In stateful agent harnesses, a state functions as a contract boundary governing phase completion, verification, and transition gating. While the state prompt defines the expected internal work of the phase, the runtime enforces explicit exit conditions before permitting a transition. An exit hook (out_hook) persists progress, updates receipts, or prepares artifacts for the subsequent phase, while a pre-transfer block (before_transfer) evaluates configured exit conditions. If a check fails, the run remains in the source state to enable inspection and repair rather than silently propagating unverified work or incomplete handoffs into later phases.