block_insert and block_set_attributes applied str() to whatever arrived, so a JSON null became the text None and a non-mapping failed after the INSERT was written. validate_attribute_values (backends/block_specs.py) runs first on both engines: strings pass the one-line rule, ints and finite floats are written as plain digits, everything else is a TypeError naming the tag. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd