Extracts a visual contract from a reference URL and saves it as JSON at outputPath. Call this once per reference design, before check_implementation or diff_pixels can be used, since both need a saved contract to compare against. Pass screenshotDir to also capture reference screenshots, which diff_pixels requires later. Returns a summary: the element count per viewport, any extraction warnings, and the path the contract was written to.

Measures an implementation URL against a previously saved visual contract: box position and size, computed styles, hover and focus states, and pseudo-elements. Call this after building or changing a UI to verify it matches the reference without a human looking at it. Needs contractPath from a prior extract_contract call. Returns a formatted deviation table (capped at 40 rows, with a note on how many were left out) plus the numeric totals, the pass rate, any missing selectors, and ok, which is true only when nothing deviated and nothing is missing.

Renders the implementation URL and compares it pixel by pixel against the reference screenshot stored in the contract. Use this for a stricter visual check than check_implementation, after the structural check passes or when a subtle rendering difference is suspected. Requires the contract to have been extracted with screenshotDir set. Returns the percentage of differing pixels, the threshold it was compared against, ok, and the filesystem path of the generated diff image.

Reads a saved contract file and describes it without opening a browser or making any network request. Use this to inspect what a contract covers, for example before deciding whether diff_pixels is possible, since that needs screenshots to already exist. Returns the original source URL, when it was extracted, the viewports it covers, the element count per viewport, whether reference screenshots exist, and any warnings recorded at extraction time.