diff --git a/src/vs/workbench/browser/layout.ts b/src/vs/workbench/browser/layout.ts
index c517d4fbd29de..7d0fcca60ea05 100644
--- a/src/vs/workbench/browser/layout.ts
+++ b/src/vs/workbench/browser/layout.ts
@@ -3050,7 +3050,18 @@ class LayoutStateModel extends Disposable {
 	}
 
 	private loadKeyFromStorage<T extends StorageKeyType>(key: WorkbenchLayoutStateKey<T>): T | undefined {
-		const value = this.storageService.get(`${LayoutStateModel.STORAGE_PREFIX}${key.name}`, key.scope);
+		let value = this.storageService.get(`${LayoutStateModel.STORAGE_PREFIX}${key.name}`, key.scope);
+
+		// TODO@bpasero remove this code in 1y when "pre-AI" workspaces have migrated
+		// Refs: https://github.com/microsoft/vscode-internalbacklog/issues/6168
+		if (
+			key.scope === StorageScope.WORKSPACE &&
+			key.name === LayoutStateKeys.AUXILIARYBAR_HIDDEN.name &&
+			this.configurationService.getValue('workbench.secondarySideBar.enableDefaultVisibilityInOldWorkspace') === true &&
+			this.storageService.get('workbench.panel.chat.numberOfVisibleViews', StorageScope.WORKSPACE) === undefined
+		) {
+			value = undefined;
+		}
 
 		if (value !== undefined) {
 			this.isNew[key.scope] = false; // remember that we had previous state for this scope
diff --git a/src/vs/workbench/browser/workbench.contribution.ts b/src/vs/workbench/browser/workbench.contribution.ts
index a971e7e18c79f..d87011805bb13 100644
--- a/src/vs/workbench/browser/workbench.contribution.ts
+++ b/src/vs/workbench/browser/workbench.contribution.ts
@@ -568,6 +568,15 @@ const registry = Registry.as<IConfigurationRegistry>(ConfigurationExtensions.Con
 					localize('workbench.secondarySideBar.defaultVisibility.maximized', "The secondary side bar is visible and maximized by default.")
 				]
 			},
+			'workbench.secondarySideBar.enableDefaultVisibilityInOldWorkspace': {
+				'type': 'boolean',
+				'default': false,
+				'description': localize('enableDefaultVisibilityInOldWorkspace', "Enables the default secondary sidebar visibility in older workspaces before we had default visibility support."),
+				'tags': ['advanced'],
+				'experiment': {
+					'mode': 'auto'
+				}
+			},
 			'workbench.secondarySideBar.showLabels': {
 				'type': 'boolean',
 				'default': true,