# What Changed in This Law?

> Compare two versions of a law or regulation section by section, with changed numbers and deadlines. Paste (or link) the old and new versions of a law, regulation, terms of service or policy. The text is split at §, Art. and Section headings, sections are aligned by their number, and every added, removed or modified section is shown with a word-level diff and the numbers, durations and obligation words (must/may/muss/kann…) that changed.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://whatchangedinthislaw.usefulweb.site/ (interactive tool at https://whatchangedinthislaw.usefulweb.site/tool)
- JSON API: POST https://whatchangedinthislaw.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://whatchangedinthislaw.usefulweb.site/api/v1/tool)
- OpenAPI: https://whatchangedinthislaw.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://whatchangedinthislaw.usefulweb.site/mcp - tool `what_changed_in_this_law`

## Inputs
- oldText (textarea): Old version (text)
- newText (textarea): New version (text)
- oldUrl (url): Or: URL of the old version - Official source (e.g. gesetze-im-internet.de, eur-lex.europa.eu, legislation.gov.uk).
- newUrl (url): Or: URL of the new version
- citation (text): Citation / amending act (optional)
- effectiveDate (date): Effective date of the new version (optional)

## Method
- Sections start at lines beginning with §, Art./Artikel/Article, Section/Sec. or Paragraph followed by a number; text before the first heading is the preamble.
- Sections are aligned by their normalised number (LCS over section ids), so renumbered sections show as removed + added.
- Inside a changed section a word-level LCS diff is computed; numbers, money amounts, percentages, durations and obligation words (must/shall/may, muss/kann/soll/darf…) present in only one version are listed.
- Whitespace-only differences are ignored.

## Limits
- Not legal advice. This is an automated, rule-based text analysis that can miss or misread clauses; it never claims completeness. For decisions with legal or financial consequences, ask a qualified professional (e.g. a lawyer, tax adviser or consumer advice centre). Always check the official consolidated text and the amending act for transitional provisions.