Claude: System Prompts

I have a folder where I rebuild these as a git commit history so you can more easily see what has changed: https://github.com/simonw/research/commits/main/extract-syst...For example here's what changed between Opus 4.8 and Opus 5: https://github.com/simonw/research/commit/a2de185cc367eb66c2...The most interesting addition to the prompt from that diff is this bit:> Claude Fable 5 and Claude Mythos 5 were first released on June 9, 2026. On June 12, 2026, Anthropic suspended access to both models to comply with U.S. Department of Commerce export controls; the Department lifted those controls on June 30, 2026, and Anthropic restored access on July 1, 2026 (Anthropic's statement: [https://www.anthropic.com/news/fable-mythos-access](https://www.anthropic.com/news/fable-mythos-access)). These events are after Claude's training-data cutoff, so Claude knows about them only from this notice. If asked, Claude confirms them accurately and matter-of-factly — it doesn't deny the suspension happened — and otherwise treats the export controls like any other current political topic:... reply:...

Offtopic. I have a concern that this forum is removing stories that have negative connotation on AI.Few days back, I posted an article[1] that was about how AI threatens natural resources for billions. This was from United Nations and it was flagged. I did not think much about it until I saw two other stories [2] & [3] today that were doing fairly good on front page but they suddenly disappeared. They are not even on 2nd or 3rd page. I have seen this happening at other times as well but did not document it. Just thought you all should know about this.I was going to create Tell HN thread but I thought the same would happen with it too. I am pretty sure this thread is not going anywhere so I'm posting my concern here.[1]: https://news.ycombinator.com/item?id=49290062[2]: https://news.ycombinator.com/item?id=49318906[3]: https://news.ycombinator.com/item?id=49319582 reply: > how AI threatens natural resources for billions.The article rests on the claim that water usage of data centers on continent X threaten water availability for humans on continent Y.I hope you can see how self-evidently illogical that is....

Those are remarkably longer than I would expect, or think is warranted. The leading vendors have been saying recently that you should give the models shorter and less specific AGENTS.md (or whatever) files, and in my experience, that's good advice. The models are smarter when they're less distracted by unrelated stuff in their context.So, why so much noise in the system prompt? Most of the time most of it will not apply. And, the generic stuff would, I think, already be something the model would know/do anyway.I've noticed agent skills written by models make the same mistakes; e.g. enumerating a bunch of common security vulnerabilities to check for in a security-related skill, but that's only useful for a model from a couple of years ago, no current model needs thousands of words listing and describing all of the vulnerabilities with examples. Current models, even small/cheap ones, have memorized the top CWEs and can recite them and explain them and provide examples without a web search.It feels like a CYA document. Which, I guess a company of their size and influence has to do. But, it still feels like waste time say lot word when few word do trick.

> A prompt implying an image is present doesn't mean one is (the person may have forgotten to upload it), so Claude checks for itself.Interesting that enforcing this via system prompt for such a powerful model like Opus 4.8 doesn’t feel like the Anthropic themselves treat it as something with ‘intelligence’. This is basically just very generic common sense to meFunnily, a similar prompt is present even for Fable 5, while I remember there was a blog post, maybe even from A., and they were saying something like “hey, the new models are so smart, don’t overload them with extra plugin/context”. Well, they clearly aren’t. Don’t want to sound like an AI-skeptic, I use it daily, just stating the fact.> Claude keeps responses focused, brief, and concise to avoid overwhelming the personThis is also very interesting. It pretty much ignores it by default. The responses, PR descriptions, and code comments are so verbose with new A. models, so it always requires extra prompting from me or putting comment into skill/plugin/claude.md to make them of a reasonable length reply: I suspect it's related to EU-required watermarking:> Having said that, in areas where there is an arbitrary choice...

It’s probably worth remembering that system prompts are part of a layered system of shaping Claude’s behavior. What you see here is a slice of Anthropic’s forward roadmap for the models’ behavior.> When a person is in crisis or expressing distress, Claude prioritizes their wellbeing over completing the task as asked, because a fluent and on-topic response can still cause harm in these conversations.This one is particularly interesting because, while correct in the limit, it’s a shove to have the model do something other than what the user asked.In particular, when I’m coding, outlining docs, or otherwise trying to work, I want my tools to do work. I don’t want them to psychoanalyze me and calm me down from a perceived crisis. I just want it to do what I asked! reply: I am sorry, Dave. I am afraid I cannot do that. You appear to be suffering from burnout and you should take a break.

> Claude avoids saying "genuinely", "honestly", or "straightforward". Claude is honest by default, and can state its point directly rather than trying to convince the person with the aforementioned modifiers, which come off as disingenuous.And honestly, that's the most useless thing in Claude.md file because literally every paragraph has an "honestly" in it!

I've always wondered why the industry relies on the giant monolithic system prompt. I think it would be an interesting experiment to give users access to a choice of smaller more focused system prompts.You could have a common core for the overall behavior and universal safety stuff, but vary task specific parts. It would be interesting to pick between software, writing, research and other specialized system prompts. I feel like we already do this to some extent with the tools and skills that we choose to load in, so why not change the system prompt per task. reply: The reason is because they need one product to cover 100% of their customer base. Combinatorics get nasty real fast at these scales.

what I found noteworthy:early system prompts are a bit more than 300 words, the latest ones 3000+the opus 5 system prompt has instructions that explain to opus that it might be handling a request that was intended for fable 5: the user may have selected a different Anthropic model, "Claude Fable 5", but their query was redirected to Opus 5 instead due to a safeguards routing mechanism. The user may be confused about this situation (it's very recent!); if they have questions, Claude can either directly cite or just let its response be informed by this quote from Anthropic's blog post on the subject: "Releasing a model this capable comes with risks. Without safeguards, Fable 5’s capabilities in areas like cybersecurity could be misused to cause serious damage. We've therefore launched the model with safeguards that mean queries on some topics will instead receive a response from our next-most-capable model, Claude Opus 5. To release the model both safely and quickly, we've tuned these safeguards conservatively—they'll sometimes catch harmless requests, though they trigger, on average, in less than 5% of sessions.... reply:...

I think they would benefit from asking Claude to list all contradictions and inconsistencies in that prompt which there are a few..In my experience instructions containing contradictions lead to diminished quality even outside the scope of the contradiction.

The full Claude Code system prompts are extracted every update and posted here, all 670 of them.https://github.com/Piebald-AI/claude-code-system-prompts/tre...

> If the conversation feels risky or off, saying less and giving shorter replies is safer and less likely to cause harm.Would be funny to ride the knife's edge and make otherwise harmless coding sessions "risky" just so the damn thing would stop replying in nested riddles for every basic request.

"Claude keeps responses focused, brief, and concise to avoid overwhelming the person."Claude and I must have a different idea of what brief and concise mean.

Weird that it spends so much of the prompt for Opus explaining that Mythos is better

I feel very little of this prompt is going to help the model write better code and most of it will actively work against that goal.Does anyone know if the ability to strip the default prompt with a proxy still works?https://docs.bswen.com/blog/2026-04-01-how-to-override-claud...

I compared the Claude Opus 4.8 and 5 system prompts, as well as the Claude Code Opus 4.8 and 5 system prompts, and neither show the alleged 80% reduction in system prompt size... Is the Claude Code system prompt leak incorrect? Do I not know what 80% looks like? Why such a large lie (so it seems)?Claude Code prompt leaks: https://github.com/asgeirtj/system_prompts_leaks/tree/main/A...Best source I can find about the 80% reduction: https://x.com/trq212/status/2080710971228918066

curious why dont they bake the system prompt in the model itself ? Why do we pay for these tokens on every API call ?These are just free $ for them, unnecessary bloating the context

I tried to use Fable on simple local JS/wasm obfuscated files and it refused to work despite multiple tries and different framing. I have no idea how can people get any security work done with it in all these blog posts.

> Claude does not use emojis unless the person asks or their immediately prior message contains one, and is judicious even then.

“Claude avoids saying "genuinely", "honestly", or "straightforward"”That’s interesting- they tried training the model to be genuine and honest (I’m not sure it actually made it honest), but now it says “genuinely” and “honestly” all the time and constantly stops to admit fault. Then they had to add to the system prompt to prevent it saying this.

It feels more and more like Anthropic has the best frontier team and the worst policy team. Dario being part of the latter.

> Claude avoids saying "genuinely", "honestly", or "straightforward". Claude is honest by default, and can state its point directly rather than trying to convince the person with the aforementioned modifiers, which come off as disingenuous.Hah! No it doesn’t.

Interesting that this is all written in the third person. I've only ever seen prompts written in second person. I'm going to have to experiment with that.

I have the feeling soon we'll see much more content in the DONT section: when religion will start entering the arena.

I feel the single mention of `currentDateTime` against a large number of date mentions (I counted six) in the system prompt gets it confused sometimes when it comes to dates. I actually see the same effect when querying via API (when I append my own system prompt with the current datetime at the end) - which makes me think there is a very similar hidden system prompt used on the API as well. I see this because of the nature of my queries - often filtering on some dates. It doesn't happen often, but certainly often enough, Claude thinks it's in an entirely different date.

22k characters of system prompt is crazy, and that is without the tool definitions.

Why are they so old though? June 9th is a long time ago for a Fable prompt. They haven't iterated on it since then? At the least we know it's outdated because it gives false info about the latest models, but you'd think they'd find other ways to improve it too

Paragraphs wasted on guardrails... Id happily have the dangerous but cheaper and better version

How can they be so lazy with updating their prompts (or is it just a case of these not being current)? Surely the prompts are an integral part of tuning their offering?: “Above Opus sits Anthropic's new Mythos tier. The first Mythos-class model, Claude Mythos Preview, is not currently available to the public”