This seam sits between planning a tranche and writing one task's brief. It exists because a role boundary is where work is lost: the planner does a deep technical pass, and without a contract its conclusions quietly fail to arrive.
What crosses — the planner's rationale for one task, written into that task's issue: what the task is and deliberately is not; why it is one task rather than three; every project and shared-package consumer in its blast radius; why each dependency and conflict edge exists; the traps the dig already found; the class of agent the work needs; when the executing agent must stop rather than improvise; and the documents this work will make incoherent. Each has exactly one named home in the brief that consumes it, so a conclusion cannot arrive without a place to land.
The hand-off is malformed when — a rationale is missing any of those parts, in which case the planner does not emit it; or when a brief drops one on the floor, in which case the brief is wrong rather than merely thin. It is malformed too when the rationale carries detail that cannot survive the wait — exact signatures, precise file lists — which goes stale between planning and dispatch; re-deriving that fresh is the brief author's half. And when the brief author's own dig contradicts the rationale, that is escalated back, never silently overridden.
What it does not carry — the brief itself, which is written later and lives elsewhere; any statement of status, which is derived from the forge and never written down; scheduling or estimates, which belong to whatever tool plans the roadmap; and the freshly pinned facts a brief asserts about current code, which belong entirely to the next seam.
How it physically runs — the carrier is the task's issue body. The planner writes the rationale there in a fixed grammar, readable by a person and checkable by a machine alike; an issue whose body does not carry every part is refused at creation, and the same check re-runs against issues already open. The brief author reads it there and writes the brief, which lands in the pull-request body at dispatch. The issue holds the reasoning; the pull request holds the instruction.