---
title: "Chapter 10: Real Tools"
description: "Expand from one tool to six — sandboxed — and understand why every additional tool multiplies the protocol surface your small model has to learn."
type: lesson
order: 10
chapter: "04-the-verifier"
---

# Chapter 10: Real Tools

## Objective

Expand to a real toolset, then build the thing everything else depends on: an evaluation harness that produces a **binary, mechanical, uncheatable** pass/fail on real tasks.

## Why This Is the Module People Skip

Without a verifier you cannot answer "did the fine-tune help?" — so you tune on vibes, and vibes on a 1B model are indistinguishable from noise. Worse: Part 5's entire premise is *filtering trajectories by whether they succeeded*. No verifier, no filter. No filter, no flywheel.

Coding is the ideal domain for this precisely because success is mechanically checkable. **Tests pass, or they don't.** You do not need a reward model, an LLM judge, or human labels. You need a test suite and an exit code. Every other domain would kill for this.

## The Toolset

```
read_file(path)
list_dir(path)
grep(pattern, path)
edit_file(path, old, new)
run_tests(target)
git_diff()
```

Six tools. **Resist adding more.** Each one multiplies the protocol surface the small model has to learn, and every tool you add needs training examples covering it.

## Sandbox the Dangerous Ones

Sandbox the write and execute tools (`edit_file`, `run_tests`). Container, git worktree, whatever — the agent will do something stupid and you want to `git checkout .` and move on.

---

[← Part 4 Index](/fine-tune-local-agent/04-the-verifier/) · [Next: Chapter 11 →](/fine-tune-local-agent/04-the-verifier/11-the-eval-set/)