How System Prompts Define Agent Behavior
February 23, 2026
Coding agents are fascinating to study. They help us build software in a new way, while themselves exemplifying a novel approach to architecting and implementing software. At their core is an AI model, but wrapped around it is a mix of code, tools, and prompts: the harness.
A critical part of this harness is the system prompt, the baseline instructions for the application. This context is present in every call to the model, no matter what skills, tools, or instructions are loaded. The system prompt is always present, defining a core set of behaviors, strategies, and tone.
Prompts may not be quite the hot topic they were three or so years ago when prompt engineering was the hot new role, but they still play a critical role in AI engineering and not just for engineers using models but actually for the model developers themselves.
Here is a breakdown of the system prompts, that is the prompts added to your own prompts when calling various models, and an analysis of the role they play in agentic systems.
