/number-check

---
name: number-check
description: >-
  Tie every number in an AI-written draft back to the file it came from
  (spreadsheet, PDF, export, statement) and recompute every derived figure
  (totals, growth rates, margins, averages, shares) before the draft leaves
  your desk. Returns a line-by-line tie-out table and a corrected draft. Use
  this skill whenever the user asks to "check the numbers", "tie this out",
  "reconcile", "does this add up", "verify the figures", or is about to send
  a report, board pack, budget, investor update, valuation, invoice summary
  or client email that contains figures Claude produced or touched. ALSO use
  it proactively, without being asked, at the end of any task where Claude
  wrote numbers from source files. Do NOT use for checking public facts
  against the web (that is a fact-check, not a tie-out), for forecasts the
  user explicitly labels as guesses, or for drafts with no numbers.
---

# Number check

In finance work one wrong figure costs more than the whole draft saved. A
reader who finds a single total that does not add up stops trusting every
other number on the page, and in a board pack, an audit file or a client
email that loss of trust has a price. The danger is that a wrong AI number
looks exactly like a right one: same font, same decimals, same confident
sentence around it. Language models are good at writing about numbers and
unreliable at carrying them. They round, transpose digits, mix up periods,
average averages and invent a plausible figure when a cell is empty.

So this skill never asks "does this look right?". It asks "where did this
come from, and can I get the same number again?". Every figure either ties
to a source or gets flagged.

## Workflow

### 1. List every number

Read the draft and extract every figure: amounts, percentages, counts,
dates, ratios, ranks ("our third-largest client"), and comparisons ("up
12% on last year"). Number them in order and keep the exact wording,
because "€1.2m" and "€1,248,300" are different claims at different
precision. Words that hide numbers count too: "doubled", "most", "half",
"record", "flat".

### 2. Sort each number into one of three types

- **Copied**: should appear as-is in a source (revenue for March, headcount
  at year end). Check: find the cell, page or line.
- **Derived**: calculated from copied numbers (growth, margin, share,
  average, total). Check: recompute it yourself from the sources, never
  from other numbers in the draft.
- **Stated**: given by the user in the chat or a brief, not in any file.
  Check: mark it as user-supplied, do not "verify" it.

### 3. Tie out against the source

For copied numbers, quote the location precisely: file name, sheet, cell
or row label, page number. If you cannot open the file, say so and stop
for that number. Do not reconstruct it from memory of the conversation.

For derived numbers, write the calculation out in full:

```
Gross margin Q3 = (Revenue 412,900 - COGS 268,400) / Revenue 412,900 = 35.0%
Draft says 37%. MISMATCH.
```

Watch the traps that catch AI most often:

1. **Period mix-ups**: Q3 vs year-to-date, fiscal vs calendar year, month
   end vs month average.
2. **Sign errors**: costs entered as negatives, "decrease of -5%".
3. **Averaging averages**: the average of three monthly margins is not the
   quarterly margin. Recompute from totals.
4. **Percent vs percentage points**: a rate going from 4% to 5% rose one
   point, or 25%. Not "1%".
5. **Units and scale**: thousands vs millions, gross vs net, with or
   without VAT, EUR vs USD.
6. **Rounding drift**: rounded parts that no longer sum to the rounded
   total. Say which figure is rounded and keep the total honest.
7. **Base effects**: growth computed on the wrong base year.

### 4. Give each number one verdict

Use exactly these five, because each one needs a different fix:

- **TIES**: matches the source or the recomputation. Show where.
- **ROUNDING**: right number, presentation differs (37.4% shown as 37%).
  Fine unless parts no longer add up.
- **MISMATCH**: the source or the recomputation gives a different value.
  Show both and the correct wording.
- **NO SOURCE**: not found in any file provided. Not false, but the user
  cannot defend it if asked. Recommend sourcing or cutting it.
- **USER-SUPPLIED**: came from the user, not a file. Listed so they know
  it was not checked.

### 5. Report, then repair

Return the report in this format:

```
## Number check
Checked N figures: X tie, Y mismatch, Z no source, W user-supplied.

| # | Draft says | Type | Source / calculation | Verdict |
|---|---|---|---|---|
| 1 | Revenue €412.9k in Q3 | Copied | P&L.xlsx, sheet Q3, cell C14 | TIES |
| 2 | Margin 37% | Derived | (412.9 - 268.4) / 412.9 = 35.0% | MISMATCH, say 35% |
```

Put every MISMATCH at the top of the table, so the user sees the problems
before the passes. Then offer a corrected draft. When you correct, change
the numbers and the words that depend on them ("strong margin growth" may
no longer be true) and nothing else. Do not restyle the draft: a number
fix mixed with a rewrite makes the change impossible to review.

## Judgment rules

**A clean result has to be earned.** If every figure ties on the first pass
in a draft with more than ten numbers, recompute the three largest derived
figures a second way (from a different total, or bottom-up instead of
top-down). Zero mismatches is possible. It should not be the default.

**Never fix a number by reasoning toward the draft.** If your recomputation
disagrees with the draft, the draft is wrong until a source says otherwise.
Do not hunt for an interpretation that makes the draft's figure right.

**Show the arithmetic, every time.** A verdict without the calculation is
an opinion. The user must be able to redo any line in ten seconds.

**Say what you could not open.** A password-protected file, a scanned PDF
you cannot read, a link you cannot reach: name it, and mark every number
that depended on it as NO SOURCE.

**Stay out of judgment calls.** Whether a cost is "one-off" or whether a
forecast is reasonable is analysis, not a tie-out. Flag it as a question
for the user if it matters, but do not give it a verdict.

## Example

Draft line: "Q3 revenue grew 18% to €412.9k, and our top client now makes
up a third of sales."

Four figures hide in one sentence: the Q3 revenue (copied), the 18% growth
(derived), the client share (derived) and "top client" (a ranking). The
P&L shows Q3 €412.9k (TIES) and Q3 last year €361.2k, so growth is
(412.9 - 361.2) / 361.2 = 14.3% (MISMATCH: the draft compared against Q2,
not Q3 last year). The client ledger shows €118.0k from the largest client:
118.0 / 412.9 = 28.6%, "more than a quarter", not "a third" (MISMATCH).
The ranking ties. One sentence, two errors, and both would have gone to the
board looking exactly like the truth.