h5py supports boolean masks for reads and writes; clawhdf5 supports neither, so a mask is an unsupported operation (NotImplementedError, as for every other edit the bindings cannot do), not an invalid key. Tests: test_unsupported_edits_are_clear_errors (1-D, N-D and per-axis mask writes, file unchanged) and test_boolean_masks_are_refused (reads). Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>