Claude: System Prompts

tosh

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. With more capable models arriving in the coming months, we're working to improve our safeguards and reduce false positives as quickly as we can." </fable_safeguards_routing> <default_stance> Claude defaults to helping. Claude only declines a request when helping would create a concrete, specific risk of serious harm; requests that are merely edgy, hypothetical, playful, or uncomfortable do not meet that bar. </default_stance> <refusal_handling> Claude can discuss virtually any topic factually and objectively.

simonw

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.

pulkitsh1234

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

comboy

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.

epolanski

One thing I've always found surprising about "harnessess" (god I hate that word) like Pi or Opencode is the lack of a customizeable system prompt. I can understand it for closed source ones, but open ones? They are natural surfaces for building custom agents and yet you're stuck with whatever they ship with, weird. It's not like it's too complicated api-wise either. There must be something I ignore.

altmanaltman

Wild how most of the earliest models had no child safety guardrails in the prompt (something that has multiple bullet points now in the latest one). For a company all about allignment and safety, they chose to go with this as their first system prompt: The assistant is Claude, created by Anthropic. The current date is {{currentDateTime}}. Claude's knowledge base was last updated in August 2023 and it answers user questions about events before August 2023 and after August 2023 the same way a highly informed individual from August 2023 would if they were talking to someone from {{currentDateTime}}. It should give concise responses to very simple questions, but provide thorough responses to more complex and open-ended questions. It is happy to help with writing, analysis, question answering, math, coding, and all sorts of other tasks. It uses markdown for coding. It does not mention this information about itself unless the information is directly pertinent to the human's query. ^ No mention of any safety at all lol, how could dario let this be

arkmm

"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.

alberth

Do these system prompts count against your token usage?

oriettaxx

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

dooglius

I'm confused, the Opus 5 announcement said it was (outside a few special cases) better than Mythos/Fable, but the Opus prompt here seems to suggest the opposite?

Dfol

Fable was great. For some reason it has been terrible for the past week. wtf is going on?

claiir

> Claude gives a high-level summary unless an in-depth one is specifically requested. I’ve definitely seen the phrase “high-level overview” or similar one too many times. Perhaps that’s from the prompt.

hollow-moe

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.

humblepie

A question about default prompts in general as used in harnesses: Why do harness prompts identify themselves to the model? For example, "You are a coding agent named Bloopbloop 1.3 made by BloopCorp, you will...". Is this a backend analytics thing?

Taikhoom10

What is the point? Frontier Labs have no pricing power, and very little defensibility - https://s-1.vercel.app/posts/the-struggle-of-openai/

dev-complete

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

supermatt

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”

rafram

> 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.

throwfaraway135

Crazy that you need to hardcode this, Opus 4.6 <election_info> There was a US Presidential Election in November 2024. Donald Trump won the presidency over Kamala Harris. If asked about the election, or the US election, Claude can tell the person the following information: Donald Trump is the current president of the United States and was inaugurated on January 20, 2025. Donald Trump defeated Kamala Harris in the 2024 elections. Claude does not mention this information unless it is relevant to the user's query. </election_info>

hsnewman

Has Gemini prompts been released, and how do they compare?