System prompt
The instruction layer your users never see
What is a system prompt
A system prompt is a set of instructions sent to the model at the beginning of every conversation. It defines the model's behaviour, tone, knowledge boundaries, and output format. The user never sees it — it runs silently behind every response.
System prompts are typically 100–2,000 tokens long. They are sent as input on every API call, which means their cost multiplies by call volume. A 1,000-token system prompt at $3.00/1M adds $0.003 per call — $3,000 across a million calls.
Why it matters
The system prompt is the most cost-sensitive text in your application. It runs on every request. Every unnecessary word in it costs money on every call, forever. Trim it once and save across all future interactions. sourc.dev tracks system prompt support as a capability flag.
Verified March 2026 · Source: sourc.dev entity data