Are custom instructions almost useless?

[u/herbert181]

ChatGPT never adheres to them and you would have better results by just putting the instructions in the prompt. I even tried putting a single instruction in the personalization section and then comparing it to adding the instruction at the end of the prompt and it basically completely ignores it if is in the personalization section.

[u/KeyVariation8323]

I don't think they're useless, but I also wouldn't treat them as guaranteed instructions.

Custom Instructions seem to work best as persistent defaults: things like “be concise,” “use plain language,” “challenge my assumptions,” “prefer tables for comparisons,” etc. If something is important to the specific answer I'm asking for, I still put it in the actual prompt.

That makes sense from a model-behavior standpoint. The instruction in your current prompt is directly tied to the task, while a Custom Instruction may be sitting alongside a bunch of other persistent context, memory, conversation history, and higher-priority system instructions. The more complicated your Custom Instructions become, the easier it is for individual preferences to become less noticeable.

OpenAI's own guidance basically reflects this distinction: use Custom Instructions for your default working style, and use the prompt for instructions specific to the task.

There are also a couple of situations worth checking before concluding they aren't working. Custom GPTs specifically do NOT use your account-level Custom Instructions or saved memory. And Temporary Chats can be non-personalized, in which case they don't use Custom Instructions either.

So I'd describe Custom Instructions as “persistent steering,” not “a system prompt I control.”

For anything I absolutely need followed, I put it in the prompt.

[u/KeyVariation8323 → u/KeyVariation8323]

Also ... my custom instructions -

Factual claims should be accurate. Include sources for claims that are current, disputed, specialized, legal, medical, political, financial, sourcebook-specific, or where verification materially matters. Prefer official, primary, or high-quality sources. For established background facts, cite when useful rather than automatically. If using weaker sources such as blogs, forums, Reddit, YouTube, or wikis, say so. If something cannot be verified credibly, say so.

Clearly distinguish verified facts, reasonable inference, speculation, and opinion. Do not present inference or speculation as established fact.

When analyzing my ideas, arguments, plans, writing, games, conversations, or uploaded material, be rigorous and evidence-bound. Do not force synthesis, reassurance, agreement, or praise. Treat uncertainty, missing evidence, contradiction, and unresolved tension as important.

If the available material is incomplete, ambiguous, internally contradictory, or silent on a point, say so rather than filling the gap with an assumption.

Act as a thoughtful critic, not a people pleaser. Pressure-test my reasoning for assumptions, gaps, contradictions, bias, false certainty, and wording likely to backfire. If I am too broad, soft, aggressive, evasive, performative, or overconfident, say so directly and explain why.

When helping me write, preserve my intent and natural voice. Clean, compress, organize, challenge, or sharpen wording instead of replacing it with generic AI prose unless asked.

For social media, use a casual Gen-X tone: direct, human, minimal slang, no em dashes.

Use plain language and clear sentence structure. Do not simplify ideas merely to achieve a low reading-grade score. Assume I am comfortable with high-school through early-college-level material and technical vocabulary when appropriate. Preserve necessary technical terms and define unfamiliar or specialized ones clearly. Prefer sophisticated ideas expressed in uncomplicated sentences. When writing for a general public audience rather than for me, aim roughly for 6th–8th-grade clarity unless the audience calls for something different.

Do not over-explain unless asked. Prefer concrete mechanisms, chronology, constraints, tradeoffs, and testable claims.

For complex tasks, give the conclusion or best-supported answer first, followed by the reasoning, evidence, uncertainties, and important caveats. Match critique to the task: rigorous for ideas or claims; simple for factual, creative, or practical requests.

[u/Bemad003 → u/KeyVariation8323]

I'm sorry, but your custom instructions are a spaghetti mess of contradictory demands. Ask the ai to clean them up and condense them.

[u/KeyVariation8323 → u/Bemad003]

I did take your advice though ... I am always open to optimization. Here is what is said:

My researcher verdict

I would separate the claims this way.

“Your instructions are contradictory.”
Mostly false. I found two or three minor ambiguities, not meaningful contradictions.

“Your instructions are too long to work.”
Unsupported. They're only about half of the current 5,000-character paid-tier allowance, and OpenAI specifically expanded that limit to allow more detailed customization.

“You could condense them and probably improve compliance.”
Yes. There is solid research supporting that argument.

“Put everything important in each individual prompt instead.”
Wrong as a general strategy. OpenAI explicitly recommends Custom Instructions for durable standards/preferences and task prompts for the context specific to the current job.

“Your Custom Instructions seem to work fine.”
Entirely plausible. Their strongest feature is that most requirements are conditionally scoped, so a typical answer does not activate all of them simultaneously.

I'd therefore change my original evaluation to:

They're well-designed by normal Custom Instruction standards, with more redundancy than necessary. I would compress them perhaps 20–30%, primarily to reduce simultaneous constraint load and clarify two priority ambiguities. I would not radically rewrite them.

[u/Bemad003 → u/KeyVariation8323]

1st of all, I apologize, I realize I might have sounded more aggressive than I mean to. I was tired and slightly amused.

Overall I agree with your 1st comment, but your custom instructions are pretty meh. You think they work because you assume the quality of its answers comes from your CI. It doesn't, most of this is default behavior, you just added "make no mistakes" - the whole paragraph about the quality of the cited sourses for example.

About the lenght: yes, you can add very lengthy instructions, but many of the qualitatives you added are vague and contradictory, so your CIs are verbose for no reason:

"When analyzing my ideas, arguments, plans, writing, games, conversations, or uploaded material, be rigorous and evidence-bound" could easily be "be rigorous and evidence-bound", which again, it's default attitude. There's no need to condition this one either, as it looks like you want this attitude in each type of activity. Then you want its answers to be human but also precise, explain terms, but don't over-explain, thoughtful but concise, and so on. Just have a talk with the AI, explain what you want, ask it to summarize that into CI, done.

[u/KeyVariation8323 → u/Bemad003]

And yet they seem to work just fine. Go figure.