Fix first plugin skill completion

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

@@ -36,6 +36,7 @@ export class AgentHostSkillCompletionProvider extends Disposable implements IAge
 		 * snapshot for the session yet.
 		 */
 		private readonly _getHostCustomizations: (session: URI | string) => readonly Customization[] | undefined = () => undefined,
+		private readonly _resolveSessionCustomizations?: (agent: IAgent, session: URI) => Promise<readonly Customization[]>,
 	) {
 		super();
 	}
@@ -88,7 +89,9 @@ export class AgentHostSkillCompletionProvider extends Disposable implements IAge
 
 	private async _getCandidates(agent: IAgent, session: URI): Promise<readonly SlashCommmandCandidate[]> {
 		const chat = URI.parse(buildDefaultChatUri(session));
-		const customizations = await agent.getChatCustomizations(chat, { configurationResource: session, resource: session }, this._getHostCustomizations(session));
+		const customizations = this._resolveSessionCustomizations
+			? await this._resolveSessionCustomizations(agent, session)
+			: await agent.getChatCustomizations(chat, { configurationResource: session, resource: session }, this._getHostCustomizations(session));
 		const result: SlashCommmandCandidate[] = [];
 		for (const c of customizations) {
 			if (c.type === CustomizationType.McpServer || (c.type === CustomizationType.Plugin ? !isCustomizationEnabled(c) : !c.enabled) || !c.children) {

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

@@ -1128,6 +1128,7 @@ export class AgentService extends Disposable implements IAgentService {
 		const provider = this._register(new AgentHostSkillCompletionProvider(
 			session => this._providerService.getProviderForSession(session),
 			session => this._hostCustomizations(URI.isUri(session) ? session : URI.parse(session)),
+			(agent, session) => this._sideEffects.refreshSessionCustomizations(agent, session.toString()),
 		));
 		this._register(this._completions.registerProvider(provider));
 	}

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

@@ -449,7 +449,7 @@ export class AgentSideEffects extends Disposable {
 		this._stateManager.dispatchServerAction(ROOT_STATE_URI, { type: ActionType.RootAgentsChanged, agents: infos });
 	}
 
-	private async _publishSessionCustomizations(agent: IAgent, session: ProtocolURI, supersededRetries: number): Promise<void> {
+	private async _publishSessionCustomizations(agent: IAgent, session: ProtocolURI, supersededRetries: number): Promise<readonly Customization[]> {
 		const currentBeforeFetch = this._stateManager.getSessionState(session)?.customizations;
 		const chat = URI.parse(this._stateManager.getSessionState(session)?.defaultChat ?? buildDefaultChatUri(session));
 		const customizations = await agent.getChatCustomizations(chat, this._chatContext(session, chat.toString()), currentBeforeFetch);
@@ -473,29 +473,39 @@ export class AgentSideEffects extends Disposable {
 			if (supersededRetries < MAX_SUPERSEDED_CUSTOMIZATION_PUBLISH_RETRIES) {
 				this._publishSessionCustomizationsSoon(agent, session, supersededRetries + 1);
 			}
-			return;
+			return current ?? [];
 		}
 		if (current && equals(current, customizations)) {
-			return;
+			return current;
 		}
 
 		this._stateManager.dispatchServerAction(session, {
 			type: ActionType.SessionCustomizationsChanged,
 			customizations: [...customizations],
 		});
+		return customizations;
 	}
 
 	private _publishSessionCustomizationsSoon(agent: IAgent, session: ProtocolURI, supersededRetries = 0): void {
+		void this.refreshSessionCustomizations(agent, session, supersededRetries).catch(() => { });
+	}
+
+	/**
+	 * Resolves and publishes the session's effective customizations in serialization order.
+	 */
+	refreshSessionCustomizations(agent: IAgent, session: ProtocolURI, supersededRetries = 0): Promise<readonly Customization[]> {
 		const previous = this._pendingSessionCustomizationPublishes.get(session) ?? Promise.resolve();
-		const publish = previous.then(() => this._publishSessionCustomizations(agent, session, supersededRetries)).catch(err => {
+		const publish = previous.then(() => this._publishSessionCustomizations(agent, session, supersededRetries));
+		const tracked = publish.then(() => undefined, err => {
 			this._logService.error('[AgentSideEffects] getChatCustomizations failed', err);
 		});
-		this._pendingSessionCustomizationPublishes.set(session, publish);
-		void publish.finally(() => {
-			if (this._pendingSessionCustomizationPublishes.get(session) === publish) {
+		this._pendingSessionCustomizationPublishes.set(session, tracked);
+		void tracked.finally(() => {
+			if (this._pendingSessionCustomizationPublishes.get(session) === tracked) {
 				this._pendingSessionCustomizationPublishes.delete(session);
 			}
 		});
+		return publish;
 	}
 
 	private _publishPendingCustomizationEnablementRefreshes(): void {

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

@@ -43,6 +43,7 @@ import { SessionDatabase } from '../../node/sessionDatabase.js';
 import { ActionType, ActionEnvelope, NotificationType, type INotification } from '../../common/state/sessionActions.js';
 import { AH_META_CREATED_BY_SESSION_DB_KEY, AH_META_IS_READ_DB_KEY, AH_META_EHCLI_ADOPTED_DB_KEY, readSessionEhcliAdopted, AH_META_IS_ARCHIVED_DB_KEY, AH_META_WORKSPACELESS_DB_KEY, ChangesetStatus, CustomizationType, MessageAttachmentKind, MessageKind, SessionActiveClient, ResponsePartKind, ROOT_STATE_URI, SESSION_META_FOLDER_PICKER_KEY, SESSION_META_MULTI_ROOT_KEY, SessionLifecycle, SessionSourceControlOutcome, SessionStatus, ToolCallCancellationReason, ToolCallConfirmationReason, ToolCallStatus, ToolResultContentType, TurnState, buildChatUri, buildDefaultChatUri, buildSubagentChatUri, buildSubagentSessionUri, createErrorResponsePart, customizationId, isDefaultChatUri, isMessageHiddenFromTranscript, isSubagentSession, parseChatUri, parseSubagentSessionUri, readSessionCreationReference, readSessionExternal, readSessionGitHubState, readSessionMultiRootMetadata, readSessionFolderPickerDecision, readSessionSourceControlState, withSessionEhcliAdoptable, withSessionExternal, withSessionMultiRootMetadata, ChatOriginKind, type ChangesetState, type ISessionFolderPickerDecision, type ISessionWithDefaultChat, type MarkdownResponsePart, type SessionState, type SessionSummary, type ToolCallCompletedState, type ToolCallResponsePart, type Turn } from '../../common/state/sessionState.js';
 import { ChatInteractivity, type MessageAttachment } from '../../common/state/protocol/state.js';
+import { CompletionItemKind } from '../../common/state/protocol/commands.js';
 import { isHostSnapshotAttachment, toHostSnapshotAttachmentMeta } from '../../common/meta/agentSnapshotAttachmentMeta.js';
 import { readAgentMessageDelegationMeta } from '../../common/meta/agentMessageDelegationMeta.js';
 import { IProductService } from '../../../product/common/productService.js';
@@ -543,6 +544,40 @@ suite('AgentService (node dispatcher)', () => {
 		}]);
 	});
 
+	test('publishes a newly discovered skill before returning its completion', async () => {
+		registerTestAgentProvider(service, copilotAgent);
+		const session = await service.createSession({ provider: 'copilot' });
+		const customization = {
+			type: CustomizationType.Plugin,
+			id: customizationId('file:///dummy'),
+			uri: 'file:///dummy',
+			name: 'dummy',
+			children: [{
+				type: CustomizationType.Skill,
+				id: customizationId('file:///dummy/skills/print-hello-world/SKILL.md'),
+				uri: 'file:///dummy/skills/print-hello-world/SKILL.md',
+				name: 'print-hello-world',
+				description: 'Calculate the sum of 3+3 and output it.',
+			}],
+		} as const;
+		copilotAgent.customizations = [customization];
+
+		const result = await service.completions({
+			kind: CompletionItemKind.UserMessage,
+			channel: session.toString(),
+			text: '/print',
+			offset: '/print'.length,
+		});
+
+		assert.deepStrictEqual({
+			items: result.items.map(item => item.insertText),
+			published: getStateManager(service).getSessionState(session.toString())?.customizations,
+		}, {
+			items: ['/dummy:print-hello-world '],
+			published: [customization],
+		});
+	});
+
 	// ---- Provider registration ------------------------------------------
 
 	suite('registerProvider', () => {