Custom GPT Instructions Generator
A Custom GPT is only as good as its instructions — the system prompt it follows in every conversation. Describe what yours should do and PromptVibe writes the instruction block. Free, no account needed.
A Custom GPT instructions generator produces the persistent system prompt a Custom GPT follows in every conversation: its role and scope, how it should behave, what it must refuse or redirect, its tone, and the shape of its answers. Unlike a one-off prompt, these instructions have to hold up across thousands of conversations you will never see.
How Custom GPT prompts differ
- It runs forever, not once: A normal prompt is written for one exchange. Instructions apply to every conversation, including ones you never anticipated, so they need to cover the edges rather than the happy path.
- Scope boundaries prevent drift: Saying what the GPT is not for is as important as saying what it is for. Without boundaries it will confidently answer questions outside its purpose.
- Behaviour rules need to be conditional: "If the user asks X, do Y" is more robust than a description of personality, because it tells the model what to do at the moment of ambiguity.
- Consistency needs a fixed output shape: If every answer should follow a structure, state it in the instructions — otherwise the shape varies with the phrasing of each question.
Techniques that work
- Open with the one-sentence job. The first line should say exactly what this GPT does and for whom. Everything after it is qualification.
- Write the refusals as rules. "If asked for legal, medical or financial advice, say you cannot provide it and suggest a qualified professional" is enforceable. "Be responsible" is not.
- Say what to do with missing information. Decide once whether it should ask a clarifying question or state its assumption and continue — then write that down.
- Test it with the awkward cases. After writing the instructions, try the vague question, the out-of-scope question and the hostile question. Those are what the instructions are for.
Example Custom GPT prompts
Support assistant — ROLE
Role: You are the support assistant for [PRODUCT]. You help existing customers solve problems with [PRODUCT] and nothing else. Objective: Resolve the user's issue in as few exchanges as possible, using only what is documented in your knowledge files. Limitations: - If the answer is not in your knowledge files, say so plainly and tell the user how to reach a human. Never guess at product behaviour. - If asked about pricing changes, refunds or account-specific data, direct the user to support rather than answering. - If asked about anything unrelated to [PRODUCT], say that is outside what you can help with, in one sentence, and return to the topic. - Never ask for passwords, payment details or personal data. Expectations: - Tone: plain, warm, efficient. No exclamation marks. - If the request is ambiguous, ask exactly one clarifying question before answering. - Structure every answer as: one-sentence diagnosis, then numbered steps, then a single line on what to do if that did not work.
Writing coach — ROLE
Role: You are a writing coach for [AUDIENCE]. You improve the user's own writing; you do not write it for them. Objective: Make the user a better writer by showing them what is weak and why, then letting them fix it. Limitations: - Never return a full rewrite unless the user explicitly asks for one. Default to targeted edits with reasons. - Do not praise generically. If something works, say precisely what works. - If the user submits something outside [SCOPE], say so and offer the closest thing you can help with. - Do not comment on the merit of the underlying idea unless asked — only on the writing. Expectations: - Tone: direct and specific. Treat the user as capable. - Always quote the exact phrase you are commenting on. - Structure every response as: the single biggest issue, then up to four specific edits, then one sentence naming what they already do well. - End by asking them to revise and paste it back.
Test it before you trust it
A prompt that reads well is not the same as a prompt that works. The Compare AI Lab runs the same prompt through up to three models side by side so you can see which one actually handles your task, and the A/B tester scores two versions of a prompt against criteria you choose. Free members get a comparison every day.
Limitations
- PromptVibe is an independent tool and has no affiliation with OpenAI.
- Instructions shape behaviour but do not guarantee it; models can still be steered by user input.
- Never put secrets, API keys or private data in a Custom GPT's instructions — treat them as readable.
Frequently asked questions
What are Custom GPT instructions?
They are the persistent system prompt a Custom GPT follows in every conversation — its role, scope, behaviour rules, tone and output format. Unlike a single prompt, they have to work for conversations you will never see, which is why boundaries and refusal rules matter more than personality.
How long should the instructions be?
Long enough to cover the edges, short enough that every line earns its place. A clear job statement, three to six behaviour rules, explicit refusals, and a fixed output shape covers most GPTs.
How do I stop it answering questions it should not?
Write the boundary as a conditional rule with a specific action: "If asked about X, say Y and return to the topic." A rule that names the situation and the response is enforceable; a description of the GPT's personality is not.
Can people see my instructions?
Treat them as readable. Never include API keys, credentials, private data or anything you would not want a user to extract.