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

#mcp #ai #api #tutorial

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.

A free public server to try it against

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" }
  }
}
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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.

The design choice worth stealing

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 visible rather than hidden inside a confident sentence.

> income_tax_estimate { country: "AU", income: 95000 }
< { incomeTax: ..., medicareLevy: ..., takeHome: ...,
    source: "ATO — ato.gov.au", dataVerifiedOn: "2026-06-01" }
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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.

The takeaway

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.

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