Anth local dev loop

Anthropic Local Dev Loop

Overview

Set up a fast local development cycle for Claude API projects with environment management, request logging, cost tracking, and hot-reload.

Prerequisites

• Completed anth-install-auth setup
• Node.js 18+ or Python 3.8+
• .env file with ANTHROPIC_API_KEY

Instructions

Step 1: Project Structure

my-claude-app/
├── .env                  # ANTHROPIC_API_KEY=sk-ant-...
├── .env.example          # ANTHROPIC_API_KEY=your-key-here
├── .gitignore            # Include .env
├── src/
│   ├── client.ts         # Singleton client
│   ├── prompts/          # System prompts as files
│   └── tools/            # Tool definitions
├── tests/
│   └── mock-responses/   # Saved API responses for testing
└── scripts/
    └── dev.ts            # Dev runner with logging

Step 2: Singleton Client with Request Logging

// src/client.ts
import Anthropic from '@anthropic-ai/sdk';

let client: Anthropic | null = null;

export function getClient(): Anthropic {
  if (!client) {
    client = new Anthropic({
      apiKey: process.env.ANTHROPIC_API_KEY,
      maxRetries: 2,
      timeout: 30_000,
    });
  }
  return client;
}

// Development logger — tracks cost per request
export function logUsage(messageId: string, usage: { input_tokens: number; output_tokens: number }, model: string) {
  const pricing: Record<string, { input: number; output: number }> = {
    'claude-sonnet-4-20250514': { input: 3.0, output: 15.0 },
    'claude-haiku-4-20250514': { input: 0.80, output: 4.0 },
    'claude-opus-4-20250514': { input: 15.0, output: 75.0 },
  };
  const rates = pricing[model] || pricing['claude-sonnet-4-20250514'];
  const cost = (usage.input_tokens * rates.input + usage.output_tokens * rates.output) / 1_000_000;
  console.log(`[${messageId}] ${model} | ${usage.input_tokens}+${usage.output_tokens} tokens | $${cost.toFixed(4)}`);
}

Step 3: Mock Responses for Tests

// tests/mock-client.ts
import { type Message } from '@anthropic-ai/sdk/resources/messages';

export function mockMessage(text: string): Message {
  return {
    id: 'msg_test_123',
    type: 'message',
    role: 'assistant',
    model: 'claude-sonnet-4-20250514',
    content: [{ type: 'text', text }],
    stop_reason: 'end_turn',
    stop_sequence: null,
    usage: { input_tokens: 10, output_tokens: 20 },
  };
}

Step 4: Hot-Reload Dev Script

# package.json scripts
"scripts": {
  "dev": "tsx watch src/index.ts",
  "dev:debug": "ANTHROPIC_LOG=debug tsx watch src/index.ts",
  "test": "vitest",
  "test:live": "LIVE_API=1 vitest --run"
}

Environment Management

# .env.example (commit this)
ANTHROPIC_API_KEY=your-key-here
ANTHROPIC_MODEL=claude-sonnet-4-20250514
ANTHROPIC_MAX_TOKENS=1024
ANTHROPIC_LOG=warn     # debug | info | warn | error

# Enable SDK debug logging
export ANTHROPIC_LOG=debug  # Logs all requests/responses

Cost Control During Development

import os

DEV_MODEL = "claude-haiku-4-20250514"      # $0.80/$4.00 per MTok
PROD_MODEL = "claude-sonnet-4-20250514"    # $3.00/$15.00 per MTok

model = DEV_MODEL if os.getenv("ENV") == "development" else PROD_MODEL

Error Handling

Issue	Cause	Solution
Hot reload triggers duplicate requests	File save causes restart	Add debounce or save-on-explicit-action
.env not loading	Missing dotenv setup	Use dotenv package or tsx --env-file=.env
Mock tests pass but live fails	Response shape changed	Update mocks from real API responses

Resources

• SDK Debug Logging
• Pricing

Next Steps

Apply patterns in anth-sdk-patterns for production-ready code.