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Chapter 5

Prepare a Server-Side TypeScript Project

The project is simple, but a couple of decisions in it are fundamental. The examples use ESM, TypeScript, and the official SDKs of all three providers. Versions aren't pinned in the book on purpose: check the release notes before installing and keep a genuinely tested build in the lockfile - API lines change faster than product logic.

Everything installs in a few commands.

Bash
npm init -y
npm pkg set type=module
npm install openai @anthropic-ai/sdk @google/genai zod
npm install -D typescript tsx @types/node

Keys and the model live in the environment, not in the code. And the model is deploy config, not a domain constant: today it's gpt-5.6, tomorrow something else, and you change it with a variable, not by editing sources.

# .env.local, never commit this file
OPENAI_API_KEY=...
ANTHROPIC_API_KEY=...
GEMINI_API_KEY=...

# The model is deploy config, not a domain constant
OPENAI_MODEL=gpt-5.6
ANTHROPIC_MODEL=claude-sonnet-5
GOOGLE_MODEL=gemini-3.6-flash

Files are laid out to survive a provider switch: the contract and runtime separately, the provider adapters separately, tools separately, endpoints and evals nearby.

src/
  agent/
    contracts.ts
    prompt.ts
    runner.ts
    tool-registry.ts
  providers/
    openai.ts
    anthropic.ts
    google.ts
  tools/
    policy-search.ts
    order-read.ts
    refund-preview.ts
    handoff-prepare.ts
  api/
    turn.ts
    approve.ts
  evals/
    cases.ts
    run.ts

And one rule whose violation costs the most.

Keys stay on the server. Neither a NEXT_PUBLIC_-prefixed variable nor a client-side fetch to a provider API will do. The browser talks to your endpoint, where session, limits, and audit apply. A key that lands in the bundle is a key handed to everyone.

The project is ready - now we assemble its core. We start with the contract that makes the agent portable across the three APIs.

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