Anthropic Local Dev Loop Skill

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

Next Steps

See clade-sdk-patterns for production client configuration.

Agent Skills: Anthropic Local Dev Loop

Install this agent skill to your local

Skill Files