Chapter 3: API Configuration

Getting Anthropic API Key

The Anthropic API Key is essential for using Claude Code. Let's walk through obtaining one:

Step 1: Register Anthropic Account

1. Visit Anthropic Console
2. Click "Sign Up"
3. Register using Email or Google account
4. Verify email address

Step 2: Add Account Balance

Important: Anthropic API is a paid service requiring prepaid credits

1. After login, go to "Billing" page
2. Click "Add Credit"
3. Select amount (recommended $20-$50 for first time)
4. Complete payment with credit card

Step 3: Create API Key

1. Go to "API Keys" page
2. Click "Create Key"
3. Set key name (e.g., claude-code-dev)
4. (Optional) Set usage limits and permissions
5. Click "Create"
6. Important: Copy and save API Key immediately (shown only once!)

API Key Format:

sk-ant-api03-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

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Configuring API Key

There are multiple ways to configure your API Key—choose what works best:

Method 1: Interactive Configuration (Recommended)

# Run configuration command
claude --set-api-key

# Enter API Key
Enter your Anthropic API key: sk-ant-api03-xxxxx

# Confirm save
✓ API key saved successfully

Method 2: Environment Variable

Temporary (Current Session):

# Linux/macOS
export ANTHROPIC_API_KEY="sk-ant-api03-xxxxx"

# Windows (PowerShell)
$env:ANTHROPIC_API_KEY="sk-ant-api03-xxxxx"

Permanent:

Linux/macOS (Bash):

# Edit .bashrc or .zshrc
echo 'export ANTHROPIC_API_KEY="sk-ant-api03-xxxxx"' >> ~/.bashrc
source ~/.bashrc

Linux/macOS (Fish):

set -Ux ANTHROPIC_API_KEY "sk-ant-api03-xxxxx"

Windows (PowerShell):

[System.Environment]::SetEnvironmentVariable('ANTHROPIC_API_KEY', 'sk-ant-api03-xxxxx', 'User')

Method 3: Configuration File

Edit configuration file:

nano ~/.config/claude-code/config.json

Add API Key:

{
  "api": {
    "key": "sk-ant-api03-xxxxx",
    "baseUrl": "https://api.anthropic.com"
  }
}

Method 4: .env File

Create .env file:

3.3 API 定价与使用

# In project or config directory
nano ~/.config/claude-code/.env

Add content:

ANTHROPIC_API_KEY=sk-ant-api03-xxxxx

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API Pricing and Usage

Model Pricing (January 2026)

Model	Input	Output	Use Case
Claude Opus 4.5	$15/M tokens	$75/M tokens	Most complex tasks
Claude Sonnet 4.5	$3/M tokens	$15/M tokens	Balanced (Recommended)
Claude Haiku	$0.25/M tokens	$1.25/M tokens	Fast simple tasks

M = Million

Cost Estimation Examples

Simple Task (Fix small bug):

• Input: ~1,000 tokens
• Output: ~500 tokens
• Cost: ~$0.01 (Sonnet 4.5)

Medium Task (Implement new feature):

• Input: ~5,000 tokens
• Output: ~2,000 tokens
• Cost: ~$0.05 (Sonnet 4.5)

Complex Task (Refactor large module):

• Input: ~20,000 tokens
• Output: ~8,000 tokens
• Cost: ~$0.20 (Sonnet 4.5)

View Usage

3.4 API 安全最佳实践

# View current session usage
claude --usage

# View account balance
claude --balance

# View detailed statistics
claude --stats

Cost Control Tips

1. Choose Right Model

   • Use Haiku for simple tasks
   • Use Sonnet for general tasks
   • Use Opus only for complex tasks

2. Optimize Prompts

   • Clear, concise task descriptions
   • Reduce unnecessary context

3. Use Subagents

   • Use Explore Agent (haiku) for exploration
   • Use Plan Agent for planning

4. Set Budget Limits

   {
     "limits": {
       "dailyBudget": 10,
       "monthlyBudget": 100,
       "warningThreshold": 80
     }
   }

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API Security Best Practices

🔒 Protect Your API Key

1. Never Commit to Version Control

# Add to .gitignore
.env
config.json
**/*api-key*

2. Use Environment Variables

# Recommended: use environment variables, not hardcoded
export ANTHROPIC_API_KEY="sk-ant-xxx"

3. Limit Key Permissions

• Set key permissions in Anthropic Console
• Create separate keys for different environments
• Rotate API Keys regularly

4. Monitor Usage

3.5 多 API Key 管理

# Regularly check for unusual usage
claude --usage-report

# Set budget alerts
claude --set-budget-alert 50

5. Secure Storage

# Limit config file permissions
chmod 600 ~/.config/claude-code/config.json

# Encrypt sensitive config (optional)
gpg -c ~/.config/claude-code/config.json

⚠️ Response to Leakage

If API Key is leaked:

1. Revoke Key Immediately

   • Log into Anthropic Console
   • Delete leaked key on API Keys page

2. Check Usage Records

   • Look for abnormal calls
   • Check billing

3. Create New Key

   • Generate new API Key
   • Update all configurations

4. Update Security Measures

   • Check .gitignore
   • Strengthen access control

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Managing Multiple API Keys

For teams or multi-project scenarios, you might need to manage multiple API Keys:

Scenario 1: Different Projects with Different Keys

Project A Config:

cd /path/to/project-a
echo "ANTHROPIC_API_KEY=sk-ant-projectA-xxx" > .env

Project B Config:

cd /path/to/project-b
echo "ANTHROPIC_API_KEY=sk-ant-projectB-xxx" > .env

Claude Code automatically reads .env from current directory

Scenario 2: Separate Dev and Production Environments

Config File (config.json):

{
  "profiles": {
    "dev": {
      "api": {
        "key": "sk-ant-dev-xxx",
        "model": "claude-haiku"
      }
    },
    "prod": {
      "api": {
        "key": "sk-ant-prod-xxx",
        "model": "claude-sonnet-4-5"
      }
    }
  }
}

Use Different Profiles:

3.6 验证配置

# Development environment
claude --profile dev "your task"

# Production environment
claude --profile prod "your task"

Scenario 3: Individual Keys for Team Members

Team Configuration Recommendations:

1. Each member uses their own API Key
2. Ignore personal configs in .gitignore
3. Provide config template file

# config.template.json
{
  "api": {
    "key": "YOUR_API_KEY_HERE",
    "baseUrl": "https://api.anthropic.com"
  }
}

# Members copy template and fill in their key
cp config.template.json config.json
# Edit config.json with personal API Key

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Verify Configuration

After configuration, verify everything works:

# 1. Test API connection
claude --test-connection

# Expected output
✓ API key valid
✓ Connection to api.anthropic.com successful
✓ Model claude-sonnet-4-5 available
✓ Account balance: $45.23

# 2. View current configuration
claude --show-config

# 3. Test simple task
claude "echo 'Configuration test successful'"

# 4. Check which API Key is loaded
claude --which-api-key

# Output: Using API key: sk-ant-***xxx (last 4 chars shown)

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Summary

In this chapter, we learned:

• ✅ How to register Anthropic account and obtain API Key
• ✅ Four methods to configure API Key
• ✅ API pricing and cost estimation
• ✅ API security best practices
• ✅ Multiple API Key management strategies
• ✅ Configuration verification methods

Important Reminders:

• 🔐 Safeguard your API Key
• 💰 Monitor costs, set budget alerts
• 🔄 Rotate API Keys regularly
• 📊 Monitor usage

Next Step: In Chapter 4, we'll start actually using Claude Code and complete your first task!

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