Solvency Invariant
A solvency invariant is a rule enforced in a protocol’s smart-contract code guaranteeing that its assets always cover its liabilities — for a yield protocol, that the backing held is never less than the tokens it has issued. Spield enforces a solvency invariant so its fixed rate can never promise more than the underlying Blend position actually earns.
A solvency invariant is a mathematical guarantee, checked on every state change, that a protocol remains fully backed. It is "solvent by construction" — the code reverts any action that would let issued value exceed real backing.
This is the antidote to the classic "fixed yield" failure mode, where a protocol quotes a rate it cannot actually back. Spield’s yield index is Blend’s real bRate, so the vault can never over-promise.