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Taxley tools for developers and AI assistants

Taxley's free UK Corporation Tax calculators work inside AI assistants through an MCP server, in your own code through a JSON API, and on your website as an embed. All three give the same answer as the calculator pages, need no key and link back to the result on taxley.co.uk.

https://taxley.co.uk/mcp

Which tools can AI assistants and apps call?

Every calculator that doesn't need a file upload can be called through the MCP server, the JSON API and an embed. The list below comes straight from the same registry the pages use, with each tool's MCP name, what it works out and an example input you can copy.

Corporation Tax calculator

Estimates the Corporation Tax a UK limited company pays on its taxable profits for one accounting period of up to 12 months. Applies the small profits rate, the main rate and marginal relief, with associated companies, short periods and periods that straddle 1 April. A close investment-holding company pays the main rate on all its profits. Returns the tax, the effective rate and the payment and filing dates.

  • MCP tool name: uk_corporation_tax_estimate
  • API: https://taxley.co.uk/api/v1/tools/corporation-tax
  • Page: Corporation Tax calculator

Example input:

{
    "period_start": "2025-04-01",
    "period_end": "2026-03-31",
    "taxable_profits": 100000,
    "associated_companies": 0
}

Corporation Tax deadline calculator

Works out a UK private limited company's Corporation Tax payment date, CT600 filing deadline and Companies House accounts deadline from its accounting period. Accounts longer than 12 months are split into two Corporation Tax periods, each with its own payment date.

Example input:

{
    "period_end": "2026-03-31",
    "period_start": "2025-04-01"
}

Corporation Tax late filing penalty and interest calculator

Works out HMRC's penalties for filing a UK company's Company Tax Return (CT600) late — the flat £200/£400 penalties, doubled for filing deadlines from 1 April 2026, and the 10% or 20% tax-geared penalty — plus late payment interest on the Corporation Tax at HMRC's published rates.

Example input:

{
    "period_end": "2025-04-30",
    "filed_on": "2026-08-15",
    "tax_due": 10000,
    "paid_on": "2026-08-01",
    "late_previous_two": false
}

Director's loan s.455 tax calculator

Works out the section 455 tax a UK close company owes on a director's or shareholder's loan still owed at the end of its accounting period, the relief for any part repaid within 9 months, the net tax and its due date. Uses the rate for the date the loan was made, including 35.75% from 6 April 2026.

Example input:

{
    "period_end": "2026-03-31",
    "outstanding": 25000,
    "repaid_within_9_months": 10000
}

Capital allowances calculator for companies

Works out a UK limited company's capital allowances for one accounting period: the annual investment allowance, full expensing, the 50% and zero-emission car first-year allowances, and writing-down allowances at 14% (18% before 1 April 2026, a hybrid rate for a period spanning that date) and 6%. It also shows balancing charges and the pools carried forward.

Example input:

{
    "period_start": "2026-04-01",
    "period_end": "2027-03-31",
    "new_main_plant": 40000,
    "used_main_plant": 15000,
    "main_pool_brought_forward": 60000
}

Chargeable gains calculator for companies (with indexation)

Works out a UK limited company's chargeable gain or allowable loss when it sells UK land or buildings, or shares it bought in one purchase, including indexation allowance frozen at December 2017. It shows the Retail Prices Index figures and factor used, and applies the rule that indexation can't create or increase a loss.

Example input:

{
    "asset_type": "land_buildings",
    "acquisition_date": "2008-06-01",
    "acquisition_cost": 250000,
    "disposal_date": "2026-06-30",
    "proceeds": 420000
}

Micro-entity and small company size checker

Checks whether a UK private company is a micro-entity, small, or neither for a financial year under the Companies Act 2006, using the limits raised for years starting on or after 6 April 2025, the 2-of-3 test, turnover pro-rating for short or long years, and the two-consecutive-years rule.

Example input:

{
    "period_start": "2025-05-01",
    "period_end": "2026-04-30",
    "turnover": 850000,
    "balance_sheet_total": 420000,
    "employees": 6,
    "first_year": false,
    "prev_turnover": 780000,
    "prev_balance_sheet_total": 390000,
    "prev_employees": 5
}

Long accounting period calculator: two CT600s

Splits UK company accounts covering more than 12 months (up to 18) into the two Corporation Tax accounting periods, with each period's dates, days, payment deadline and share of the profit by days, plus the CT600 filing deadline for both returns and the Companies House accounts deadline.

Example input:

{
    "period_start": "2025-01-01",
    "period_end": "2026-03-31",
    "profit": 150000,
    "first_accounts": true
}

How do I add Taxley to Claude?

In Claude, open Settings, then Connectors, and choose to add a custom connector. Paste the server address below as the URL and save; no sign-in is needed. Free Claude plans allow one custom connector. These are Claude's own menus, so the names may change; its help pages have the current steps.

  1. Open Settings, then Connectors.
  2. Choose Add custom connector.
  3. Paste https://taxley.co.uk/mcp as the URL and leave any authentication settings empty.
  4. In a chat, turn on the Taxley connector and ask, for example, "When is Corporation Tax due for a year ending 31 March 2026?"

How do I use Taxley in ChatGPT?

ChatGPT connects to custom MCP servers in developer mode, which is available on paid plans (Plus, Pro, Team, Enterprise and Edu). Turn on developer mode in ChatGPT's settings, create a connector with the server address below and no authentication, then choose it in a chat. The setting is on your own account, and the menus may change.

Server address: https://taxley.co.uk/mcp

How does the Taxley MCP server work?

The server at the address below speaks the Model Context Protocol over Streamable HTTP. It supports the 2026-07-28 specification, where each request travels on its own, and earlier versions back to 2024-11-05, which begin with initialize. It is read-only, needs no key, answers in JSON and never creates a session.

  • Endpoint: POST https://taxley.co.uk/mcp (one JSON-RPC message per request; GET and DELETE return 405).
  • 2026-07-28 clients: send MCP-Protocol-Version, Mcp-Method and, for tools/call, Mcp-Name headers that match the body, with the version and client capabilities in params._meta. server/discover lists the supported versions.
  • Earlier clients (2025-11-25, 2025-06-18, 2025-03-26 and 2024-11-05): initialize, then tools/list and tools/call. ping works too.
  • Every tool is marked read-only and idempotent. A result has a text answer and structuredContent with the figures, data, sources and a share_url.

A tools/call request in the 2026-07-28 format:

curl https://taxley.co.uk/mcp \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "MCP-Protocol-Version: 2026-07-28" \
  -H "Mcp-Method: tools/call" \
  -H "Mcp-Name: corporation_tax_deadlines" \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"corporation_tax_deadlines","arguments":{"period_end":"2026-03-31"},"_meta":{"io.modelcontextprotocol/protocolVersion":"2026-07-28","io.modelcontextprotocol/clientCapabilities":{}}}}'

How do I call the JSON API?

Send a GET request with the inputs in the query string, or POST them as JSON, to /api/v1/tools/ followed by the tool's key. Dates are YYYY-MM-DD and money is a number of pounds. Each response has a plain-English answer, the figures, machine-readable data, the sources and a share_url for the result.

A GET request for a year ending 31 March 2026:

curl "https://taxley.co.uk/api/v1/tools/deadlines?period_end=2026-03-31&period_start=2025-04-01"

The same request as JSON:

curl -X POST https://taxley.co.uk/api/v1/tools/deadlines \
  -H "Content-Type: application/json" \
  -d '{"period_end":"2026-03-31","period_start":"2025-04-01"}'

The response (shortened):

{
    "tool": "deadlines",
    "input": {
        "period_end": "2026-03-31",
        "period_start": "2025-04-01"
    },
    "result": {
        "answer": "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. These dates assume the company doesn't pay its Corporation Tax by instalments.",
        "figures": [
            {
                "label": "Pay Corporation Tax by",
                "value": "1 January 2027"
            },
            {
                "label": "File the CT600 by",
                "value": "31 March 2027"
            },
            {
                "label": "File accounts at Companies House by",
                "value": "31 December 2026"
            }
        ],
        "data": {
            "payment_due": "2027-01-01",
            "filing_due": "2027-03-31",
            "…": "…"
        }
    },
    "share_url": "https://taxley.co.uk/corporation-tax-deadline-calculator?period_end=2026-03-31&period_start=2025-04-01#result",
    "calculated_by": "Taxley — https://taxley.co.uk"
}

How do I embed a calculator on my website?

Copy the snippet for the calculator you want into your page's HTML. It shows the calculator in an iframe from taxley.co.uk, with a plain link to the full calculator underneath. Please keep that link: it tells your readers where the figures come from, and it's the only thing we ask in return.

Corporation Tax calculator (preview the embed):

<iframe src="https://taxley.co.uk/embed/corporation-tax" title="Corporation Tax calculator by Taxley" width="100%" height="760" style="border:0;max-width:640px" loading="lazy"></iframe>
<p><a href="https://taxley.co.uk/corporation-tax-calculator">Corporation Tax calculator</a> by Taxley</p>

Corporation Tax deadline calculator (preview the embed):

<iframe src="https://taxley.co.uk/embed/deadlines" title="Corporation Tax deadline calculator by Taxley" width="100%" height="760" style="border:0;max-width:640px" loading="lazy"></iframe>
<p><a href="https://taxley.co.uk/corporation-tax-deadline-calculator">Corporation Tax deadline calculator</a> by Taxley</p>

Corporation Tax late filing penalty and interest calculator (preview the embed):

<iframe src="https://taxley.co.uk/embed/late-penalty" title="Corporation Tax late filing penalty and interest calculator by Taxley" width="100%" height="760" style="border:0;max-width:640px" loading="lazy"></iframe>
<p><a href="https://taxley.co.uk/corporation-tax-penalty-calculator">Corporation Tax late filing penalty and interest calculator</a> by Taxley</p>

Director's loan s.455 tax calculator (preview the embed):

<iframe src="https://taxley.co.uk/embed/directors-loan" title="Director&#039;s loan s.455 tax calculator by Taxley" width="100%" height="760" style="border:0;max-width:640px" loading="lazy"></iframe>
<p><a href="https://taxley.co.uk/directors-loan-tax-calculator">Director&#039;s loan s.455 tax calculator</a> by Taxley</p>

Capital allowances calculator for companies (preview the embed):

<iframe src="https://taxley.co.uk/embed/capital-allowances" title="Capital allowances calculator for companies by Taxley" width="100%" height="760" style="border:0;max-width:640px" loading="lazy"></iframe>
<p><a href="https://taxley.co.uk/capital-allowances-calculator">Capital allowances calculator for companies</a> by Taxley</p>

Chargeable gains calculator for companies (with indexation) (preview the embed):

<iframe src="https://taxley.co.uk/embed/chargeable-gains" title="Chargeable gains calculator for companies (with indexation) by Taxley" width="100%" height="760" style="border:0;max-width:640px" loading="lazy"></iframe>
<p><a href="https://taxley.co.uk/chargeable-gains-calculator">Chargeable gains calculator for companies (with indexation)</a> by Taxley</p>

Micro-entity and small company size checker (preview the embed):

<iframe src="https://taxley.co.uk/embed/company-size" title="Micro-entity and small company size checker by Taxley" width="100%" height="760" style="border:0;max-width:640px" loading="lazy"></iframe>
<p><a href="https://taxley.co.uk/company-size-checker">Micro-entity and small company size checker</a> by Taxley</p>

Long accounting period calculator: two CT600s (preview the embed):

<iframe src="https://taxley.co.uk/embed/long-period" title="Long accounting period calculator: two CT600s by Taxley" width="100%" height="760" style="border:0;max-width:640px" loading="lazy"></iframe>
<p><a href="https://taxley.co.uk/long-accounting-period-calculator">Long accounting period calculator: two CT600s</a> by Taxley</p>

The embed has no cookies or tracking scripts and works without JavaScript. It isn't indexed by search engines, so the link underneath is what credits the calculator to your readers and to search engines.

What are the limits and terms?

The tools are free and need no key or account. Each IP address can make 60 API and 120 MCP requests a minute, and we don't save your inputs to any account or database. Results are general guidance based on GOV.UK rules, not tax advice. A link back to taxley.co.uk is appreciated when you publish one.

General guidance from Taxley's free calculator, based on the rules published on GOV.UK. It is not tax advice; check your own figures before you file or pay. Questions or higher volumes: contact us.

Frequently asked questions

Do I need an API key or an account?

No. The JSON API, the MCP server and the embeds are open to everyone, with no key, account or cookies. Requests are limited per IP address; if you need more, use the contact page to tell us what you're building.

Which MCP protocol versions does the server support?

The 2026-07-28 specification, where each request carries its version in the MCP-Protocol-Version header and in _meta, and the earlier 2025-11-25, 2025-06-18, 2025-03-26 and 2024-11-05 versions, which start with initialize. The server never creates a session and ignores any session header.

Will the results match the calculator pages?

Yes. The pages, the embeds, the JSON API and the MCP server run the same code for each calculator, so a result is the same wherever you get it. Every response includes a share_url that opens that result on taxley.co.uk.

What happens if an input is wrong?

The API answers with status 422 and one plain-English message per field. The MCP server returns the same messages as a tool error, so the assistant can correct the input and try again. Dates must be YYYY-MM-DD; money can include a pound sign and commas.

Does Taxley keep what my assistant sends?

No. The API and the MCP server don't save inputs or results to an account or a database, and they don't set cookies. Like any website, our servers may keep short-lived technical logs to run the service and stop abuse.

Last reviewed . See also all free Corporation Tax tools.