When should a startup rewrite its prototype or MVP from scratch?

A full rewrite is tempting whenever the code feels messy, but it is one of the riskiest decisions you can make. Consider a rewrite only when the current system blocks almost every change, or when a hard technical requirement cannot be met with the existing architecture. Even then, prefer an incremental approach where you gradually replace critical parts behind stable interfaces while keeping the product running. Rewrites often take longer than expected and freeze feature delivery, which can hurt traction and morale. If the main problem is inconsistent patterns or lack of tests, a focused refactor is usually safer than throwing everything away.