Claude Api

# Claude API

Build applications with the Anthropic Claude API and SDKs.

## When to Activate

- Building applications that call the Claude API
- Code imports `anthropic` (Python) or `@anthropic-ai/sdk` (TypeScript)
- User asks about Claude API patterns, tool use, streaming, or vision
- Implementing agent workflows with Claude Agent SDK
- Optimizing API costs, token usage, or latency

## Model Selection

| Model | ID | Best For |
|-------|-----|----------|
| Opus 4.1 | `claude-opus-4-1` | Complex reasoning, architecture, research |
| Sonnet 4 | `claude-sonnet-4-0` | Balanced coding, most development tasks |
| Haiku 3.5 | `claude-3-5-haiku-latest` | Fast responses, high-volume, cost-sensitive |

Default to Sonnet 4 unless the task requires deep reasoning (Opus) or speed/cost optimization (Haiku). For production, prefer pinned snapshot IDs over aliases.

## Python SDK

### Installation

```bash
pip install anthropic
```

### Basic Message

```python
import anthropic

client = anthropic.Anthropic()  # reads ANTHROPIC_API_KEY from env

message = client.messages.create(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Explain async/await in Python"}
    ]
)
print(message.content[0].text)
```

### Streaming

```python
with client.messages.stream(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Write a haiku about coding"}]
) as stream:
    for text in stream.text_stream:
        print(text, end="", flush=True)
```

### System Prompt

```python
message = client.messages.create(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    system="You are a senior Python developer. Be concise.",
    messages=[{"role": "user", "content": "Review this function"}]
)
```

## TypeScript SDK

### Installation

```bash
npm install @anthropic-ai/sdk
```

### Basic Message

```typescript
import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic(); // reads ANTHROPIC_API_KEY from env

const message = await client.messages.create({
  model: "claude-sonnet-4-0",
  max_tokens: 1024,
  messages: [
    { role: "user", content: "Explain async/await in TypeScript" }
  ],
});
console.log(message.content[0].text);
```

### Streaming

```typescript
const stream = client.messages.stream({
  model: "claude-sonnet-4-0",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Write a haiku" }],
});

for await (const event of stream) {
  if (event.type === "content_block_delta" && event.delta.type === "text_delta") {
    process.stdout.write(event.delta.text);
  }
}
```

## Tool Use

Define tools and let Claude call them:

```python
tools = [
    {
        "name": "get_weather",
        "description": "Get current weather for a location",
        "input_schema": {
            "type": "object",
            "properties": {
                "location": {"type": "string", "description": "City name"},
                "unit": {"type": "string", "enum": ["celsius", "fahrenheit"]}
            },
            "required": ["location"]
        }
    }
]

message = client.messages.create(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    tools=tools,
    messages=[{"role": "user", "content": "What's the weather in SF?"}]
)

# Handle tool use response
for block in message.content:
    if block.type == "tool_use":
        # Execute the tool with block.input
        result = get_weather(**block.input)
        # Send result back
        follow_up = client.messages.create(
            model="claude-sonnet-4-0",
            max_tokens=1024,
            tools=tools,
            messages=[
                {"role": "user", "content": "What's the weather in SF?"},