What belongs in a handoff file when one AI agent has to continue another agent's work?

A transcript contains too much noise, while a short summary can omit the exact files, links, and decisions needed to continue safely. A useful handoff might separate the objective, current state, authoritative files and assets, constraints, decisions with rationale, failed approaches, validation already completed, permissions, unresolved risks, and the next concrete action. It should also mark assumptions, identify outputs that may be overwritten, and preserve source links instead of copying unsupported claims into the summary. Which fields consistently prevent repeated work or version drift in multi-agent workflows? Is a human-readable Markdown file enough, or is structured JSON worth maintaining alongside it for status, dependencies, and machine validation?