The format in ten minutes

This page walks through a real file: the forecast workbook from the benchmark, as the writer produces it. The specification is the full definition.

File layout

Front matter declares the version. Each sheet gets a # Sheet: heading, and each rectangular region of the sheet a ## heading with its range. A # Names section closes the file.

---
cellprose: 0.3
---

# Sheet: Forecast

## A1:N5
…grid, then formulas and formats…

## A7:B10
…

# Names
…

Keep the Sheet: prefix

Without it, Haiku read # Report as a document title and invented a sheet named Sheet1.

Regions and tables

Each region is a compact table without outer pipes. The first column holds row numbers and the header row holds column letters, so every value has an address a model can cite.

#|A|B|C|D|E|F|G|H|I|J|K|L|M|N
-|-|-|-|-|-|-|-|-|-|-|-|-|-|-
1|Line|Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec|FY
2|Bookings|100000|104000|108160|112486|117000|121680|126547|131609|140000|145600|151424|157481|1515990
4|Headcount|40|42|44|46|48|50|52|54|56|58|60|62

The formulas block

Formulas live in a fenced formulas block after the table, never inline. Identical formulas shifted across cells collapse into one fill range, written with the formula of its top-left cell.

```formulas
C2:E2 = B2*(1+$B$7)
G2:I2 = F2*(1+$B$7)
K2:M2 = J2*(1+$B$7)
N2:N3 = SUM(B2:M2)
C4:G4 = B4+$B$10
I4:M4 = H4+$B$10
```

C2:E2 = B2*(1+$B$7) means C2 is B2*(1+$B$7), D2 is C2*(1+$B$7), E2 is D2*(1+$B$7): relative references move with the cell, $ references stay put.

A gap is a typed-in number

C2:E2 then G2:I2 means F2 is not a formula. Here it holds 117,000 where the formula would give 116,986. C4:G4 then I4:M4 means H4 is typed in too, even though it holds exactly the value its formula would give, which no values-only format can show.

Numbers and rounding

• Formula results are rounded to 6 significant figures. They are recomputed from the formulas on decode, so nothing is lost.
• Typed-in values are written exactly. They are the source data.
• Text that would read as something else gets a ' prefix: '2026-10-07 plan, 'TRUE.
• The serialization is canonical: the same workbook always produces the same bytes.

Formats and names

Number formats follow the formulas block, as ranges. Defined names go in a names block at the end.

```formats
B4:F4 = $#,##0
```

# Names

```names
Growth = Assumptions!$B$2
```

Design decisions

Question	Decision	Evidence
Formulas inline or in a block?	A block, with fill ranges	Inline cost Sonnet +47% tokens and dropped its accuracy to 94%
Column letters or the sheet's own labels?	Column letters	Labels added 20% tokens on Sonnet and gained no accuracy
Table style	Compact table, no outer pipes	Renders on GitHub; 24–38% fewer characters than v0.1, and E19 found no accuracy loss without the outer pipes
Formula result precision	6 significant figures	Lookups stayed at 100%
Records like TOON?	Rejected	Address accuracy fell to 43–50% on Haiku

The evidence for each is in the design notes.

Cheatsheet

Write	Means
cellprose: 0.3	Format version, in the front matter
# Sheet: Name	Start of a sheet
## A1:F9	A rectangular region
|2|Units|1200|	Row 2: A2 = Units, B2 = 1200
B4:E4 = B2*B3	B4 = B2B3, C4 = C2C3, … (filled right)
B4:F4 = $#,##0	Number format for a range
# Names	Defined names, at the end