The clawhdf5-tools interop tests compare h5rs with h5ls, h5stat, h5dump and h5diff, and CLAWHDF5_REQUIRE_INTEROP=1 turns a missing tool into a failure. Also hold clawhdf5-tools to the no-C-in-the-default-build check. Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>