> What to give an AI coding assistant or agent so it can use the Check Take Home Pay API: a key an agent gets for itself, llms.txt, Markdown copies of every page and a prompt.

Web version: https://checktakehomepay.co.uk/docs/agents · Last updated: 2026-10-09

# 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](https://checktakehomepay.co.uk/llms.txt) |
| The product, plans and API in one file | [/llms-full.txt](https://checktakehomepay.co.uk/llms-full.txt) |
| Any page as Markdown | Add `.md` to its address, such as [/docs/quickstart.md](https://checktakehomepay.co.uk/docs/quickstart.md), or send `Accept: text/markdown`. |
| Every endpoint, request and response | [openapi.json](https://api.checktakehomepay.co.uk/openapi.json) |
| Every rate and threshold, no key | [/v1/rates](https://api.checktakehomepay.co.uk/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

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

Node.js

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

Python

```
import requests

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