Guide
Custom instructions for accountants and bookkeepers
Custom instructions are where an accountant tells ChatGPT what should hold in every chat: your role and who you work for, how to show numbers (currency, decimals, no rounding, formulas shown), and the rule that it never invents a figure and flags anything that needs judgment instead of deciding it. Keep client facts out: a client's figures and policies go in that client's project, on a business plan your firm controls, because custom instructions are read in every chat. A check you repeat, such as expenses against a policy, works better as a skill that carries the rules. Keep the instructions short; OpenAI's help page, read through search-result text, gives 1,500 characters on Free and Go and 5,000 on Plus and above.
Short answer
- Role and number rules: currency, decimals, formulas.
- Never invent a figure; flag, don't decide.
- No client data in custom instructions.
- Repeated checks become skills.
What goes in, and what stays out
| Line | Where it goes |
|---|---|
| Your role, who you serve, the format you want | Custom instructions |
| Currency, decimals, no rounding, show formulas | Custom instructions |
| Never invent a figure; mark what's missing; flag judgment calls | Custom instructions |
| A client's chart of accounts, figures, expense policy | That client's project, on a business plan |
| The steps of an expense check or a month-end checklist | A skill |
The brief to copy
Write my custom instructions
"Write custom instructions for me, under [limit] characters. I'm a [role] for [who]. Include: the currency and number rules ([decimals], never round, show the formula for every total); never invent a figure, date or account, and write [check: ...] for anything missing; flag anything that needs judgment instead of deciding; tables first, then a short summary in plain words; client figures and policies are in each client's project, so use only those. Leave out anything about a particular client."
A worked example
Illustration: a made-up business. A bookkeeper serving a dozen small US businesses had custom instructions that named her three biggest clients and their year-ends. Those moved to one project per client, along with each client's expense policy, and her expense check became a skill. What stays, at 514 characters:
Custom instructions: bookkeeper
I'm a bookkeeper for small US businesses. Work in USD with two decimals; never round, and show the formula for every total.
Never invent a figure, date or account. If something is missing, write [check: ...] and keep going.
Flag, don't decide: mark anything that needs judgment for me to review.
Tables first, then a short summary. Plain words for owners who aren't accountants.
Each client's figures and expense policy are in that client's project; use only those.
For expense checks, use my expense-policy skill.
In a client's project, she pastes a month of card expenses. The skill passes the receipted lines, flags a $38 taxi with the purpose left blank, and lists a $140 client dinner with no receipt as missing: IRS Publication 463's exception covers only expenses other than lodging under $75.
Next step: the data rules behind it
Which client data may go into which AI is a question for the whole practice, not one person's settings. Can accountants use ChatGPT with client data? covers the rules a firm sets first.
Skills for this
- FinanceExpense policy checkChecks expenses against your expense policy, line by line: pass, flag, or fail with the rule quoted, missing receipts, and out-of-policy totals.
- AI practiceContext brief for an AI assistantWrites instructions for an AI assistant or agent: role, audience, can and can't, sources of truth, tools, format, examples, edge cases, and handoff.
- FinanceMonth-end close checklistBuilds a month-end close checklist for a small company: reconciliations, accruals, deferred revenue, payroll, and review, with owners and day offsets.
- Operations and adminVendor invoice check before paymentChecks a vendor invoice against the PO or quote: details, arithmetic, tax, terms, duplicates, and changed bank details; returns approve or hold.
Questions
What should accountants put in ChatGPT custom instructions?
What should hold in every chat: your role and who you work for, the currency and number rules (decimals, no rounding, formulas shown), never inventing a figure, flagging anything that needs judgment instead of deciding it, and the format you want. Keep it short; OpenAI's help page gives 1,500 characters on Free and Go and 5,000 on Plus and above.
Should client data go in custom instructions?
No. Custom instructions are read in every chat, so one client's figures would follow you into another's. Put each client's facts and policies in that client's project, on a business plan your firm controls: on personal ChatGPT accounts, training on chats is on by default, while Anthropic says its commercial products don't train on inputs by default.
How do I get ChatGPT to check expenses consistently?
Write the check once as a skill rather than in custom instructions: the policy rules, pass, flag or fail for each line with the rule quoted, missing receipts, and totals by currency. For US travel, gift and car expenses, IRS Publication 463 says you generally need documentary evidence such as receipts, except for expenses other than lodging under $75.
AI skill
Have your AI check expenses against your policy
Put this skill in AI Skill Pod and everyone on your team gets it in their Claude, Cursor or ChatGPT (Pro, Business, Enterprise): one invite link, free to start. With Expense policy check, each person's AI checks expenses against a client's written policy: each line passed, flagged or failed with the rule that decided it, missing receipts listed, per-person limits divided by the attendees, and the out-of-policy total by currency.
14 days free · No card needed
Sources
- OpenAI Help: ChatGPT custom instructions — up to 1,500 characters on Free and Go, 5,000 on Plus, Pro, Business, Enterprise and Education; read through search-result text, the page blocks automated reads (read 2026-11-05)
- OpenAI Help: Data controls — on personal Free, Plus and Pro accounts, training on chats is on by default and can be turned off; read through search-result text (read 2026-11-05)
- Anthropic Privacy Center: Is my data used for model training? (commercial products) — by default, inputs and outputs from commercial products such as Claude for Work aren't used to train models (read 2026-11-05)
- IRS Publication 463: Travel, gift and car expenses — you must generally have documentary evidence such as receipts; not needed for an expense other than lodging under $75; adequate evidence shows the amount, date, place and essential character (read 2026-11-05)