localecheck

Locale-correctness tools for AI agents. Agents are systematically wrong about everyday non-US data — they read 03/04 as March 4 in the UK, apply today's VAT rate to a historical invoice, format €1,234.50 for a German reader, and miss bank holidays in delivery dates. localecheck gives them verified answers and flags impossible inputs instead of guessing.

Coverage: UK + USA (v1), expanding. Free, no API key.

Packages

	Package	Install	Use
TypeScript (primary)	packages/localecheck-ts	npx -y localecheck (live on npm)	MCP server and importable library
Python	packages/localecheck-python	from source (PyPI release coming soon)	MCP server, importable library, REST API

Both wrap the same authoritative libraries (libphonenumber, CLDR/Intl, date-holidays) and share the same hand-curated tax data.

What it does

parse_date · validate_phone · format_currency · tax_rate (VAT/sales-tax by date) · is_holiday / next_holiday · parse_address

Quick start (in an AI app)

{ "mcpServers": { "localecheck": { "command": "npx", "args": ["-y", "localecheck"] } } }

Then ask naturally — the agent picks the tool: "A UK invoice dated 1 June 2009 — what VAT rate?" → 15%.

Repo map

• packages/localecheck-ts — primary TypeScript package (MCP + library)
• packages/localecheck-python — Python package + FastAPI REST API
• eval/ — benchmark harness: how often raw LLMs get this wrong
• benchmark.html — shareable results page · BUSINESS_MODEL.md — strategy · PUBLISH.md — release steps · LOCAL_TEST.md — test before publishing

Status & caveats

UK VAT history is reliable for the modern era (standard rate). US state sales-tax values are 2025 state base rates (local taxes add on top) — verify against an authoritative feed before live invoicing. Addresses parse + extract a valid postcode/ZIP but don't verify deliverability.

Code: Apache-2.0. Curated tax data: see BUSINESS_MODEL.md.