What
Artifact pages can now share a per-tab presence object (what viewers are doing in the page) and Claude sees it, after aggressive sanitizing. Cursor and identity fields are stripped, and the block is framed in the prompt as data produced by the page, not as instructions from your user.
Details
cursorandwhoare removed outright.- Rejected: prototype-polluting keys, non-finite numbers, lone surrogates, private-use code points and most format/control code points.
- Hard caps on nesting depth, array length, key count, and a total size ceiling of 12288 bytes.
Evidence
Each presence object above is data the artifact page's own code shares with everyone viewing it, produced in your user's browser and not typed by them as a message
Strings lifted out of the shipped bundle, so the claim above can be checked against them.