diff --git a/backend/src/app.ts b/backend/src/app.ts
index 25e4ff51..fdc8ac35 100644
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -99,7 +99,19 @@ export function buildApp(config: AppConfig, deps?: BuildAppDeps): FastifyInstanc
   };
   const getSettings = async (principal: PrincipalContext): Promise<Settings> => {
     if (deps?.getSettings) return await deps.getSettings(principal);
-    return effectiveSettings(config, loadSettings(config));
+    const stored = loadSettings(config);
+    const effective = effectiveSettings(config, stored);
+    // In the registry system the legacy `harness/workspaces/*.yaml` default is obsolete: when no
+    // installation workspace is pinned, default to the first active registry workspace.
+    if (!stored.workspace) {
+      try {
+        const revisions = await workspaceRegistry.list();
+        if (revisions.length > 0) effective.workspace = revisions[0].id;
+      } catch {
+        // Registry not bootstrapped yet; keep the legacy fallback.
+      }
+    }
+    return effective;
   };
   const piManagement = deps?.piManagement ?? createPiManagement(config, { listModels });