UseAI glossary
System prompt
The set of instructions an AI assistant receives before the user's first message, and which the user usually never sees. Written by the service provider, it defines the AI's role, its tone, what it can do and what it must refuse. It explains why the same model behaves differently from one app to another, but it is not a foolproof security barrier.
In other words
The instructions given to a sales assistant before the shop opens: customers never hear them, but they shape every answer.
Alyriaz articles that mention it
- Claude Fable 5.1 and Mythos 5.1: agentic scores double and cache drops by 75%
Anthropic released Claude Fable 5.1 and Mythos 5.1 on 1 September, with a 75% drop in cache read pricing and sharply higher agentic scores. We analyse the release, the unusual structure of the two models, and what the distribution of gains reveals.
- Why doesn't the same AI respond differently depending on the app?
The same model produces very different responses depending on the application using it. We explain the five factors behind this gap, and why it's useful to know.
- What is a "system prompt"? The invisible instructions that shape every response
Every AI assistant receives, before your message, an invisible set of instructions that defines its role and limits. We explain what a system prompt is, what it contains, and why it accounts for many surprising behaviours.
- Why Claude, ChatGPT and Gemini don't look alike
Large models rely on very similar technologies, but adopt clearly distinct tones. We explain where this personality comes from, at which stage it is crafted, and what it implies when choosing a tool.
- What is "prompt caching"? The setting that could halve your AI bill
Prompt caching lets you avoid paying full price for the same context sent with every request. We explain the mechanism, the real savings, the pitfalls of cache lifetime, and why it's the first lever to pull before September's price hikes.
- On 1 September, Claude Sonnet 5 increases by 50%. And there's a catch almost no one mentions.
The promotional rate for Claude Sonnet 5 ends on 31 August 2026: from 1 September, input and output costs rise by 50%. But the real trap lies elsewhere: the new tokenizer counts more tokens for the same text. We explain the actual calculation and how to prepare now.