The FleCSI 2.0 programming system supports multiphysics application development through a runtime abstraction layer, and by providing core topology types that can be customized for specific nu-merical methods. The abstraction layer provides a single-source programming interface for distributed and shared-memory data parallelism through task and kernel execution, and has been demonstrated to introduce virtually no runtime overhead. FleCSI’s core topology types represent a rich set of basic data structures that can be specialized to create application-facing interfaces for a variety of different physics packages. Using the FleCSI control and data models, it is straightforward to compose multiple packages to create full multiphysics applications. When used with the Legion backend, FleCSI offers extended runtime analysis that can increase task concurrency, facilitate load balancing, and allow for portability across heterogeneous computing architectures.