agentHost: observe capabilities lazily in session adapters

src/vs/sessions/contrib/providers/agentHost/AGENT_HOST_SESSIONS_PROVIDER.md

@@ -96,7 +96,7 @@ A single agent host session uses several distinct identifiers:
 
 ## Architecture
 
-- **`AgentHostSessionAdapter`** (`baseAgentHostSessionsProvider.ts`) is the `ISession` implementation. It wraps an `IAgentSessionMetadata` from the backend and exposes the observable session surface (`status`, `title`, `workspace`, `mainChat`, `mode`, …). The base provider keeps a `_sessionCache` of adapters keyed by `rawId`. Adapter capabilities derive from a shared provider-to-capabilities lookup, so one root-state event listener and one catalog scan serve the entire cache; root-state errors and disconnects clear the lookup so stale capabilities are not retained.
+- **`AgentHostSessionAdapter`** (`baseAgentHostSessionsProvider.ts`) is the `ISession` implementation. It wraps an `IAgentSessionMetadata` from the backend and exposes the observable session surface (`status`, `title`, `workspace`, `mainChat`, `mode`, …). The base provider keeps a `_sessionCache` of adapters keyed by `rawId`. Adapter capabilities derive from a shared provider-to-capabilities lookup, so one root-state event listener and one catalog scan serve the entire cache; an adapter observes that lookup only after receiving a chat catalog that may need reapplication. Root-state errors and disconnects clear the lookup so stale capabilities are not retained.
 - **`NewSession`** is a disposable draft (pre-creation) session. Several can be in flight simultaneously; the management layer tears down superseded drafts via `deleteNewSession`. A draft eagerly creates its backend session once authentication settles, then **graduates** into a committed `AgentHostSessionAdapter` on first send.
 - The base provider is abstract; concrete providers supply: `connection`, `authenticationPending`, `resourceSchemeForProvider`, `_formatSessionTypeLabel`, `_adapterOptions` (workspace builder), `resolveWorkspace`, and optionally `_diffUriMapper`.
 

src/vs/sessions/contrib/providers/agentHost/browser/baseAgentHostSessionsProvider.ts

@@ -648,9 +648,10 @@ export class AgentHostSessionAdapter extends Disposable implements ISession {
 	/**
 	 * The last {@link SessionState} applied to the chat catalog, retained so the
 	 * catalog can be re-reconciled when {@link capabilities} change after the
-	 * fact (see the capability autorun in the constructor).
+	 * fact.
 	 */
 	private _lastCatalogState: SessionState | undefined;
+	private readonly _chatCatalogCapabilitiesObserver = this._register(new MutableDisposable());
 	private readonly _rawId: string;
 	private readonly _resourceScheme: string;
 
@@ -905,19 +906,6 @@ export class AgentHostSessionAdapter extends Disposable implements ISession {
 				supportsDelete: true,
 			};
 		});
-
-		// Re-apply the chat catalog when advertised capabilities change (e.g. the
-		// agent host's root state arrives after the session's first state update).
-		// Without this, a multi-chat session whose state was processed while
-		// `supportsMultipleChats` was still `false` would stay collapsed to
-		// `[defaultChat]` until the next session-state update.
-		this._register(autorun(reader => {
-			this.capabilities.read(reader);
-			const state = this._lastCatalogState;
-			if (state) {
-				this._applyChatCatalog(state);
-			}
-		}));
 	}
 
 	/**
@@ -938,7 +926,17 @@ export class AgentHostSessionAdapter extends Disposable implements ISession {
 	 */
 	applyChatCatalog(state: SessionState): void {
 		this._lastCatalogState = state;
-		this._applyChatCatalog(state);
+		if (this._chatCatalogCapabilitiesObserver.value) {
+			this._applyChatCatalog(state);
+		} else {
+			this._chatCatalogCapabilitiesObserver.value = autorun(reader => {
+				this.capabilities.read(reader);
+				const currentState = this._lastCatalogState;
+				if (currentState) {
+					this._applyChatCatalog(currentState);
+				}
+			});
+		}
 	}
 
 	private _applyChatCatalog(state: SessionState): void {

src/vs/sessions/contrib/providers/agentHost/test/browser/localAgentHostSessionsProvider.test.ts

@@ -9,7 +9,7 @@ import { DeferredPromise, raceTimeout, timeout } from '../../../../../../base/co
 import { Codicon } from '../../../../../../base/common/codicons.js';
 import { Emitter, Event } from '../../../../../../base/common/event.js';
 import { DisposableMap, DisposableStore, ImmortalReference, toDisposable, type IReference } from '../../../../../../base/common/lifecycle.js';
-import { autorun, constObservable, ISettableObservable, observableValue, type IObservable } from '../../../../../../base/common/observable.js';
+import { autorun, constObservable, ISettableObservable, observableFromEvent, observableValue, type IObservable } from '../../../../../../base/common/observable.js';
 import { URI } from '../../../../../../base/common/uri.js';
 import { isEqual } from '../../../../../../base/common/resources.js';
 import { mock } from '../../../../../../base/test/common/mock.js';
@@ -45,7 +45,7 @@ import { IActiveSession } from '../../../../../services/sessions/common/sessions
 import { ISessionsService } from '../../../../../services/sessions/browser/sessionsService.js';
 import { IAgentCustomizationScope, IAgentHostActiveClientService } from '../../../../../../workbench/contrib/chat/browser/agentSessions/agentHost/agentHostActiveClientService.js';
 import { LocalAgentHostSessionsProvider } from '../../browser/localAgentHostSessionsProvider.js';
-import { AgentHostSessionAdapter } from '../../browser/baseAgentHostSessionsProvider.js';
+import { AgentHostSessionAdapter, type IAgentHostAdapterOptions } from '../../browser/baseAgentHostSessionsProvider.js';
 import { IAutomationStorageService } from '../../../../automations/common/automationStorageService.js';
 import { TestAutomationStorageService } from '../../../../automations/test/browser/automationTestUtils.js';
 import { ILabelService } from '../../../../../../platform/label/common/label.js';
@@ -4143,6 +4143,60 @@ suite('LocalAgentHostSessionsProvider', () => {
 			});
 		});
 
+		test('session adapters observe capabilities only after receiving a chat catalog', () => {
+			let listenerCount = 0;
+			let agentCapabilities = new Map<string, AgentInfo['capabilities']>([['copilotcli', {}]]);
+			const capabilitiesChanged = disposables.add(new Emitter<void>({
+				onDidAddListener: () => listenerCount++,
+				onWillRemoveListener: () => listenerCount--,
+			}));
+			const capabilitiesObs = observableFromEvent(disposables, capabilitiesChanged.event, () => agentCapabilities);
+			const instantiationService = disposables.add(new TestInstantiationService());
+			instantiationService.stub(IGitHubService, new class extends mock<IGitHubService>() { });
+			instantiationService.stub(ISessionsService, new class extends mock<ISessionsService>() {
+				override readonly activeSession = constObservable<IActiveSession | undefined>(undefined);
+			});
+			instantiationService.stub(IPullRequestIconCache, new class extends mock<IPullRequestIconCache>() { });
+			const options: IAgentHostAdapterOptions = {
+				icon: Codicon.copilot,
+				loading: constObservable(false),
+				buildWorkspace: () => undefined,
+				instantiationService,
+				getConnection: () => undefined,
+				agentCapabilities: capabilitiesObs,
+			};
+			const adapters = Array.from({ length: 200 }, (_, index) => disposables.add(instantiationService.createInstance(
+				AgentHostSessionAdapter,
+				createSession(`lazy-capabilities-${index}`),
+				'local-agent-host',
+				'agent-host-copilotcli',
+				'copilotcli',
+				options,
+			)));
+			const sessionUri = AgentSession.uri('copilotcli', 'lazy-capabilities-0').toString();
+			const defaultChat = buildDefaultChatUri(sessionUri);
+			const peerChat = buildChatUri(sessionUri, 'peer-1');
+
+			const listenerCountBeforeCatalog = listenerCount;
+			adapters[0].applyChatCatalog(makeState([
+				makeChatSummary(defaultChat, ''),
+				makeChatSummary(peerChat, 'Peer'),
+			], { defaultChat }));
+			const listenerCountAfterCatalog = listenerCount;
+			agentCapabilities = new Map([['copilotcli', { multipleChats: { fork: true } }]]);
+			capabilitiesChanged.fire();
+
+			assert.deepStrictEqual({
+				listenerCountBeforeCatalog,
+				listenerCountAfterCatalog,
+				chatFragmentsAfterHydration: adapters[0].chats.get().map(chat => chat.resource.fragment),
+			}, {
+				listenerCountBeforeCatalog: 0,
+				listenerCountAfterCatalog: 1,
+				chatFragmentsAfterHydration: ['', 'peer-1'],
+			});
+		});
+
 		test('forkChat forwards the source chat and turn to the host and surfaces a new peer chat', () => runWithFakedTimers<void>({ useFakeTimers: true }, async () => {
 			const provider = createProvider(disposables, agentHost);
 			const session = setupMultiChatSession(provider, 'multi-fork');