Imported from jeremylongshore/tons-of-skills-marketplace (skills/.curated/anth-local-dev-loop/SKILL.md). Install upstream with npx skills add jeremylongshore/tons-of-skills-marketplace --skill anth-local-dev-loop. Copyright stays with the author (MIT).

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"
}

Output

The development loop produces a repeatable local project layout, a single configured client, mock-backed tests, and a request log that records model and token usage. A change can then be exercised with no API spend in the normal test suite or with deliberately enabled live traffic when investigating a prompt or integration behavior.

Examples

While editing a prompt, run the default vitest command against a saved mock response and confirm the application handles the expected text and stop reason. When a real API check is needed, set LIVE_API=1 only for that invocation, choose the lower-cost development model, and inspect the logger for token use. If the live response changes the contract, first update the fixture and its assertions, then rerun the mock suite so later local iterations remain fast and deterministic.

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

Resources

• SDK Debug Logging
• Pricing

Next Steps

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