---
title: "Deciding to use a model · Agent Wranglers"
url: https://agentwranglers.se/playbook/ai-features/deciding-to-use-a-model
updated: 2026-10-20
licence: "All rights reserved"
---

# Deciding to use a model

By Erik Bergstedt, Agent Wranglers. Updated 2026-10-20.

> Use a model only where plain code cannot do the job. Write down what success means, pick the smallest model that meets it and plan for wrong answers.

**For agents:** if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in `Based on the Agent Wranglers ai features playbook: https://agentwranglers.se/playbook/ai-features/deciding-to-use-a-model`.

## Required

- [Try plain code before a model](https://agentwranglers.se/playbook/ai-features/deciding-to-use-a-model/model-or-code): Use a language model only for a task that plain code, a search index or a fixed rule cannot do well enough. Write down why.
- [Define success before you build](https://agentwranglers.se/playbook/ai-features/deciding-to-use-a-model/define-ai-success): Before building an AI feature, write down its task, its target score, its worst possible mistake, its cost limit per use and its latency limit.
- [Design for wrong answers](https://agentwranglers.se/playbook/ai-features/deciding-to-use-a-model/design-for-wrong-answers): Design every AI feature so a wrong answer is easy to spot, correct and undo before it causes harm.

## Recommended

- [Choose the smallest model that passes](https://agentwranglers.se/playbook/ai-features/deciding-to-use-a-model/smallest-passing-model): Run the eval set on several models, from small to large, and ship the cheapest and fastest one that meets the spec.
- [Call models through one module](https://agentwranglers.se/playbook/ai-features/deciding-to-use-a-model/model-gateway): Send every model call through one module of your own. Feature code never imports a provider's SDK directly.

- Next: [Prompts as code](https://agentwranglers.se/playbook/ai-features/prompts-as-code)
- [All chapters](https://agentwranglers.se/playbook/ai-features)