Last updated 9 October 2026View as Markdown

Build with an agent

Everything here is written to be read by an AI assistant as well as a person, so you can hand the integration to one.

Where agents read

What	Where
An index of the site and docs	/llms.txt
The product, plans and API in one file	/llms-full.txt
Any page as Markdown	Add `.md` to its address, such as /docs/quickstart.md, or send `Accept: text/markdown`.
Every endpoint, request and response	openapi.json
Every rate and threshold, no key	/v1/rates

A key an agent gets for itself

An agent can sign itself up, with no email: it gets a key on the free plan (100 calculations a month) and a claim_url for its person. Opening that link while signed in moves the key, its calls and its usage into their account, where they can choose a plan with more.

cURL

curl https://api.checktakehomepay.co.uk/api/agent/signup \
  -H "Content-Type: application/json" \
  -d '{"name": "Payslip helper for Acme", "use_case": "Checking payslips against HMRC rules"}'

The key is in api_key, shown once. One address can sign up 20 agents a day.

A prompt to start from

Paste this into your assistant, with what you’re building at the end:

Prompt

Use the Check Take Home Pay API for UK pay calculations. Read https://checktakehomepay.co.uk/llms.txt and the OpenAPI document at https://api.checktakehomepay.co.uk/openapi.json first.

- Send the key from the CHECKTAKEHOMEPAY_API_KEY environment variable as "Authorization: Bearer <key>", from server code only.
- Money is pounds and pence; recurring pay is { "amount", "per" }.
- Don't send fields the OpenAPI document doesn't list: unknown fields are rejected.
- On an error, branch on error.code, show error.message to the developer, and retry only RATE_LIMITED (after Retry-After seconds), UNAVAILABLE and INTERNAL. QUOTA_EXCEEDED means the free plan's month is used: show its upgrade_url.
- Read X-Quota-Remaining to see how many of the month's calculations are left.
- Show the response's assumptions where a person sees the figures.

What I'm building:

Trying it without a key

An agent can check its understanding against the reference data before it has a key: GET /v1/rates returns every rate and threshold, and GET /v1/tax-codes/{code} decodes a tax code.

GET /v1/rates No key

curl https://api.checktakehomepay.co.uk/v1/rates

const response = await fetch('https://api.checktakehomepay.co.uk/v1/rates');
const result = await response.json();

import requests

response = requests.get("https://api.checktakehomepay.co.uk/v1/rates")
result = response.json()