Chapter 01: Environment Setup

What We’re Doing

We’re setting up a reproducible development environment using mise (a version manager), bun (a fast TypeScript runtime), and getting an API key for OpenRouter so we can talk to LLMs.

Step 1: Install mise

mise manages your development environment. It’s like asdf or nvm but faster and simpler.

Open a terminal and run:

curl https://mise.run | sh

You may need to restart your terminal afterward. Verify it’s working:

mise --version

Step 2: Install bun

With mise installed, getting bun is a single command:

mise use bun

This installs the latest version of bun and makes it available in your shell. Verify:

bun --version

Step 3: Get an OpenRouter API Key

OpenRouter is an API gateway that gives you access to hundreds of models through a single API key. Instead of managing separate keys for Anthropic, Google, and OpenAI, you have one key that routes to all of them.

1. Go to openrouter.ai/settings/keys
2. Create an account
3. Add a credit card if required (you’ll spend maybe $1–5 for this entire tutorial)
4. Create a new API key

Save the key in a .env file in your project directory:

echo 'OPENROUTER_API_KEY=sk-or-v1-your-key-here' > .env

Never commit .env to git. Make sure it’s in your .gitignore.

Step 4: Choose Your Model

Set the model you want to use:

echo 'AGENT_MODEL=google/gemini-3-pro-preview' >> .env

Alternative: if you prefer Claude, use anthropic/claude-opus-4.5.

As of this writing, Gemini 3 Pro Preview and Opus 4.5 work best for this tutorial. But feel free to experiment — one of the benefits of OpenRouter is that swapping models is just changing an environment variable.

Step 5: Create mise.toml

Create a mise.toml to lock in your tool versions:

[tools]
bun = "latest"

Step 6: Create a Project Directory

mkdir weekend-coding-agent
cd weekend-coding-agent

Copy your .env and mise.toml into this directory.

What We Have Now

weekend-coding-agent/
├── .env              # OPENROUTER_API_KEY and AGENT_MODEL
├── .gitignore        # includes .env
└── mise.toml         # bun version locked

You’re ready to write your first agent. Let’s go.

The Source Files (for Reference)

The complete project structure is available at src/. Here’s what we’re building toward:

src/
├── index.ts           # Entry point with REPL
├── agent.ts           # Core agent loop (runTurn)
├── lib/
│   ├── api.ts         # OpenRouter client, model stats
│   ├── logger.ts      # JSONL session logging
│   ├── readline.ts    # Terminal input handling
│   └── types.ts       # Message, CompletionResponse interfaces

But we’ll build everything step by step. The agent will help.

← Part 1 Index · Next: Chapter 02 →