Stress-Test the Assumptions Behind a Financial Model

A financial model is only as credible as its assumptions, yet those assumptions usually get the least scrutiny — they're buried in an inputs tab and rarely challenged. This prompt acts as a critical reviewer, going line by line to judge whether each assumption is aggressive, conservative, or reasonable, and explaining the reasoning. The most useful output is the sensitivity ranking: which three assumptions the model's result hinges on. Most outputs are dominated by a small number of inputs, and knowing which ones lets you focus your validation effort where it actually matters instead of polishing assumptions that barely move the answer. It also catches internal inconsistencies — growth that doesn't square with headcount, margins that don't match the cost structure — which are the errors that quietly discredit a model in front of investors. It reasons qualitatively rather than inventing benchmark numbers, so use it to pressure-test and prioritize, then source-document the assumptions it flags before you share the model widely.

What you can expect back

Assumption review (qualitative):

- 12% MoM growth held flat for 36 months — aggressive. Sustaining a constant high growth rate over three years is unusual; rates typically decay as the base grows. Compounding makes this the single biggest swing factor.
- Gross margin 75% — reasonable for SaaS, though verify it's net of support and infrastructure at scale.
- Sales rep ramp 4 months — reasonable to slightly optimistic for a product moving from founder-led to a sales team; first reps often ramp slower.
- CAC flat at $1,200 — optimistic. CAC usually rises as you exhaust easy channels; holding it flat while scaling headcount is internally inconsistent with the growth curve.
- 2% monthly churn — reasonable if logo-based; confirm whether it's logo or revenue churn.

Most sensitive: (1) sustained growth rate, (2) churn, (3) CAC — small changes compound across 36 months.

Pressure-test first: the flat 12% growth and flat CAC, then the ramp. These carry the most output risk and the weakest stated basis.

Illustrative example — your results will vary by tool and inputs.

Get sharper results

• 01List assumptions with their exact values and basis if you have one — the reviewer can judge 'reasonable' far better when it knows where a number came from.
• 02Pay closest attention to the sensitivity ranking; fixing a barely-relevant assumption is wasted effort.
• 03Watch for the consistency flags — a flat CAC alongside aggressive growth is the kind of contradiction investors catch.
• 04After the review, document a source or rationale for each high-sensitivity assumption before the model leaves your hands.

Adapt it for your case