The assistant and MCP apps are told to build pages from the site's blocks
(with section, grid, embed and video) and to use a custom HTML ("code")
block only when the user asks for it, or for something no other block can
do. create_page, set_page_blocks and add_block refuse a new code block
until the call passes "custom_html": true, which the assistant may only do
once the user has agreed. The "Custom code blocks" permission now says
it's granted to no role by default.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>