System Prompt

Concept

Also known as: System Instruction

Instruction text sent to the model before the user’s input that establishes a framework without enforcing it

A System Prompt is a text that precedes the actual user prompt and defines the role, tone, or rules for the model’s responses. The model processes the System Prompt and the user’s input within the same context. The System Prompt establishes a framework rather than an immutable rule.

What is a System Prompt?

A System Prompt is a text that precedes the actual user prompt and defines the role, tone, or rules for the model’s responses. The model processes the System Prompt and the user’s input within the same context—the System Prompt establishes a framework rather than an immutable rule.

How it works

Many chat models distinguish the System Prompt and the user’s input through special control tokens or chat templates with different roles. This distinction influences the model’s learned behavior, but it does not enforce the instruction like an access control rule implemented outside the model—the System Prompt merely shifts which continuations become more likely.

Why a System Prompt is sometimes ignored

A System Prompt can be overridden by later context when that context is long and strongly focused on a different subject—for example through foreign-language technical terminology or quoted source code. Prompt Injection follows a related principle: an instruction placed later in the context competes with the System Prompt for influence over the same probability distribution.

System Prompts do not change the model’s parameters—they affect only the current inference context. Every new conversation starts without previous System Prompts unless they are explicitly set again.

Why System Prompts have practical implications

Not access control: A System Prompt is not a hard access control rule like a server-side permission check, but a shift in weighting within the same computation as the rest of the context.
Competition for probability: Long or conflicting user inputs can weaken the influence of a System Prompt.
No memory: Unless it is explicitly set again, a System Prompt does not automatically apply to a new conversation.

Typical example

A System Prompt such as “Always answer in German” noticeably increases the probability of German tokens but does not exclude English tokens completely. If the surrounding context is strongly dominated by English, the instruction can lose effectiveness.

Sources

Hauptquellen

Vertiefung

Attention Is All You Need (Transformer Paper) [extern]

Sources archived on: 2026-08-02