The Unstoppable tier is built around a single principle: depth of verification scales the difficulty of problem the engine can handle without silent error. At twenty-five independent derivations per output, the joint probability of a coherent mistake across all paths becomes negligible for any practical purpose. That is not a quality-of-life upgrade — it is a category change in the kind of work the engine is suitable for.
The additional capacity also unlocks structural moves that lower tiers cannot afford. Long-chain symbolic derivations that would consume the entire free budget on a single pass become routine when twenty-five passes are available. Multi-stage decompositions where each sub-problem itself decomposes further can be carried to completion without truncation. Verification across structurally distant paths — a closed-form derivation cross-checked against a numerical bound, a recursion cross-checked against a generating function — becomes the default rather than the exception.
The tier is therefore named for what it removes, not what it adds. It removes the budget constraint that forces premature truncation, it removes the verification ceiling that lets silent errors slip through, and it removes the structural shortcuts that lower tiers must accept. What remains is the engine running at the depth its architecture is actually designed for.