feat: add autoAttachReferences option to chat session extension point (#302508)

* feat: add autoAttachReferences option to chat session extension point

This allows us to opt-in/out in to auto-attaching references.

Some external agents will manage auto attaching references by themselves, this allows them to do so without Core injecting stuff automatically.

* fix: treat autoAttachReferences as enabled by default, fix description

Address PR review feedback:
- Only skip instruction auto-attachment when autoAttachReferences is
  explicitly false, preserving existing behavior for session types that
  don't specify the flag.
- Update schema description to accurately reflect what the flag controls
  (instruction files only, not implicit context) and correct the default.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: update autoAttachReferences default to false and improve description

src/vs/workbench/contrib/chat/browser/chatSessions/chatSessions.contribution.ts

@@ -216,6 +216,11 @@ const extensionPoint = ExtensionsRegistry.registerExtensionPoint<IChatSessionsEx
 					description: localize('chatSessionsExtPoint.requiresCustomModels', 'When set, the chat session will show a filtered model picker that prefers custom models. This enables the use of standard model picker with contributed sessions.'),
 					type: 'boolean',
 					default: false
+				},
+				autoAttachReferences: {
+					description: localize('chatSessionsExtPoint.autoAttachReferences', 'Whether to automatically attach instruction files to chat requests for this session type.'),
+					type: 'boolean',
+					default: false
 				}
 			},
 			required: ['type', 'name', 'displayName', 'description'],

src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts

@@ -2714,6 +2714,11 @@ export class ChatWidget extends Disposable implements IChatWidget {
 	 * - instructions referenced in an already included instruction file
 	 */
 	private async _autoAttachInstructions({ attachedContext }: IChatRequestInputOptions): Promise<void> {
+		const contribution = this._lockedAgent ? this.chatSessionsService.getChatSessionContribution(this._lockedAgent.id) : undefined;
+		if (!contribution?.autoAttachReferences) {
+			this.logService.debug(`ChatWidget#_autoAttachInstructions: skipped, autoAttachReferences is disabled`);
+			return;
+		}
 		this.logService.debug(`ChatWidget#_autoAttachInstructions: prompt files are always enabled`);
 		const enabledTools = this.input.currentModeKind === ChatModeKind.Agent ? this.input.selectedToolsModel.userSelectedTools.get() : undefined;
 		const enabledSubAgents = this.input.currentModeKind === ChatModeKind.Agent ? this.input.currentModeObs.get().agents?.get() : undefined;

src/vs/workbench/contrib/chat/common/chatSessionsService.ts

@@ -94,6 +94,11 @@ export interface IChatSessionsExtensionPoint {
 	 */
 	readonly customAgentTarget?: Target;
 	readonly requiresCustomModels?: boolean;
+	/**
+	 * Decides whether to automatically attach instruction files to chat requests
+	 * for this session type. Defaults to false when not specified.
+	 */
+	readonly autoAttachReferences?: boolean;
 }
 
 export interface IChatSessionItem {