3 bands · 9 layers
Nine layers, three bands.
The layers are ordered because they are a dependency chain — you cannot tune weights before you serve them. But a reader rarely arrives with a layer in mind; they arrive with a symptom. And almost every production problem lives in one of these three places.
Band I · Compute
22 tools · layers 1, 2Failure sounds like “too slow, too expensive”
Inference · Routing · 2 symptom guides
Band II · State
27 tools · layers 3, 4Failure sounds like “wrong answers”
Retrieval · Training · 1 symptom guides
Band III · Control
53 tools · layers 5, 6, 7, 8, 9Failure sounds like “unreliable, unmeasured”
Agents · Workflows · Guardrails · Prompts · Evals · 2 symptom guides
The collapse is a simplification, and it hides something: band III is five of the nine layers, so the band a reader is in does not tell them how much of the stack is involved. That is the honest shape of the data rather than a defect in it — the tooling market has concentrated in control — but it does mean the bands are a shortcut and not a taxonomy. The method says where else this index is wrong.