fn.yml spec

A function is defined by a single fn.yml file. It is the source of truth for its interface and behavior.

name: classify
version: 3
model: claude-opus-4-8

input:
  ticket: file

output:
  category: string
  priority: enum[low, high]
  confidence: number

eval:
  dataset: ./tickets.jsonl
  threshold: 0.95
  runs: 5

Fields

Field	Type	Required	Description
`name`	string	yes	Function identifier, lowercase, URL-safe
`version`	integer	yes	Monotonic version number
`model`	string	yes	Target model id
`input`	map of typed fields	yes	Named, typed inputs
`output`	map of typed fields	yes	Named, typed outputs (the structured result)
`eval`	object	no	Dataset, threshold, and run count

Field types

Inputs and outputs are typed. Supported types:

Type	Notes
`string`	UTF-8 text
`number`	Floating point
`boolean`	`true` / `false`
`enum[a, b, ...]`	One of a fixed set of string values
`file`	A file reference (PDF, CSV, log, image)
`array[<type>]`	A list of any of the above

TypeScript types

The compiled function's I/O is also available as TypeScript, generated by fn types:

export interface ClassifyInput {
  ticket: File;
}

export interface ClassifyOutput {
  category: string;
  priority: "low" | "high";
  confidence: number;
}