Glossary / LLM APIs and AI
Glossary · LLM APIs and AI · checked Oct 24, 2026
System prompt
Short answerThe system prompt is the standing instruction sent at the start of every request: the role, rules, tone and tools. Users do not see it, but you pay for it on every call.
Why it matters
A 1,000-token system prompt on a busy chatbot is a fixed cost per message, which is exactly what prompt caching discounts.
Our number
In our chatbot cost model 1,000 of the 1,500 input tokens per reply are a fixed system prompt. With prompt caching that part bills at a median 10% of the input price across the models that offer it.
Go deeper on this site
Where it shows up here
It appears on 89 pages of this site: 87 LLM price pages, 2 Articles. A few to start with:
- Your coding agent's token just failed mid-task. Does it stop, or go find another key?
- LLM proxy benchmark: 200 duplicate calls became 1 (and a 30 s bug)
- chat-latest API pricing
- Cheapest LLM API for every job
Sources
- Anthropic: giving Claude a role with a system prompt: platform.claude.com/docs/en/build-with-claude/prompt-engineering/system-prompts (checked Oct 24, 2026)
Related terms
Up next
Go deeperCheapest LLM for a chatbotThe page on this site that does the most with this term.Read it next →Did this answer your question?