Mortgage Tools for AI Agents
Six WebMCP tools, live on every page, built by an FCA-authorised UK mortgage broker
Six tools that an AI agent can call directly from this website. UK stamp duty, bridging costs, the FCA high net worth test, and how more than forty named UK lenders treat contractor income, retained profit and one year of accounts. No sign-in, no charge, nothing collected.
Fox Davidson publishes six mortgage tools that AI agents can call directly from this website. An agent running in the ChatGPT desktop app's built-in browser, or in Chrome, can calculate UK stamp duty, cost a bridging loan, run the FCA high net worth test, and compare how more than forty named UK lenders treat contractor income, retained profit, one year's accounts and foreign currency pay. There is no sign-in, no charge, and no personal data is collected.
The tools
What can an AI agent do on this site?
Six tools are registered on every page of foxdavidson.co.uk using the WebMCP imperative API. An agent discovers them automatically when it opens the site. Three are calculation tools that mirror our public calculators. Three read our lender criteria reference and answer questions that no individual lender publishes.
Converts a contractor day rate into the assessed annual income each lender would actually use, sorted highest to lowest, with the borrowing that produces and a flag against any lender whose published minimum the contractor does not meet.
Looks up how named lenders treat one underwriting question: day rate annualisation, retained profit, one year's accounts, manual underwriting, bonus and commission, foreign currency income, income multiples, maximum age at end of term, or private bank thresholds.
Puts two to six lenders side by side across their published criteria and returns the topics where they take different positions. The differences are the point.
SDLT, LBTT and LTT across England, Northern Ireland, Scotland and Wales. Handles first-time buyer relief, the additional dwelling surcharge, the non-UK resident surcharge and the corporate flat rate, with a band by band breakdown.
Full cost of a UK bridging loan on rolled-up, retained or serviced interest, with gross facility, net advance, loan to value and total cost of finance.
Runs the FCA high net worth mortgage customer test on a single or joint application and returns the routing that follows from it.
The reasoning
Why does a mortgage broker publish tools for AI agents?
Because the comparison is the part nobody publishes. Every lender documents its own criteria on its own intermediary site, usually as a PDF, often behind a login. No lender publishes how it compares with the twenty lenders it competes against, and no lender ever will, because the comparison is rarely flattering.
So an agent asked a simple question, such as which lender will lend most against a £500 a day contract, has no good way to answer it. It has to find and read twenty separate criteria documents, and even then it will miss the conditions attached, which is where the answer usually lives. We hold that comparison already, because we cannot place a case without it. Publishing it as a callable tool costs us nothing and saves the agent twenty searches.
What we have found over the years is that the question contractors ask is always what they can borrow, and the honest answer is that it depends less on the day rate than on which lender is approached. Almost nobody believes that until they see the figures side by side.
Worked example
What does the contractor day rate tool actually show?
UK lenders annualise a contractor day rate over anything from 41 weeks to 52 weeks. The contract does not change. The contractor does not change. The assessed income changes by a quarter.
| Day rate | Lender basis | Weeks used | Assessed income | Borrowing at 4.5x |
|---|---|---|---|---|
| £500 | Coventry | 41 | £102,500 | £461,250 |
| £500 | Halifax, NatWest, Barclays, Skipton and others | 46 | £115,000 | £517,500 |
| £500 | Kensington, Bluestone, Vida | 48 | £120,000 | £540,000 |
| £500 | Nationwide umbrella route, Aldermore fixed term | 52 | £130,000 | £585,000 |
The number that matters
On an identical £500 a day contract, lender selection alone moves assessed income by £27,500, a swing of 26.8%, and indicative borrowing by £123,750 at a flat 4.5 times income. That is the difference between a purchase working and not working, and it has nothing to do with the borrower.
The tool returns all nineteen lenders in that dataset, sorted, with the published basis for each and the conditions attached. Ask it about a £180 a day contract and it also flags the three lenders whose published minimum income or minimum day rate that contractor fails, which is the kind of detail a page of prose tends to bury.
Using them
How do I use these tools?
In the ChatGPT desktop app
- Open the ChatGPT desktop app and choose View, then Browser, then Open Browser Tab.
- Load any page on foxdavidson.co.uk in that browser tab.
- Ask the agent a question, for example: list the site tools on this page, then work out what a £650 a day contractor could borrow.
- Approve the site when ChatGPT asks. The tools then run on the live page and return structured results.
In Chrome
- Use Chrome 149 or later. The tools are live on this domain under the WebMCP origin trial, so no flag is needed.
- For local testing without the trial, enable chrome://flags/#enable-webmcp-testing and restart.
- The Model Context Tool Inspector extension will list all six tools and let you call them by hand.
Under the bonnet
How is this built?
One script, loaded on every page, registering six tools through the WebMCP imperative API. Tools are registered on the top-level page rather than in an iframe, which is what ChatGPT's browser requires. Every tool declares a JSON schema for its inputs and carries a read-only annotation, because none of them writes anything, changes anything or touches an account.
// document.modelContext, with a navigator fallback for Chrome 149 to 151
mc.registerTool({
name: 'fd_contractor_day_rate',
description: 'Convert a UK contractor day rate into the assessed annual income each named lender would actually use...',
inputSchema: { type: 'object', properties: { day_rate_gbp: { type: 'number' } }, required: ['day_rate_gbp'] },
annotations: { readOnlyHint: true, untrustedContentHint: false },
execute: async (input) => { /* returns the full lender spread */ }
});
Every response carries a source block naming Fox Davidson, our FCA firm reference number 600427 and a link to the FCA register, so an agent quoting the figures can attribute them and a user can verify who produced them. The full source is public and MIT licensed at github.com/foxdavidson/property-mortgage-mcp.
The same calculation logic is also published as a Model Context Protocol server on npm and as a hosted endpoint, so an agent that is not in a browser can reach the same tools. WebMCP covers the case where the user and the agent are looking at the same page together. MCP covers everything else.
Limits
What these tools do not do?
They do not give advice. A tool result is a calculation or a criteria lookup, not a recommendation, and nothing here is a lending decision. They do not check affordability, which is a full assessment of income against committed expenditure and stress rates, not a multiple. They do not submit anything or collect anything.
Lender criteria also move constantly. The figures here were reviewed on 31 August 2026 and are as published by each lender at that date. Anything material to a real case should be confirmed on the case.
Questions
Frequently asked questions
What is WebMCP?
WebMCP is a proposed web standard that lets a website offer structured tools to an AI agent, so the agent calls a defined function instead of guessing its way around the page. It is in origin trial in Chrome from version 149, and the ChatGPT desktop app supports it in its built-in browser, where it is called site tools.
Do I need to install anything to use the Fox Davidson tools?
No. The tools are registered by the website itself. If you are using the ChatGPT desktop app's built-in browser or Chrome 149 or later, opening any page on foxdavidson.co.uk is enough for the agent to find them. There is no extension, no account and no charge.
Why do lenders annualise a contractor day rate differently?
There is no rule requiring a common basis, so each lender sets its own assumption about how many weeks a contractor works in a year. The published range across UK lenders runs from 41 weeks to 52 weeks. On a £500 a day contract that is the difference between £102,500 and £130,000 of assessed income, a swing of 26.8%.
Which lenders will use limited company retained profit?
Twenty-one lenders are held in the dataset with their published position on retained profit or share of net profit, including the conditions attached, such as whether every shareholder must be party to the mortgage. Metro Bank is the only one in that set that works from profit before taxation rather than after. Ask the fd_lender_criteria_lookup tool with the retained_profit topic for the current list.
Are the tools safe to let an agent call?
All six are read-only and declare that in their annotations. They perform a calculation or return published criteria. None of them writes data, changes an account, submits a form or takes payment. No personal information is required to call any of them.
Is a tool result the same as a mortgage offer?
No. Every result is indicative and carries that statement in the response. Figures are not a quote, an offer of finance or regulated advice. A lending decision follows a full application and underwriting on the specific case.
Can I use these tools without an AI agent?
Yes. The same calculations are published as free web calculators, and the lender criteria are published in full as a reference page. The tools exist so an agent can reach the same information without reading a page designed for a person.
Where does the lender data come from?
Each figure traces to a named lender's own current published intermediary criteria. Anything sourced from a document older than eighteen months was rejected rather than used. The dataset holds one hundred rows across nine underwriting topics and is reviewed on the same cycle as the criteria reference page.
Figures produced by these tools are indicative and are not a quote, an offer of finance, regulated advice or tax advice. Lender criteria and rates are subject to change and are correct at the date of review stated on this page. Always speak to your broker for current criteria on your specific case. Your home may be repossessed if you do not keep up repayments on your mortgage.
Complex income is what we do
If a lender has annualised your contract at 41 weeks and the purchase no longer works, the problem is usually the lender, not the contract. That is a five minute conversation.