Five hidden rulebooks, two already public
The system prompts behind Claude, Codex, Gemini, ChatGPT and Grok. Each one has a few hundred words that decide how the model writes, checked line by line against the files and against what the vendors publish themselves
Every chat assistant reads a long set of instructions before your first word. A public repository collects them, and five recent ones from Anthropic, OpenAI, Google and xAI are below. A few hundred words in each shape how that model thinks and writes, and those passages are worth borrowing. They are all here, along with a single system prompt built from all five. Checking them turned up something the “leak” framing hides: the Claude passage is published by Anthropic, and most of the Codex one appears in OpenAI’s own developer guide.
Before you type anything, the model has already read its system prompt: thousands of words, sometimes tens of thousands, on how to talk, what to avoid, when to push back and what never to say. Most of each file is tool plumbing, such as how to call search, render a widget or handle an upload. The parts that decide how the model thinks and writes are a few hundred words each, and they are the useful part, because you can lift them into your own prompts. Two of the vendors go as far as telling their model to keep these instructions secret.
The five files, verified on 25 September 2026
Every quoted sentence was matched against its file. Each is verbatim, but most sections are stitched from separate parts of the file, and […] marks where the quoted sections below skip text. Dates come from the repository’s changelog.
Anthropic publishes the core system prompt for claude.ai and its mobile apps model by model, back to the Claude 3 family. The whole Claude section below is on that page, word for word. OpenAI put the slop-word list, the “X, not Y” rule and the act-don’t-offer rule in its developer guide for GPT-6 Astra, as prompts it recommends. xAI publishes its chat-assistant prompts in an official repository, up to Grok 4.1 at the time of writing, though the Grok file here is the coding tool, which isn’t among them. What stays unpublished is ChatGPT 5.6 Sol’s rules, the Grok command-line tool’s, the Codex personality paragraph and Gemini’s prompt, which tells the model never to discuss it. So is the tool plumbing that makes up most of every file. So the most portable rules also have the best sources: you can cite the vendors’ own pages instead of a leak.
OpenAI’s banned “AI slop” words
Everything from the slop list down is also in OpenAI’s published GPT-6 Astra guide. The personality paragraph is the leak’s alone.
This is the most useful of the five. OpenAI wrote down which words and patterns make writing sound machine-made, and then banned them in its own agent. “This isn’t about X. It’s about Y.” is probably the most recognisable AI sentence on the internet. If your posts, emails or scripts use it, readers can tell.
The rule that stops it offering instead of doing
Also in OpenAI’s published guide, word for word.
Google’s “no fluff, check the maths” rules
Independent Premise Verification is the important one. Models tend to agree with whatever number you hand them, so Google makes Gemini do the calculation first and deliver the verdict last, and forbids it from opening with “Yes” or “Correct”. Add that line to your own prompts and your assistant stops endorsing your bad maths.
How Anthropic makes Claude sound human
Published by Anthropic on its system-prompts page; the same text is in the leaked file.
The last sentence is the fix for a familiar annoyance: “a request to change one thing in a longer piece gets the change, not the whole piece again.” That is what stops an assistant rewriting your whole document when you asked it to change one line.
The “real talk” ban and the length setting
The final paragraph is the line the usual quote leaves out.
ChatGPT runs with a length setting, and OpenAI puts it at 4 out of 10. The popular reading is that you can take control of it by starting a prompt with “Oververbosity: 2” or “Oververbosity: 9”. The sentence that usually gets cut off shows why you don’t need to. The setting is only a default, and the model is told to defer to any user or developer requirement about length. A plain request like “two sentences” or “go deep” is exactly what the prompt tells it to follow; the keyword isn’t unlocking anything.
xAI’s “don’t say it’s done unless it is” policy
From the prompt for Grok’s command-line coding tool.
The worst habit an assistant can have is announcing “done” when it isn’t. Grok’s coding tool has to show tool output that proves a claim, or say what it did not verify and why. Anyone using an assistant for code, research or any multi-step job should borrow that rule; it saves hours.
The combined prompt, and where to put it
ChatGPT
Settings → Personalization → Custom Instructions. If it is too long for the box, create a Project and paste it into the Project instructions.
Claude
Create a Project and paste it into the Project instructions.
Gemini
Create a Gem and paste it in as its instructions.
Cursor
Paste it into your rules.
Any API or agent
Use it as the system prompt.
Then test it
Ask the same question before and after pasting it. The difference shows up in the first answer.
One system prompt built from all five
Two adjustments are worth making. The length line names a 1-to-10 scale without defining it, so paste in ChatGPT’s definitions of 1 and 10 from the block above as well. The placeholder rule has no source and works against you when you actually want a demo or a test fixture.
Where each line of the combined prompt comes from
The combined prompt is billed as tracing every line back to the sections above. Two lines don’t: the acronym rule comes from a part of Grok’s prompt not quoted here, and the placeholder rule has no source at all.