Contracts

Introduced in v0.1 (Milestone 3).

Every component declares a contract: its anatomy, variants, states, accessibility role, slots and events. The contract is a JSON block (comments allowed) and everything else, such as types, docs and accessibility tests, is derived from it.

{
  "name": "Button",
  "role": "button",
  "parts": ["root", "icon", "label"],
  "variants": {
    "tone": ["primary", "neutral", "danger"], // first value is the default
    "size": ["sm", "md", "lg"]
  },
  "states": ["hover", "focus-visible", "pressed", "disabled"],
  "behavior": "press",
  "slots": { "default": "label content", "icon": "optional leading icon" },
  "events": { "press": "fired on click, Enter, or Space" },
  "contrast": [{ "foreground": "label", "background": "root" }]
}

Field	Required	Meaning
name	yes	PascalCase; must equal the file name (Button.tess).
role	yes	An ARIA role.
parts	yes	Public class names. Must include root; each must appear as a class in the template.
variants	no	Name to allowed values. The first value is the default. Becomes typed props.
states, slots, events	no	Declared states, slot names and event names.
behavior	no	A headless behavior (press, toggle, disclosure). The contract must declare the parts and events it needs.
contrast	no	Foreground and background parts to check for contrast. Both must be declared parts.
description, since, deprecated, renamedFrom, forkedFrom	no	Docs and versioning.

Unknown fields are errors, with a suggestion for typos.

Errors

ERROR  T3002  Button.tess:5:3
  Unknown contract field "varients".
  Did you mean "variants"?
  Run `tessera explain T3002` for details.

Contract codes run from T3001 to T3013.

Editor support

packages/contract/contract.schema.json is a JSON Schema for the contract block, for autocomplete and inline errors. It cannot express every rule (name matches file, contrast parts exist, behavior requirements), so the compiler remains the source of truth.

Derived types

generateTypes(contract) produces ButtonProps (variants as unions of allowed values), ButtonSlots, ButtonEvents and ButtonPart.