AI
System Prompts
A practical guide to understanding what system prompts are and how they shape AI behaviour in products.
What system prompts do, how they define an AI's role and constraints, and what product and design teams need to know when working with them.
What it is
A glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term is a set of instructions given to an glossaryModelA model is a system or representation used to process data and generate outputs, often trained to perform specific tasks.Open glossary term before a user interaction begins. It defines how the model should behave throughout the conversation.
glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term typically specify the AI's role, tone, glossaryConstraintsConstraints are limitations or restrictions that impact how a product or solution can be designed or built.Open glossary term, goals, and any rules it should follow. They are invisible to the user but shape every response the AI produces.
For example, a glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term might instruct the AI to act as a customer support agent for a specific brand, respond only in formal language, never discuss competitors, and always recommend contacting a human for billing queries.
Unlike a user's message, which changes with every glossaryInteractionInteraction refers to any action a user takes within a product and how the system responds. It includes clicks, taps, gestures, and inputs that drive the user experience.Open glossary term, the glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term is consistent and persistent across all conversations.
glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term design is a core product and design responsibility — not just a technical one.
When to use it
Understand when glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term design matters most.
It is most relevant when:
It is less relevant when:
Key takeaway
If you are building an AI feature, the system prompt is your first and most important design decision. Get it wrong and no amount of user-facing design will fix the behaviour.
How it works
Understand the basic mechanism. When a conversation begins, the glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term is passed to the glossaryModelA model is a system or representation used to process data and generate outputs, often trained to perform specific tasks.Open glossary term as context before any user input. The model uses this to establish a framework for how it will respond.
A well-written glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term makes the glossaryModelA model is a system or representation used to process data and generate outputs, often trained to perform specific tasks.Open glossary term's behaviour more predictable, consistent, and appropriate to the context. A poorly written one leads to inconsistent responses, off-brand behaviour, or outputs that do not meet user needs.
glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term can include role definitions, behavioural rules, tone guidance, content restrictions, output format requirements, and information the AI needs to know about the glossaryContextThe surrounding conditions that shape behaviour and decisions.Open glossary term it is operating in.
What this means for designers and product teams. glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term design is iterative. You write, test, observe the outputs, and refine — the same way you would iterate on any other product design decision.
Changes to a glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term can have significant and unpredictable effects on the AI's glossaryBehaviourBehaviour refers to how users interact with a system, including actions, patterns, and responses.Open glossary term. Testing after every change is essential.
glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term also need to be maintained. As the product evolves, the glossaryContextThe surrounding conditions that shape behaviour and decisions.Open glossary term changes, or new edge cases emerge, the system prompt will need updating.
What to look for
Focus on:
Where it goes wrong
Most issues come from: Vague instructions produce unpredictable glossaryBehaviourBehaviour refers to how users interact with a system, including actions, patterns, and responses.Open glossary term — precision in glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term design is not optional.
What you get from it
Understanding glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term gives you:
Key takeaway
The system prompt is the instruction set for your AI's behaviour. It deserves the same rigour as any other design decision.
FAQ
Common questions
A few practical answers to the questions that usually come up around this method.
What is a system prompt?
A glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term is a set of instructions given to an glossaryModelA model is a system or representation used to process data and generate outputs, often trained to perform specific tasks.Open glossary term before the user starts interacting with it. It defines the AI's role, tone, constraints, and rules. Users typically do not see it, but it shapes every response the AI gives.
How is a system prompt different from a user prompt?
A user glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term is what the person types during a conversation. A glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term prompt is what the product team writes in advance to configure the AI's glossaryBehaviourBehaviour refers to how users interact with a system, including actions, patterns, and responses.Open glossary term. The system prompt is persistent and consistent — the user prompt changes with every interaction.
Can users see or change the system prompt?
Not in most products. glossarySystemA system is a collection of interconnected components that work together to achieve a specific function or outcome.Open glossary term glossaryPromptA prompt is the input or instruction given to an AI system to guide its output or response.Open glossary term are set by the product team and are not visible to users. In some glossaryDeveloperA developer builds and maintains the technical implementation of a product.Open glossary term tools and API contexts they may be more visible, but in consumer products they are typically hidden.
How long should a system prompt be?
Long enough to cover the AI's role, key glossaryConstraintsConstraints are limitations or restrictions that impact how a product or solution can be designed or built.Open glossary term, and any important glossaryContextThe surrounding conditions that shape behaviour and decisions.Open glossary term — but not so long that it becomes contradictory or dilutes the most important instructions. glossaryClarityClarity is how easily users can understand what is happening and what they need to do.Open glossary term and precision matter more than length. A focused, well-written system prompt will outperform a long, vague one.
Who is responsible for writing system prompts?
It is a shared responsibility between product, design, and engineering. Product and design should own the behavioural and experience decisions — what the AI should and should not do, what tone it should use, what glossaryConstraintsConstraints are limitations or restrictions that impact how a product or solution can be designed or built.Open glossary term it should operate within. Engineering typically handles the glossaryIntegrationIntegration is the process of connecting different systems, tools, or services to work together.Open glossary term. In practice, all three need to collaborate on testing and glossaryIterationIteration is the process of repeatedly improving a product through cycles of testing, feedback, and refinement.Open glossary term.
Quick take
System prompts are where you define how an AI behaves before the user ever types a word — and they are one of the most important design decisions in any AI product.
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