GlossaryFloor 2 · The Harnessthe block and its bolted-on plates: what gets added to itFloor 2 · The Harness
harness
No. 003 · v2026-08FR: harnaisThe harness is everything built around a model to make it useful: instructions, documents, tools, guardrails. Like a car around an engine: without a body, seats or steering wheel the mechanics still turn, but nobody goes anywhere.
What it is not
The harness is not the model, and this is the most expensive confusion of all. When a product retrieves a document, cites its sources or refuses a request, it is almost never the model doing so: it is the harness. The model only knows how to produce text from what it is given to read, everything else has been built around it. Changing model therefore does not repair a badly designed harness.
In depth
What it assembles
The harness is the layer that turns a model into a product. On every request, it is the harness that assembles what the model will read: the standing instructions, the history of the exchange, the documents retrieved for the occasion, the description of the available tools. It then receives the text produced, interprets it, possibly carries out an action, then starts again. The model itself keeps nothing between two calls: any continuity the user perceives is a construction of the harness.
Harness or model
The richness of a harness is often mistaken for the power of a model, because both improve the answer. The distinction is easy to check, though: whatever depends on the harness can be corrected without retraining anything, by changing an instruction, a document or a rule. That is where memory, document retrieval, tools, moderation and output format live. It is also where the greater part of the engineering work happens, while public attention goes to the model.
It is not neutral
The first trap is to believe that a harness is neutral: every assembly choice steers the answer, and two products built on the same model behave differently. The second is to forget that everything the harness inserts becomes text read by the model, with no guaranteed hierarchy between an internal instruction and the content of a document: this is the door through which attacks by hidden instruction come. The third is cost: the more the harness gives to read, the more each request is paid for, and the more the answer risks being diluted. A harness is therefore designed by subtraction as much as by accumulation.
Relations where the neighbours live
- Requires
- Floor 1 · The Modela solid block on its own: the prediction machinemodelFloor 2 · The Harnessthe block and its bolted-on plates: what gets added to itpromptFloor 2 · The Harnessthe block and its bolted-on plates: what gets added to itcontext
- Often confused with
- Floor 1 · The Modela solid block on its own: the prediction machinemodel
- Related comparisons
- Model or Product
Check 3 questions · click your answer
Level 1 · Recognise
A question-and-answer service retrieves the right page of an internal manual before answering. Where does this capability sit?
Level 2 · Distinguish
Your product answers badly on your internal procedures. What falls to the harness first?
Level 2 · Distinguish
Two products use exactly the same model and give very different answers on the same subject. How is that possible?
Try it 1 practice
Concrete things to try where this term comes up, in ten minutes.
Who works with this 3 roles
The roles for which this term is part of the ordinary work.
Lexigraph, "Harness", v2026-08, https://www.lexigraph.org/en/harness/, CC BY 4.0.