Spreadsheet Audit Skill

Spreadsheets fail silently: the SUM that stops at row 40 while data runs to 60, the hardcoded 1.1 someone typed over a formula in March, the column that's monthly in one section and annual in another. The sheet still displays numbers — wrong ones, confidently. This skill audits the way inherited models deserve: hunt the classic error classes, map what breaks when the sheet grows, and rank findings by damage-if-wrong — because the audit's job is protecting the decision the sheet feeds, not achieving cosmetic tidiness.

What This Skill Produces

• The findings, ranked by damage — each: location, what's wrong, what it's currently mis-stating
• The fragility map — the formulas that break on the next added row/column, before they do
• The verified/suspect ledger — which outputs were traced clean and which remain unverified (unverified ≠ wrong; the label is the honesty)
• The fix list — ordered, with the make-it-robust upgrades (structured ranges, input isolation) where they matter

Required Inputs

Ask for these if not provided:

• The sheet — the file, or its formulas/structure described; audits work on the actual formulas, not the values screenshot
• The stakes — what decisions this sheet feeds (a budget approval? pricing? a board number?) — depth and ranking follow the damage potential
• The lineage — author available? Inherited from a departed colleague? Known past incidents? Inherited orphans get the deeper hardcode-hunt
• The growth pattern — does data get appended? The fragility map keys on how the sheet evolves

Framework: The Hunt Rules

1. Hardcode hunt first: values typed over formulas are the deadliest class — invisible, intentional-once, wrong-forever. Scan for constants where columns are otherwise formulaic (inconsistent-formula warnings, or eyeball the pattern breaks). Every hardcode found gets asked: override or accident?
2. Range-edge check on every aggregate: SUMs/AVERAGEs/LOOKUPs vs. the data's actual extent — the stops-at-row-40 error. The robust fix where growth is real: full-column ranges or tables/structured references, so appended rows join automatically.
3. Unit and time-grain consistency: monthly-vs-annual mixes, currencies, thousands-vs-units — checked at every junction where sections meet. The tell is a ratio that's ~12× or ~1000× off; the fix is a stated grain per section, labeled in headers.
4. Trace the load-bearing outputs: the 3–5 numbers the sheet exists to produce get full precedent-traces (follow every input to its source). Everything else gets the class-level checks — tracing everything is how audits never finish; tracing nothing is how meetings get corrected from the floor.
5. Rank by damage, report honestly: a $2 rounding artifact and a double-counted revenue line are different findings; the report leads with what changes decisions. The verified/suspect ledger states what the audit did not cover — an audit that implies total coverage it didn't do is itself an error class.

Output Format

Spreadsheet Audit: [sheet] — feeds: [the decision]

Findings (damage-ranked)

Fragility Map

[What breaks on the next row/column · the structured-range upgrades worth making]

Verified / Suspect Ledger

[Outputs traced clean: … · checked at class level: … · not covered: … — labeled, not implied]

Fix Order

[Damage-first, with the two structural upgrades (input isolation, structured ranges) if the sheet has a future]

Quality Checks

• The hardcode hunt ran across all formulaic columns
• Every aggregate was checked against the data's true extent
• Load-bearing outputs got full traces; the ledger says which
• Findings are ranked by decision-damage, not discovery order
• Coverage limits are stated — no implied total audit

Anti-Patterns

• Do not audit the displayed values — the formulas are the sheet; the display is its costume
• Do not fix silently while auditing — findings first, fixes as their own reviewed pass
• Do not treat every finding as a crisis — the $2 artifact and the double-count get different fonts
• Do not imply coverage you didn't do — the suspect ledger is the audit's integrity
• Do not leave growth-fragile ranges unflagged in a sheet that grows — today's clean audit is next month's row-41 error