System Prompt in plain English.

A system prompt is a set of instructions sent to an AI model before the user's actual message, invisibly setting the model's persona, rules, tone, and boundaries for the whole conversation. When ChatGPT tells you it's "a helpful assistant" or refuses to discuss certain topics, that behavior is usually defined by a system prompt written by the company deploying it, not something the underlying model does unprompted. Developers building on top of GPT-5.6, Claude, or Gemini write custom system prompts to turn a general-purpose model into a specific product — a customer support bot that only answers questions about one company's products, or a coding assistant that always responds in a certain style. System prompts are powerful but not airtight: a sufficiently determined user can sometimes get a model to reveal or override its system prompt, which is why sensitive logic, like real security rules, should never rely on the system prompt alone.