.add(createTestAgentService(new NullLogService(), fileService, createSessionDataService(), { _serviceBrand: undefined } as IProductService, createNoopGitService()));
+			const agent = disposables.add(new ConfigurableProvisionalAgent('copilot'));
+			registerTestAgentProvider(localService, agent);
+			const registryEpoch = () => (localService as unknown as { _registryEpoch: number })._registryEpoch;
+			const initialEpoch = registryEpoch();
+
+			const provisional = await localService.createSession({ provider: agent.id });
+			const afterProvisionalCreate = registryEpoch();
+			await localService.disposeSession(provisional);
+			const afterProvisionalDispose = registryEpoch();
+
+			agent.provisional = false;
+			const materialized = await localService.createSession({ provider: agent.id });
+			const afterMaterializedCreate = registryEpoch();
+			await localService.disposeSession(materialized);
+
+			assert.deepStrictEqual({
+				initialEpoch,
+				afterProvisionalCreate,
+				afterProvisionalDispose,
+				afterMaterializedCreate,
+				afterMaterializedDispose: registryEpoch(),
+			}, {
+				initialEpoch,
+				afterProvisionalCreate: initialEpoch,
+				afterProvisionalDispose: initialEpoch,
+				afterMaterializedCreate: initialEpoch + 1,
+				afterMaterializedDispose: initialEpoch + 2,
+			});
+		});
+
 		test('listSessions overlays live workspace metadata over a stale provider snapshot', async () => {
 			class DelayedListAgent extends MockAgent {
 				readonly listStarted = new DeferredPromise<void>();
@@ -6769,6 +6811,7 @@ suite('AgentService (node dispatcher)', () => {
 					{ git: gitState },
 				);
 			});
+
 		});
 
 		test('subscribe to a registered session changeset URI returns a changeset snapshot', async () => {