This usually means your EF Core version is too old, ExecuteDeleteAsync() was only added in EF Core 7. Check your Microsoft.EntityFrameworkCore.* package versions; if you're on 6.x or lower, you'll need to upgrade to at least 7.x for MassTransit 8.4.x outbox cleanup to work. Alternatively, downgrade MassTransit.EntityFrameworkCoreIntegration to match your EF Core version. Double-check your dependencies and make sure they're aligned.