Anthropic Local Dev Loop

Overview

Set up a fast, cheap development workflow for building with Claude.

Prerequisites

• Node.js 18+ or Python 3.10+
• ANTHROPIC_API_KEY environment variable set
• npm or pip package manager

Instructions

Step 1: Project Setup

mkdir my-claude-app && cd my-claude-app
npm init -y
npm install @claude-ai/sdk dotenv tsx

# Create .env (never commit this)
echo 'ANTHROPIC_API_KEY=sk-ant-api03-...' > .env
echo '.env' >> .gitignore

Step 2: Create a Test Script

// src/test-prompt.ts
import 'dotenv/config';
import Anthropic from '@claude-ai/sdk';

const client = new Anthropic();

async function main() {
  const message = await client.messages.create({
    model: 'claude-haiku-4-5-20251001', // Use Haiku for dev — 20x cheaper than Opus
    max_tokens: 512,
    messages: [{ role: 'user', content: 'Summarize this in one sentence: ...' }],
  });

  console.log(message.content[0].text);
  console.log(`Cost: ~$${((message.usage.input_tokens * 0.80 + message.usage.output_tokens * 4) / 1_000_000).toFixed(4)}`);
}

main();

Step 3: Run with Hot Reload

# Watch mode — re-runs on file changes
npx tsx watch src/test-prompt.ts

# Or one-shot
npx tsx src/test-prompt.ts

Cost-Saving Dev Tips

Tip	Savings
Use claude-haiku-4-5-20251001 during development	20x cheaper than Opus
Set max_tokens: 256 for testing	Fewer output tokens billed
Cache your system prompt with prompt caching beta	90% off cached input tokens
Use Message Batches for bulk testing (50% off)	Half price, 24h turnaround
Log responses locally so you don't re-call for the same input	100% savings on repeats

Mock Client for Unit Tests

// tests/mock-anthropic.ts
export function createMockClient() {
  return {
    messages: {
      create: async (params: any) => ({
        id: 'msg_test',
        type: 'message',
        role: 'assistant',
        model: params.model,
        content: [{ type: 'text', text: 'Mock response for testing' }],
        stop_reason: 'end_turn',
        usage: { input_tokens: 10, output_tokens: 5 },
      }),
    },
  };
}

// In your test:
import { createMockClient } from './mock-anthropic';
const client = process.env.MOCK ? createMockClient() : new Anthropic();

Python Dev Loop

pip install anthropic python-dotenv ipython

# Interactive exploration
ANTHROPIC_API_KEY=sk-ant-... ipython
>>> import anthropic
>>> c = anthropic.Anthropic()
>>> r = c.messages.create(model="claude-haiku-4-5-20251001", max_tokens=100, messages=[{"role":"user","content":"hello"}])
>>> r.content[0].text

Output

• Project scaffolded with SDK, dotenv, and tsx for hot reload
• Test script running against Claude Haiku (cheapest model)
• Mock client available for unit tests without API calls
• Cost estimate printed per request

Error Handling

Issue	Fix
ANTHROPIC_API_KEY not loading	Make sure dotenv/config is imported first
Slow iteration	Use Haiku, reduce max_tokens
High dev costs	Log responses, use mocks for unit tests

Examples

See Step 1 (project setup), Step 2 (test script with cost tracking), Step 3 (hot reload), Mock Client section, and Python Dev Loop section above.

Resources

• Quickstart Guide
• TypeScript SDK

Next Steps

See clade-sdk-patterns for production client configuration.