Definition
A prompt is input used to condition a model's next output. It can contain a request, instructions, examples, source material, output constraints, and data to transform. In a simple interface, the prompt may be the sentence a user types. In an API or agent harness, people also use the word for the complete input assembled from multiple messages and other context.
That ambiguity matters. A user prompt is not necessarily the full prompt the model receives. The runtime may prepend system or developer instructions, include previous turns, serialize tool definitions, retrieve documents, and append tool results. When debugging behavior, inspect the assembled request and its trust boundaries rather than only the visible user message.
Operational significance
A prompt influences model behavior but does not enforce it. Text that says "only use approved tools" is not a substitute for an allowlist, and text that requests JSON is not a substitute for schema validation. Untrusted documents can also contain instructions that compete with the intended task. The application must preserve the difference between instructions and data even though both may arrive as input tokens.
Prompts are model- and version-dependent. Record the model, relevant decoding controls, context assembly, and evaluation cases when a prompt matters to production behavior.
Distinguish it from nearby terms
- A system prompt is an instruction layer supplied by the application. It is one possible part of the assembled prompt, not a security boundary by itself.
- Context is everything available to the model for the current call. A prompt may refer to one message or, less precisely, to that whole assembled context.
- A specification defines required behavior and acceptance criteria. A prompt communicates intent to a probabilistic model.
- Prompt engineering is the process of designing and evaluating prompts.
Check your understanding
The user sends "summarize this file," but the model follows an instruction embedded inside the file. Explain which prompt boundary was confused and which control belongs outside prompt text.