Dimension::size of an unlimited dimension was its dimension scale's extent, which netCDF-C leaves at 0, so it read 0 for a dimension holding records. It is now what netCDF-C reports (nc4_find_dim_len): the largest current extent of the variables using it in any group, found through the scale's REFERENCE_LIST, a coordinate variable's own extent included; 0 when nothing has been written. interop_tests::unlimited_dimension_lengths_match_netcdf4_python compares with netCDF4-python (variables of different lengths, one in a subgroup, an unwritten dimension, a coordinate variable shorter than another variable on its dimension, a subgroup's own dimension); before the fix it got time 0/6, rec 3/5, srec 0/1. The known-issues entry moves to Fixed; the crate README's warning goes. A new open entry records a related bug found meanwhile: variables' dimensions are matched by size, not DIMENSION_LIST. Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>