Today, we released a small refresh to our 2.1 SDK and 5.1 runtime. This update fixes a series of bugs in those releases, which are described in the detailed release notes. If you already have the 2.1.150 SDK, you can install the update from here. If you are new to Service Fabric or upgrading from an earlier release, you should perform a full upgrade to get the latest Service Fabric tools, either for Visual Studio 2015 or Visual Studio "15" Preview.
The updated runtime (5.1.163) has been rolled out to all Azure clusters worldwide. If your cluster was not upgraded, it likely means that your cluster or one of your deployed applications is unhealthy or that it became unhealthy during the upgrade and was automatically rolled back. You can read more about the cluster upgrade process here.
Going forward, we intend to release new minor versions of the runtime, SDK, and tools every 2-3 months. These releases will include new platform and tooling features, potentially including additions to the API surface. Between these larger releases, you should watch out for smaller refreshes like this one that fix bugs reported and discovered after we've shipped.
The Service Fabric Team