Structures spreadsheet models a second analyst can audit - separated input/calc/output sheets, one-formula-per-row hygiene, built-in error checks, and a documented cover sheet. Use when someone asks "how should I structure this model", "clean up this spreadsheet before the board sees it", "why does my model break when I insert a column", or is building any forecast or calculator others will review. Do NOT use for the SaaS revenue logic itself - use revenue-modeling instead; for a full FP&A operating model use fpa-model; for personal or department budgets use budget-builder; for cash timing use cash-flow-forecast; for GAAP statement construction use financial-statement-builder.
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name: Spreadsheet Model Builder
description: Structures spreadsheet models a second analyst can audit - separated input/calc/output sheets, one-formula-per-row hygiene, built-in error checks, and a documented cover sheet. Use when someone asks "how should I structure this model", "clean up this spreadsheet before the board sees it", "why does my model break when I insert a column", or is building any forecast or calculator others will review. Do NOT use for the SaaS revenue logic itself - use revenue-modeling instead; for a full FP&A operating model use fpa-model; for personal or department budgets use budget-builder; for cash timing use cash-flow-forecast; for GAAP statement construction use financial-statement-builder.
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# Spreadsheet Model Builder
A spreadsheet model is judged by whether a second analyst can verify it without a guided tour. The costly failure is not a wrong formula - it is a *hidden* wrong formula: a hardcoded number buried in cell F47, an inconsistent row that Ctrl+\ would have caught, a scenario someone ran by overtyping inputs and never restoring. This skill builds models where errors have nowhere to hide.
## Operating procedure
Structure comes before formulas: retrofitting sheet separation onto a finished tangle costs more than the model did.
### Step 1: gather inputs
- What question the model answers and who reviews it. A model only the builder reads can cut corners; a reviewed model cannot.
- The time axis: granularity (monthly/quarterly), horizon, and history vs forecast boundary.
- Every assumption the user already holds, each with a source. Anything unsourced gets written down anyway - labeled a guess in the note column, never silently blended with sourced numbers.
- How many scenarios are needed. Decide now: each scenario is a column on the Inputs sheet, never a separate file.
### Step 2: lay out the sheet structure
Separate sheets with single roles:
- **Inputs** (or Assumptions) - every editable variable lives here, and only here.… install to load the full skill