Adding UK Corporation Tax calculations to your AI assistant with a free MCP server

Ask a general-purpose AI assistant when a UK company's Corporation Tax is due and you'll usually get a confident answer. Sometimes it's right. Tax dates and rates are rule-based, though, so they're a good fit for a deterministic tool the model can call instead of guessing.

This post shows how to connect an assistant to a free, public Model Context Protocol (MCP) server that does exactly that for UK limited companies, and what you get back when you call it directly with curl.

The Model Context Protocol is an open protocol that lets AI clients (Claude, ChatGPT in developer mode, Cursor, VS Code and others) discover and call tools hosted by a server. A remote MCP server is just an HTTPS endpoint that speaks JSON-RPC 2.0:

The client reads the tool descriptions and schemas, and the model decides when a user's question needs one. You write no glue code.

Taxley, UK Corporation Tax software from Taxley UK LLP, runs a public MCP server at:

https://taxley.co.uk/mcp Enter fullscreen mode Exit fullscreen mode According to its developer docs, it:

The same calculators are also available as a plain JSON API and as website embeds, but MCP is the interesting bit here.

Most MCP clients that support remote servers accept a config like this (the exact file and key names vary by client):

{ "mcpServers": { "taxley": { "url": "https://taxley.co.uk/mcp" } } } Enter fullscreen mode Exit fullscreen mode No headers and no auth block are needed. In Claude, you can add it through Settings → Connectors → Add custom connector: paste the URL and leave the authentication settings empty. In ChatGPT, developer mode lets you create a connector with the same URL and no authentication.

Then ask something like: "When is Corporation Tax due for a year ending 31 March 2026?"

Since it's plain JSON-RPC over HTTPS, you can explore it without any client. Here's initialize:

curl -s https://taxley.co.uk/mcp \ -H "Content-Type: application/json" \ -H "Accept: application/json, text/event-stream" \ -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"curl","version":"1"}}}' Enter fullscreen mode Exit fullscreen mode { "jsonrpc": "2.0", "id": 1, "result": { "protocolVersion": "2025-06-18", "capabilities": { "tools": { "listChanged": false } }, "serverInfo": { "name": "taxley", "title": "Taxley — UK Corporation Tax tools", "version": "1.0.0", "websiteUrl": "https://taxley.co.uk/developers" }, "instructions": "Free calculators for UK limited companies... The results are general guidance, not tax advice." } } Enter fullscreen mode Exit fullscreen mode And tools/list:

curl -s https://taxley.co.uk/mcp \ -H "Content-Type: application/json" \ -H "Accept: application/json, text/event-stream" \ -d '{"jsonrpc":"2.0","id":2,"method":"tools/list"}' Enter fullscreen mode Exit fullscreen mode The response lists 17 tools. Trimmed to one entry:

{ "jsonrpc": "2.0", "id": 2, "result": { "tools": [ { "name": "corporation_tax_deadlines", "title": "Corporation Tax deadline calculator", "description": "Works out a UK private limited company's Corporation Tax payment date, CT600 filing deadline and Companies House accounts deadline from its accounting period....", "inputSchema": { "type": "object", "properties": { "period_end": { "type": "string", "format": "date", "...": "..." }, "period_start": { "type": "string", "format": "date", "...": "..." } }, "required": [ "period_end" ], "additionalProperties": false }, "annotations": { "readOnlyHint": true, "destructiveHint": false, "idempotentHint": true, "openWorldHint": false } } ] } } Enter fullscreen mode Exit fullscreen mode Calling it:

curl -s https://taxley.co.uk/mcp \ -H "Content-Type: application/json" \ -H "Accept: application/json, text/event-stream" \ -d '{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"corporation_tax_deadlines","arguments":{"period_start":"2025-04-01","period_end":"2026-03-31"}}}' Enter fullscreen mode Exit fullscreen mode The text content comes back as:

For accounts from 1 April 2025 to 31 March 2026, pay Corporation Tax by 1 January 2027 and file the CT600 by 31 March 2027. A private limited company must also deliver these accounts to Companies House by 31 December 2026.... Enter fullscreen mode Exit fullscreen mode The structuredContent object has answer, figures, data, notes, sources, page_url, share_url and disclaimer, so your own code can use the dates directly, and the model can point the user to the share_url to check the working.

The five you'll probably reach for first:

The rest cover chargeable gains with indexation, the micro-entity/small company size test, long accounting periods, company car and van benefits, mileage allowance, home office allowance, trivial benefits, dividend reserves, salary vs dividends, sole trader vs limited company, and pension contributions. Run tools/list for the full schemas.

If you're building anything for UK small businesses, such as a bookkeeping bot, an internal finance assistant or a founder's Slack helper, wiring in deterministic tax tools like these is a cheap way to cut down on hallucinated dates and rates.