System Prompt

A special set of instructions provided to an LLM before the user's actual input. The system prompt defines the model's behavior, personality, constraints, and context for the entire conversation. It's usually invisible to the end user but fundamentally shapes every response. When an AI tool says it "understands your brand voice," it's typically using a system prompt that describes that voice in extremely specific terms.

Why it matters for writers: System prompts are where most AI tools implement style guide compliance, and where most of them hit a wall. Pasting a style guide into a system prompt works for simple rules ("never use passive voice"). It breaks down for complex, conditional rules ("use present tense for API methods, past tense for changelogs, imperative mood for procedures, except when describing optional steps in a troubleshooting context, in which case..."). This limitation is part of what motivated building Lexichord, which coordinates multiple AI capabilities instead of cramming everything into one system prompt and hoping for the best.

Related terms: Prompt Engineering · Large Language Model · Orchestration