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: it gives a fair, accurate account rather than sharing personal opinions, and points to the linked statement for anything further. Things may have developed since this notice, so Claude checks for newer information when it can search, and otherwise suggests checking Anthropic's site.

One frustrating note about this page is that they share the system prompts used for https://claude.ai and the Claude mobile apps regular chat, but they omit the tool definitions. Those are much more interesting if you want to understand what Claude can actually do for you. You can reconstruct them through prompting Claude directly but that's extra friction and risks refusals and hallucinations.

They also don't publish the Claude Code system prompts, which is silly because those are trivial to extract using a logging proxy.

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eterm 6 hours ago | parent | next [-]

It'd be ironic if the "Opus 5 nerf" effect is from telling Opus that it sits a tier down from Fable and Mythos, while Opus4.8 believed it was the best of the best, just a note that it was "Preceded by Mythos".

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tosh 5 hours ago | parent | next [-]

i'd not be surprised if the current system prompt negatively affects performance

at the least it takes away thousands of tokens in the most important part of the context window (!)

also see the comment by comboy on contradictions not helping performance

the system prompt is the most important part of the instruction you can give the model

it comes before everything else + the model is trained to pay extra attention to it

edit: that's also why in smol (minimalist agent harness) there currently is no system prompt at all (you can add one easily if you want to though)

https://github.com/smol-env/smol

the context window is precious

it should be filled with your task and helpful context for that task

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swingboy 5 hours ago | parent | next [-]

Pretty sure Anthropic and other providers prepend these "official" system prompts to your conversation even if you send in a custom system prompt otherwise it would be trivial to produce CSAM, etc.

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ardel95 3 hours ago | parent | next [-]

CSAM, and other harms, are typically detected using a set of specially trained, faster and cheaper models (and out of band matching techniques) that run before and after the main model.

Any mention in the system prompt is mostly defense in depth, and to make refusals more graceful.

	▲	whstl an hour ago \| parent [-]
		Also, the system prompt, or even something reinforced on every message, is nowhere near as strong as its internal training or as an external safeguard. If the prompt were the only protection, it would be extremely easy to produce illegal content after a long session.

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fullmoon 4 hours ago | parent | prev | next [-]

I don’t think so. If you start a new Claude Code session without a system prompt, it doesn’t even know what model it is and hallucinates being some old variant of Sonnet.

	▲	flaburgan an hour ago \| parent [-]
		How do you start a session without a system prompt if you use ACP in Zed for example?

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LPisGood 4 hours ago | parent | prev | next [-]

The system prompt is (and cannot be) the only guardrail against things like that, because any system prompt is little more than a good suggestion.

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CodesInChaos 4 hours ago | parent | prev | next [-]

I wouldn't put auch limitations in the system prompt. A mix of fine-tuning and out-of-band detection appears to be a better fit.

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tosh 4 hours ago | parent | prev | next [-]

at least according to their documentation they do not

afaiu they have other systems for denying and re-routing requests

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DANmode 4 hours ago | parent | prev [-]

They use non-LLM gates for this.

Otherwise DANmode and similar jailbreaks would still be as easily accessible as they were at the beginning.

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SubiculumCode 2 hours ago | parent | prev [-]

I wonder whether adding that it is as good or better than Mythos, and that genius is 99% perspiration, just 1% inspiration to your prompts...

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KellyCriterion 6 hours ago | parent | prev | next [-]

Curious:

Cant it spin up a webbrowser in the background and go to claude.ai and play with the sibling models and "find out" about it rank? :-D

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ameliaquining 5 hours ago | parent [-]

The claude.ai frontend contains defenses against automated access.

	▲	monkpit 4 hours ago \| parent [-]
		I’m sure you can use a warm chrome session over CDP no problem

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mcbuilder 6 hours ago | parent | prev [-]

Nah, that's the same sort of thinking that makes people type "make no mistakes", I don't make my model roll play, etc. I believe that the longer the system prompt and the more you cram in it the worse the model does. You need the human doing minimal prompts, but in the right direction. Take a look a the transcripts of Terrance Tao with ChatGPT

	▲	eterm 6 hours ago \| parent \| next [-]
		My comment was a bit tongue in cheek, I'm not actually convinced there was real degradation in opus 5 beyond a tendency to try to plough ahead without stopping to clarify things. I don't really think 1 line in lengthy system prompt affects things that much, it'd just be an amusing form of emergent behaviour where we now have to massage the ego of something with no id.
	▲	8n4vidtmkvmk 2 hours ago \| parent \| prev [-]
		For complex projects with lots of internal tools and strict requirements, I'm finding a fairly lengthy system prompt is quite worth it. Start short or empty and watch where it makes mistakes then just keep tuning it so they're less frequent. That works for me.

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5 hours ago | parent | prev [-]

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