Grounding an LLM in real tax data with a public MCP server

2Fin has launched a free public MCP server at taxmcp.ai2fin.com that grounds LLM tax queries in real data from 88 countries, with eight tools including tax rate lookup and income tax estimates. Every response includes the source authority and a dataVerifiedOn date, ensuring auditable outputs. The same engine powers the company's web calculators, preventing drift between the MCP answers and the site.

We all know the failure mode: ask an LLM for a tax rate and it'll hand you a confident number with zero provenance. Fine for a demo, useless for anything you'd ship. The interesting question is how you ground the model in a source of truth — and MCP has quietly made that a one-liner. Quick recap for anyone who missed it: Model Context Protocol is an open standard Anthropic, late 2024 for connecting an assistant to external tools and data. Client support is now broad — Claude Desktop, Claude Code, ChatGPT, Cursor, VS Code, Windsurf. 2Fin runs a free public MCP server for tax at taxmcp.ai2fin.com . No key, no auth. Point any MCP client at it: { "mcpServers": { "tax": { "url": "https://taxmcp.ai2fin.com" } } } You get eight tools: tax rate lookup , compute gst vat , income tax estimate , company tax estimate , cgt estimate , superannuation estimate , student loan repayment , compare countries — across 88 countries. Every response includes its source authority and a dataVerifiedOn date. That's the part most "AI + data" integrations skip, and it's exactly what makes the output auditable instead of vibes. A wrong or stale number is income tax estimate { country: "AU", income: 95000 } < { incomeTax: ..., medicareLevy: ..., takeHome: ..., source: "ATO — ato.gov.au", dataVerifiedOn: "2026-06-01" } The same engine backs their web calculators, so the MCP answers and the site can't drift apart — which, if you've ever maintained two copies of the same rate table, you'll appreciate. Good worked example of the pattern: don't make the model remember facts, give it a tool that fetches them with a citation attached. The tax domain just makes the stakes obvious — but the same design applies to any data your assistant has no business memorising. If you want to poke at it, it's free and needs no account: taxmcp.ai2fin.com https://taxmcp.ai2fin.com .