A riskiest assumption test isolates the single belief your product most depends on — the one that kills the idea if false — and tests it as cheaply as possible. Where an MVP tests a product, a RAT tests one load-bearing assumption.
List what must be true for the product to work, then rank by impact-if-wrong times uncertainty.
Most killer assumptions can be tested without a product: a landing page for demand, a pre-order for pricing, a concierge run for the workflow, a technical spike for feasibility.
Practical answers
A RAT is narrower: one assumption, one test, days not weeks. Several passed RATs are what justify spending on the MVP.
That's the test working — you just saved the build budget. Reshape the idea around what you learned and test the next version's riskiest belief.
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