You'll notice
Creating an Artifact from a type_url now also rejects a stray deadline field
What
When an Artifact is created from a type_url (a type-based template), Claude Code already stripped out several fields that don't belong on that kind of creation, such as url, pr_review, capabilities, contract, lang, and force. Now deadline is rejected the same way.
Why
A type-based Artifact always gets its settings from its type, so a deadline value supplied alongside it would be meaningless or conflicting. Rejecting it keeps creation requests consistent and catches a mistake early instead of silently ignoring it.