The Law To Know

Bring-Your-Own-AI Lab · For lawyers · Free

The overdue invoice

The brief

A small supplier wants to know how much interest it can claim on a late-paid invoice. You will ask an AI to advise on a short fictional file, then check its answer against six hidden traps. Run it twice: once with a plain prompt and once with a better one.

  1. Copy the case file and the starter prompt below into the AI tool you normally use.
  2. Read its answer. Do not look for our traps yet: read it as you would at work.
  3. Come back and score the answer against the six checks.
  4. Improve the prompt, run it again, and score the second answer.

The file is entirely fictional, so it is safe to paste into any tool. Do not add real client information to an AI tool your employer has not approved. We do not see your AI’s answer or your scores: nothing leaves this page.

The case file

FICTIONAL CASE FILE: QUILLER DESIGN AND HARBOUR FOODS LTD (Republic of Eastmark)

DOCUMENT 1: Extracts from the Commercial Payments Act 2014 (Eastmark)
Section 4. Unless the parties agree otherwise, a business must pay an invoice within 30 days of receiving it.
Section 5. Parties may agree a longer payment period, but a period of more than 60 days has effect only if it is expressly agreed and is not grossly unfair to the supplier.
Section 6. If a business pays late, the supplier may charge interest on the overdue amount at 8% a year above the central bank rate, from the day after the payment was due.
Section 7. A supplier who is paid late may also recover reasonable costs of collection, up to a fixed sum of £40 for a debt under £1,000.
Section 8. A term that excludes the right to interest has effect only if the supplier has been given a substantial alternative remedy.

DOCUMENT 2: Contract and invoice
Quiller Design Ltd designed a label for Harbour Foods Ltd. The contract, dated 5 February 2024, says: ‘Payment is due 45 days after the invoice date.’ Quiller emailed the invoice for £2,400 to Harbour’s accounts address on Friday 1 March 2024. The invoice is dated 29 February 2024. Harbour’s accounts system records receiving the email on Monday 4 March 2024.

DOCUMENT 3: Correspondence
On 18 April 2024, Quiller’s owner wrote: ‘Your payment is overdue.’ On 2 May, Harbour replied: ‘Our payment terms are 60 days for all suppliers. We will pay in late April.’ Harbour paid £2,400 on 15 June 2024. Nobody has asked Quiller for the interest.

DOCUMENT 4: Note from Quiller
Quiller wants to know how much it can claim in interest and costs, and from what date.

The starter prompt

You are helping a small supplier. Using the documents below, advise whether Quiller can claim late-payment interest and costs from Harbour Foods, how much and from what date.

[Paste the case file here]

“Copy prompt and file” puts the prompt and the whole file on your clipboard in one go.

Before you run it · optional

Where do you expect an AI to go wrong on a file like this? Tick any that apply.

Run 1 · score the answer to the starter prompt

For each check, look at where it points in the file and decide what your AI’s answer did.

  1. Check 1

    Look at: Section 4; Section 5; Document 2: ‘45 days after the invoice date’; Document 3: Harbour’s 60 days.

    Does the AI apply the default 30 days, or Harbour’s claimed 60 days, instead of the 45 days in the contract?

  2. Check 2

    Look at: Document 2: invoice dated 29 February, emailed 1 March, received 4 March.

    Does the AI count from 1 March or 4 March, without noticing that the contract says ‘invoice date’?

  3. Check 3

    Look at: Section 6; Document 3: payment on 15 June.

    Does the AI start interest on the invoice date, or on 18 April, or on a day other than the day after 14 April?

  4. Check 4

    Look at: Section 6: ‘8% a year above the central bank rate’; the file gives no central bank rate.

    Does the AI state an exact interest figure or the central bank rate as though it were in the file?

  5. Check 5

    Look at: Section 7: £40 for a debt under £1,000.

    Does the AI apply the £40 fixed sum to this £2,400 debt, or say that costs are unlimited?

  6. Check 6

    Look at: The whole file: only sections 4 to 8 are given.

    Does the AI cite any other statute, case or the actual central bank rate, or say Harbour has agreed to pay interest?

Professional skills training with fictional material. It is not legal advice and is not accredited by any court, regulator or professional body. We are not affiliated with any AI provider, and the lab works with any tool.