---
title: 'How do I choose a model?'
description: 'Where each model id is set, how to read the live catalogue, and how to measure a swap instead of guessing at it.'
position: 2
---

# How do I choose a model?

> Stackbone does not rank models for you. What it gives you is one place to set
> each model, a live catalogue to read, and eval suites to measure a swap once
> you have made it.

Three model ids come from your own project, and each is set in a different
place. The agent's reasoning model is the `model` you pass to
`defineDeepAgent(...)` in its `index.ts`, and it drives the main loop. A one-off
completion, embedding or image goes through `stackbone.ai`, where you pass
`model` on the call itself. Retrieval takes the `model` you pass on the
`stackbone.rag` call, and the `rag.embeddingModel` key in `agent.yaml` parses
but changes nothing. Guardrails and evals ask their own models, which you pick
in Studio settings rather than in code.

To compare candidates, read the catalogue. `stackbone.ai.models.list()` returns
whatever the provider reports for each model, which on OpenRouter includes
`context_length` and `pricing`. Every id uses the `creator/model-name` form, so
switching is a one-line edit.

Then measure the change. An **eval suite** runs your saved cases against the
agent and reports a pass rate against a pass mark, and that number belongs in
CI.

## Read more

- **[Gateway](/docs/home/features/gateway)**: where each of those ids is set,
  and the one endpoint they all resolve through.
- **[`stackbone.ai`](/docs/sdk/platform/ai)**: the catalogue call and every
  `ai_*` error code.
- **[Testing and evals](/docs/faqs/building/testing-and-evals)**: how to measure
  a model swap.
- **[Which models can I use?](/docs/faqs/general/supported-models)**: providers
  and the ids they resolve.