Harness Engineering: Tools, Memory, and Context

An agentic system is a reasoning model operating inside an engineered environment. The model generates decisions. The harness gives it the tools to act on those decisions, the memory to carry information forward, and the context to reason accurately at each step. The two parts are inseparable: the model decides what to do, and the harness determines whether that decision can be executed safely and reliably.

The harness is where most enterprise deployment decisions live. A team that focuses on model selection and treats the harness as an afterthought builds a system that works in demonstrations and fails under real operational conditions. Production failures in agentic systems almost always trace back to a harness component that was underdefined, misconfigured, or simply missing.

For the FDE, understanding how each harness component operates is the foundation for scoping agentic projects accurately, validating behavior during the pilot, and handing off a system the customer’s team can own.

The reasoning loop

The reasoning loop is the operating cycle that drives every agentic system forward. Each iteration moves through four phases:

• Perceive: The agent receives the current state of the task: what it knows so far, what tools have returned, any new input from the environment, and what it still needs to accomplish.

• Reason: The model processes that state and determines the next action. This is where the model contributes: selecting what to do next based on everything in its current context.

• Act: The agent executes the chosen action, typically by calling a tool, retrieving information, or generating an output.

• Observe: The agent receives the result of that action and incorporates it into its updated picture of the task.

The loop repeats from the top until the agent reaches its ...