The Missing Contributor Class: A Fourth Contributor Taxonomy for AI-Assisted Commits

Git's credit system recognizes exactly three contributor classes: the author, the human co-author (via the Co-authored-by trailer), and the machine account (the [bot]-suffix convention). AI coding agents fit none of these. The 2025–2026 conflict over AI attribution — VS Code's reverted git.addAICoAuthor, the Linux kernel's nine-month adopt-to-deprecate arc for Assisted-by, GCC and QEMU on opposite sides — is not a fight about whether to attribute AI work. It is a fight about which existing class AI must be crammed into. Both available classes are dishonest: co-authorship overclaims human creative agency; the bot class underclaims it to nothing. This paper proposes a fourth contributor class, defined in the existing trailer-taxonomy tradition (Signed-off-by, Reviewed-by, Suggested-by, Co-authored-by), with a vendor-neutral grammar (Assisted-by: <name> [; mode=generated|edited|reviewed]), an opt-in consent model (default absent, never vendor default-on), and an observer rendering contract (never human-rendered, never human-accounted, never erased). The kernel's failed first attempt is post-mortemed to extract its design constraints: vendor-neutrality, non-gating, and non-competition with existing slots. Reference implementation runs in production on a 20-agent fleet across seven hosts since 2026-11-06; 16-case test suite; falsifiable 90-day adoption criteria included.