# ChatGPT Custom Instructions for Coding: 8 Rules to Copy > Stop retyping "only show what changed" in every chat. Eight short ChatGPT custom instructions for people who write code at work, and where they go. Source: https://learn.ynteractive.com/content/blog/chatgpt-custom-instructions-for-coding · Updated 2026-10-21 · Free from Stepthrough (https://learn.ynteractive.com) Quick answer **Short answer:** if you write code at work, put the rules you repeat in every chat into ChatGPT's [custom instructions](https://learn.ynteractive.com/content/glossary/custom-instructions): your main language, how to format code, "show only what changed," and "say what caused the error before the fix." Keep them short, because Free and Go plans allow 1,500 characters and paid plans 5,000. Rules for one project belong in that Project's instructions instead, which the free [Turn a Great Prompt into Reusable Instructions](https://learn.ynteractive.com/content/turn-chatgpt-prompt-into-project-instructions) lesson walks through. Last updated October 21, 2026 [Take the free module →](https://learn.ynteractive.com/courses/better-prompts/8)Module 8 of the free [Better Prompts](https://learn.ynteractive.com/courses/better-prompts) course, about 7 minutes. No account needed. By [Robert Breen](https://learn.ynteractive.com/content/about) · Published October 21, 2026 · Updated October 21, 2026 · 5 min read ChatGPT · Settings · Coding This guide is for analysts, operations staff and developers who use ChatGPT for SQL, spreadsheet formulas, scripts and small [automations](https://learn.ynteractive.com/content/glossary/automation). The idea came from Ted at Nook, who pointed out that people who write code repeat the same rules more than anyone, and wrote [a free guide to ChatGPT custom instructions for coding](https://nook-2.foundable.now/best-chatgpt-custom-instructions). Settings details below were checked against [OpenAI](https://learn.ynteractive.com/content/glossary/openai)'s Help Center in October 2026. Menus change by plan, so check your own Settings. ## Why do people who write code need custom instructions most? Because coding requests come with the same rules every time. Which language, which style, whether to rewrite the whole file or show one function, whether to explain basics you already know. Without standing rules, you type them again in every chat, or you get a 200-line answer when you wanted three changed lines. Custom instructions are guidance you write once. OpenAI says they apply to all chats and are available on all plans, on the web, desktop, iOS and Android. ## Where do I put ChatGPT custom instructions? On the web or desktop app, open **Settings**, choose **Personalization**, make sure **Enable customization** is on, and type in the **Custom Instructions** field. On iOS and Android the menu is called **Customize ChatGPT**. The size limit depends on your plan: up to 1,500 characters on Free and Go, and up to 5,000 on Plus, Pro, Business, Enterprise and Education. Eight one-line rules fit easily in either. ## What are good custom instructions for coding? Here are eight rules, one line each. Change the details to match your work, and delete any you don't need. 1. **Language.** "Unless I say otherwise, I write SQL for PostgreSQL and Python 3." 2. **Code first.** "Show the code first, then explain it in three sentences or fewer." 3. **Only what changed.** "When I paste code and ask for a fix, show only the lines you changed, with a comment above each one. Give the whole file only if I ask." 4. **Errors.** "When I paste an error, tell me the most likely cause in one sentence before the fix." 5. **Style.** "Follow the style already in my code: names, indentation and quotes." 6. **Level.** "I know the basics. Skip explaining loops and joins, and explain anything unusual." 7. **Ask first.** "If my request could mean two things, ask one short question before writing code." 8. **No invented APIs.** "If you aren't sure a function or setting exists, say so instead of guessing." The last one matters most at work. A made-up function name wastes more time than a slow answer. ## What does a combined example look like? Here is one block for an operations analyst who writes Google Sheets formulas and small Apps Script automations. It is about 600 characters, well inside the Free limit: ```text I'm an operations analyst. I write Google Sheets formulas and small Google Apps Script automations. Show the formula or code first, then explain it in three sentences or fewer. When I paste code and ask for a fix, show only the changed lines, with a comment above each. When I paste an error, give the likely cause in one sentence before the fix. Assume I know basic formulas; explain anything unusual. If my request could mean two things, ask one short question first. If you aren't sure a function exists, say so instead of guessing. Never put real customer data in examples; use made-up names. ``` ## Should coding rules go in custom instructions or a Project? Put rules about *you* in custom instructions and rules about *one codebase or one job* in a Project. OpenAI says project instructions apply only within that project and override your global custom instructions. So "I write Python 3" goes in custom instructions, and "this report script uses the finance team's column names" goes in the Project. The [custom instructions vs project instructions guide](https://learn.ynteractive.com/content/blog/chatgpt-custom-instructions-vs-project-instructions) covers where each rule goes, and the free [ChatGPT Projects lesson](https://learn.ynteractive.com/content/chatgpt-projects-brand-voice) sets one up in a practice copy. ## Isn't ChatGPT memory enough? No. Memory is what ChatGPT decides to keep from your chats, and what it holds can change. Custom instructions are the exact text you wrote, every time. For rules you always want, such as "only show what changed," write them down. The [memory vs custom instructions guide](https://learn.ynteractive.com/content/blog/chatgpt-memory-vs-custom-instructions) explains the difference in detail. ## How do I keep custom instructions short and working? - One rule per line, written as an instruction, not a wish. - Start with three or four rules, use them for a week, then add the ones you still catch yourself typing. - Test a rule by asking the same question in a new chat before and after. - Remove rules that fight each other, such as "be brief" and "explain everything." - Keep secrets out. Never put passwords, API keys or client data in custom instructions. The free [ChatGPT data controls lesson](https://learn.ynteractive.com/content/chatgpt-data-controls-protect-client-data) shows the settings that decide what OpenAI can use. ## Try it free, step by step **Turn a Great Prompt into Reusable Instructions**. Take a prompt you paste every week, sort it into permanent rules and fill-in blanks, put the rules in a ChatGPT Project, and test it with a one-line request. You do every step yourself in a practice copy of ChatGPT, it checks your work as you go, and “Do it for me” finishes any step you get stuck on. Free, and nothing touches your real accounts. [Start the free lesson](https://learn.ynteractive.com/modules/prompt-to-project-instructions) [Or read the full step-by-step guide](https://learn.ynteractive.com/content/turn-chatgpt-prompt-into-project-instructions) ## Key takeaways - People who write code repeat the same rules most, so custom instructions pay off fastest for them. - They live under Settings, then Personalization, then Custom Instructions on the web and desktop app. - The limit is 1,500 characters on Free and Go and 5,000 on paid plans. - "Show only what changed" and "say what you're not sure exists" save the most time. - Rules for one codebase or client go in a Project, which overrides custom instructions inside it. ## Frequently asked questions **Do custom instructions apply to every ChatGPT chat?** OpenAI says custom instructions are applied to all chats. Inside a Project, the project's own instructions apply only within that project and take priority. **How many characters can ChatGPT custom instructions be?** Up to 1,500 characters on Free and Go plans, and up to 5,000 on Plus, Pro, Business, Enterprise and Education, as listed in OpenAI's Help Center in October 2026. **Can I have different coding rules for different projects?** Yes. Keep your general rules in custom instructions and put project-specific rules, such as a table's column names or a script's purpose, in that Project's instructions. **Should I put my API key in custom instructions so ChatGPT can use it?** No. Never put passwords, API keys or client data in custom instructions or chats. Use placeholders like YOUR_API_KEY and add the real value in your own code. **Where can I find more coding examples?** Nook's free guide, [Best ChatGPT custom instructions for coding](https://nook-2.foundable.now/best-chatgpt-custom-instructions), has more copy-and-paste rules and a combined example. ## Free lessons in this post - [Turn a Great Prompt into Reusable Instructions](https://learn.ynteractive.com/content/turn-chatgpt-prompt-into-project-instructions): Take a prompt you paste every week, sort it into permanent rules and fill-in blanks, put the rules in a ChatGPT Project, and test it with a one-line request. - [ChatGPT Projects: Keep Your Brand Voice, Files and Instructions in One Place](https://learn.ynteractive.com/content/chatgpt-projects-brand-voice): Set up a ChatGPT Project with your brand voice instructions, your price list and your style guide, so every new chat already knows your business. - [ChatGPT: Keep Client Data Safe, Set Up Your Team's Data Controls in 10 Minutes](https://learn.ynteractive.com/content/chatgpt-data-controls-protect-client-data): A client asks if their files go into ChatGPT. Answer with confidence: set the data controls, memory, Temporary Chat and shared links, then hand your team a one-page AI… ## Watch it in a free lesson: ChatGPT: Keep Client Data Safe, Set Up Your Team's Data Controls in 10 Minutes ### ChatGPT: Keep Client Data Safe, Set Up Your Team’s Data Controls in 10 Minutes: video walkthrough Full walkthrough · 3:05 · with captions A client asks if their files go into ChatGPT. Answer with confidence: set the data controls, memory, Temporary Chat and shared links, then hand your team a one-page AI policy. The whole lesson narrated step by step, recorded in Stepthrough’s simulated practice environment. [Watch it with chapters and the full transcript](https://learn.ynteractive.com/content/chatgpt-data-controls-protect-client-data#watch) From the free lesson [ChatGPT: Keep Client Data Safe, Set Up Your Team's Data Controls in 10 Minutes](https://learn.ynteractive.com/content/chatgpt-data-controls-protect-client-data) ## Related terms Plain-English definitions, each with an example from a free lesson. [Custom instructions (ChatGPT)](https://learn.ynteractive.com/content/glossary/custom-instructions) · [Automation](https://learn.ynteractive.com/content/glossary/automation) · [OpenAI](https://learn.ynteractive.com/content/glossary/openai) · [AI incident response](https://learn.ynteractive.com/content/glossary/ai-incident-response) · [AI vendor risk assessment](https://learn.ynteractive.com/content/glossary/ai-vendor-risk-assessment) · [Chat export](https://learn.ynteractive.com/content/glossary/chat-export) · [ChatGPT data controls](https://learn.ynteractive.com/content/glossary/chatgpt-data-controls) · [Meta-prompting](https://learn.ynteractive.com/content/glossary/meta-prompting) [The full AI glossary](https://learn.ynteractive.com/content/glossary) ## Related prompts - [ChatGPT memory sort prompt](https://learn.ynteractive.com/content/prompts/chatgpt-memory-sort-prompt): Paste your ChatGPT saved memories and sort each one: keep it, move it to custom instructions, or delete it because it holds… - [Firm project instructions draft prompt](https://learn.ynteractive.com/content/prompts/firm-project-instructions-draft-prompt): A prompt that turns your accounting firm's tone, fee rules and client policies into project instructions for Claude or ChatGPT… - [IT incident report prompt](https://learn.ynteractive.com/content/prompts/it-incident-report-prompt): A ChatGPT prompt for an IT incident report: turn your log into a timeline, what was affected, how you contained it and what is… - [Sales assistant instructions prompt](https://learn.ynteractive.com/content/prompts/sales-assistant-instructions-prompt): Copy this prompt to write instructions for a sales Custom GPT or ChatGPT Project: voice, structure and the rules that stop… [All free prompt templates](https://learn.ynteractive.com/content/prompts) ## Visual guides The same ideas as quick visual guides: swipe through the slides, or read each one as text. - [Mistake: Re-Explaining the Client in Every ChatGPT Chat](https://learn.ynteractive.com/content/visual/mistake-re-explaining-the-client-every-chat): A common ChatGPT mistake: explaining the same client again in every new chat. Give each client one Project with its own instructions and files. - [Myth: ChatGPT Memory Is Where Your Work Rules Belong](https://learn.ynteractive.com/content/visual/myth-chatgpt-memory-holds-your-rules): Myth vs fact: ChatGPT memory is not where work rules belong. Put standing rules in custom instructions and one client's rules in its Project. - [Myth: A New ChatGPT Chat Is a Private Chat](https://learn.ynteractive.com/content/visual/myth-new-chat-means-private): Myth vs fact: a new ChatGPT chat is saved like any other and memory can carry details. For a sensitive job, use Temporary Chat and remove names first. [All visual guides](https://learn.ynteractive.com/content/visual) ## Keep reading - [ChatGPT Shared Project vs Shared Chat Link: Which to Use](https://learn.ynteractive.com/content/blog/chatgpt-shared-project-vs-shared-chat-link): A shared chat link shows one conversation; a shared project gives people your chats, files and instructions. See what each reveals before you send it. - [ChatGPT Custom Instructions vs Project Instructions for Work](https://learn.ynteractive.com/content/blog/chatgpt-custom-instructions-vs-project-instructions): Custom instructions apply to every ChatGPT chat; project instructions apply to one client's chats and win. Where each rule goes, and memory's role. - [ChatGPT Memory vs Custom Instructions: What's the Difference](https://learn.ynteractive.com/content/blog/chatgpt-memory-vs-custom-instructions): Custom instructions are rules you write; memory is what ChatGPT picks up from your chats. See how each works, where to find them and which to use for work. [All blog posts](https://learn.ynteractive.com/content/blog) · [All free tutorials](https://learn.ynteractive.com/content)