Agent Architecture Concepts › What a shape is, and what it is not 12 of 12 steps written
01 — part 1, conceptsconcept

What a shape is, and what it is not

Agent architecture is the practice of deciding who asks whom, in what order, and what has to be true before that arrangement is worth building.

The definition

A shape is the arrangement of the parts: who asks the model, how many times, in what order, and who decides what happens next. That is the whole of it.

Six shapes appear in this volume. They run the same job, on the same input, against the same checks, using the same model with the same instructions. The only thing that differs between them is the arrangement — and that difference is worth up to a factor of four in cost and the difference between a right and a wrong answer.

the job the same lines in, the same nine boxes out, every time lines in nine boxes out one agent with tools shape 1 of 6 fixed stages shape 2 of 6 sort first, then handle shape 3 of 6 plan, then carry out shape 4 of 6 do, then check shape 5 of 6 split and rejoin shape 6 of 6 the arrangement changes completely the model is the same the prompt is the same the tools are the same and it is worth a factor of four
Lines in, nine boxes out, unchanged the whole way through. The arrangement in the middle changes completely and nothing about the job does. The model, the prompt and the tools are identical in all six.
Nothing in this scene is measured. It is what a shape is; step 5 prices the six.

Four things it is not

why it is not the shape
the modelhold it fixed and the six shapes still cost different amounts. Every figure in this volume is measured with one scripted model precisely so the arrangement is the only variable
the promptthe wording is the same in every shape here. What differs is how many times it is sent and what is in scope each time
the toolsall six shapes read the same files and look rates up in the same table. Reading a file is free; asking about it is not
the frameworka framework is an implementation of a shape, usually of one shape. Choosing a framework before choosing a shape is choosing the answer before the question

That last row is the reason this volume exists. Most writing on the subject names a shape and recommends it. Six articles will name six different shapes with equal confidence, and all six can be right about some workload — which is a fact about workloads, not about shapes.

Why the arrangement is the expensive decision

It is the one you cannot change later without rebuilding. The model can be swapped in an afternoon and the prompt in a minute. Who asks whom is the thing you have to live with — which is why step 4 argues that four of the eight decisions should be made before you commit to it.