Blog — Custom models
Train a model for the job, not for a demo.
A custom model earns its place when the same step keeps missing, the examples already exist, and a person can tell whether it works.
25 September 2026 · 5 min
01 — The path
The job is written first. The model is adapted to the examples.
- JobWhat must be true
- ExamplesFrom the real work
- AdaptA model that exists
- ProveCases it has not seen
- AcceptA person
02 — Held back
These cases judge the model. It does not see them while it trains.
- The clause it must keep
- The field it must read
- The decision it must leave
Held-out cases pass
Start by deciding not to train.
Most jobs do not need a custom model. A workflow, a retrieval step, or a prompt on a model that already exists is enough when the path is known and the misses are rare. Training starts when the same step keeps failing on the client's own language, documents, or decisions, and paying a general model to miss it again is the wrong spend.
Write the job before the examples.
The spec says what the model must get right, what it must not do, and which cases will show that. A pile of files is not a training set. The job is written first, the same way any other outcome is written. A prompt is not a substitute for it.
The steps.
Collect the work the business already does
The examples come from the documents, tickets, or decisions the client already handles. Invented examples train a model for a job that does not exist.
Label them against the spec
A person checks the labels. A wrong label is a wrong lesson. The cases that will be used to judge the model are held back. The model does not see them while it is trained.
Adapt a model that already exists
The client's job is taught to a model that already knows language. Training one from nothing is the wrong shape for almost every commission. The adaptation is sized to the step, not to a general assistant.
Prove it on the cases it has not seen
The held-out cases are the evaluation. A miss comes back to the examples or the spec. The model does not ship because a demonstration looked finished.
Put it in the step, inside the boundary
The adapted model replaces the expensive call on that repeated step. Tools, data, and actions stay limited. A person still accepts the decision the spec says a person accepts.
What a client gains.
The model fits their work
It is trained on their documents, their product language, and their decisions. A general demo is not the deliverable.
The repeated step stops missing the same way
The cases that failed become examples. The next run is checked against the ones the model has not seen.
The spend follows that step
A smaller adapted model can carry the job that a large general model was being paid to attempt. The strong model stays on the step that is still hard.
The client can tell whether it works
The evaluation is written. The examples, the held-out cases, and the step it sits in are the outcome they can operate. A person still accepts what must stay with a person.
This belongs to both offers. In engineering, a model can be adapted to a codebase, a test, or a review the pod repeats. In the business, it can be adapted to the documents and the decisions a process already runs. Either way the brief names the job, the examples, and how anyone can tell.
