Hopi MCP server: use the tools in any AI assistant

Hopi runs a remote MCP server so any AI assistant can use its calculators, converters and validators as tools. One address, no account, no key, nothing stored. This page is the human guide; the endpoint itself is https://mcp.hopi.co.uk/mcp. If you want the in-browser route instead, where an agent uses a Hopi page you already have open, see WebMCP tools for AI agents.

What it is

MCP, the Model Context Protocol, is the open standard that lets an assistant call outside tools. The assistant reads a list of tools, each with a name, a description and a precise input schema, and when your question needs one it sends the inputs and gets a structured answer back. Hopi's server exposes the same tool logic that runs on this site, so the assistant gets the 2026/27 tax bands, the exact conversion factors and the same worked answers a visitor would, instead of guessing from training data. Every result includes a source_url pointing at the tool's page here, so you can check the figure or change the inputs yourself.

Connect it

  • Claude (web or desktop): Settings, then Connectors, then Add custom connector. Name it Hopi, paste https://mcp.hopi.co.uk/mcp, leave authentication empty, save.
  • Claude Code: claude mcp add --transport http hopi https://mcp.hopi.co.uk/mcp
  • Cursor: Settings, MCP, Add new server, type HTTP, the same URL.
  • VS Code (Copilot agent mode): MCP: Add Server, choose HTTP, the same URL.
  • Windsurf, Cline, Zed and others: add an HTTP (Streamable HTTP) MCP server with the same URL.
  • ChatGPT: where your plan allows custom connectors, add the URL as a remote MCP connector.

Then just ask: "what is 20 per cent VAT on 1,250 pounds", "how many working days until 20 December", "what would I take home on 48,000 a year", "is this IBAN valid". The assistant picks the tool, calls it and cites the answer.

Calling it directly

Developers and agent frameworks can speak to the endpoint without any client software. It is stateless JSON-RPC over HTTPS, following the MCP Streamable HTTP transport. Two requests cover everything:

POST https://mcp.hopi.co.uk/mcp
{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}
POST https://mcp.hopi.co.uk/mcp
{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"hopi_vat_calculator","arguments":{"amount":1250,"mode":"add","rate":20}}}

The first returns every tool with its JSON Schema; the second runs one and returns both a readable text block and a structuredContent object. Tool names follow the site: hopi_ plus the tool's slug with hyphens as underscores. A plain browser visit to the endpoint returns a short JSON description rather than an error, and https://mcp.hopi.co.uk/tools lists names and descriptions without the protocol.

Smaller collections

Two hundred tool definitions is a lot for an assistant to hold in mind at once. If you only need one area, connect a collection instead; each is the same server filtered to one category, so nothing else changes:

  • https://mcp.hopi.co.uk/mcp/money tax, pay, VAT, stamp duty, loans, savings, currency
  • https://mcp.hopi.co.uk/mcp/date dates, working days, week numbers, time zones
  • https://mcp.hopi.co.uk/mcp/convert units, measurements, cooking and data sizes
  • https://mcp.hopi.co.uk/mcp/dev encoding, formatting, hashing, validators
  • https://mcp.hopi.co.uk/mcp/maths, /mcp/text, /mcp/colour, /mcp/health, /mcp/random, /mcp/crypto

Tool names are identical across collections, and the full endpoint always lists everything. Every tool also declares an output schema, and where a calculation has a known limit (bank holiday data covers 2026 and 2027; exchange rates refresh hourly) the result carries explicit complete and warnings fields so software reading the numbers is not misled.

What is served, and what is not

Around two hundred tools: money (take-home pay, National Insurance, VAT, stamp duty, mortgages, loans, savings, inflation, currency with live rates, day rates), dates (days between, working days with bank holidays, week numbers, time zones), units, health, text, colour, encoding, hashing, JWT decoding, and validators for IBANs and Bitcoin and Ethereum addresses. A few stay web-only on purpose:

  • The two tools that parse HTML and XML need a real browser.
  • The two AES tools use deliberately slow key stretching that exceeds what the server platform allows.
  • Tools that create or transform private keys, seed phrases, passwords, passphrases or secret shares are kept off the server, because anything a tool returns through an assistant passes through that assistant's conversation history. The same applies to tools that take a secret as input: the TOTP generator, JWT signer, HMAC generator and public-key message decrypter. Use those on the site, where they run only in your browser.

Privacy and limits

The server keeps nothing you send. Each call computes an answer from the inputs and forgets them. It sets no cookies and needs no login. It does count calls in Hopi's own cookieless analytics: the tool name, the name of the calling software and whether the call succeeded, so we can see what is useful and fix what fails. Inputs and outputs are never logged. Calls are limited to about 120 a minute per connection, which is far more than any assistant needs and enough to stop a runaway loop.

Remember that your assistant provider sees what you ask it, including the inputs it passes to tools. That is true of every MCP server; Hopi simply adds nothing to it.

Two routes, one set of tools

Hopi offers agents two ways in. The remote MCP server on this page works from anywhere and needs no browser. WebMCP works the other way round: when you already have a Hopi tool page open in a browser with an in-page agent, the page registers its tool with the browser and the agent can use it on the spot. Both routes run the same code and give the same answers. The full machine-readable catalogue, with every input schema, is at /webmcp.json.

Links