Claude Api

---
name: claude-api
type: reference
description: "Provides code patterns for the Anthropic Claude API including streaming, tool use, and prompt caching. Use when working with Anthropic SDK files or when the user mentions Claude API, Anthropic client, or LLM integration."
origin: ECC
paths: ["**/*.py", "**/*.ts", "**/anthropic*", "**/claude*"]
effort: 3
allowed-tools: Read, Glob, Grep
user-invocable: true
when_to_use: "When building applications using the Anthropic Claude API, SDK, or implementing agent workflows with tool use or streaming"
---

# 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?"},
                {"role": "assistant", "content": message.content},
                {"role": "user", "content": [
                    {"type": "tool_result", "tool_use_id": block.id, "content": str(result)}
                ]}
            ]
        )
```

## Vision

Send images for analysis:

```python
import base64

with open("diagram.png", "rb") as f:
    image_data = base64.standard_b64encode(f.read()).decode("utf-8")

message = client.messages.create(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    messages=[{
        "role": "user",
        "content": [
            {"type": "image", "source": {"type": "base64", "media_type": "image/png", "data": image_data}},
            {"type": "text", "text": "Describe this diagram"}
        ]
    }]
)
```

## Extended Thinking

For complex reasoning tasks:

```python
message = client.messages.create(
    model="claude-sonnet-4-0",
    max_tokens=16000,
    thinking={
        "type": "enabled",
        "budget_tokens": 10000
    },
    messages=[{"role": "user", "content": "Solve this math problem step by step..."}]
)

for block in message.content:
    if block.type == "thinking":
        print(f"Thinking: {block.thinking}")
    elif block.type == "text":
        print(f"Answer: {block.text}")
```

## Prompt Caching

Cache large system prompts or context to reduce costs:

```python
message = client.messages.create(
    model="claude-sonnet-4-0",
    max_tokens=1024,
    system=[
        {"type": "text", "text": large_system_prompt, "cache_control": {"type": "ephemeral"}}
    ],
    messages=[{"role": "user", "content": "Question about the cached context"}]
)
# Check cache usage
print(f"Cache read: {message.usage.cache_read_input_tokens}")
print(f"Cache creation: {message.usage.cache_creation_input_tokens}")
```

## Batches API

Process large volumes asynchronously at 50% cost reduction:

```python
import time

batch = client.messages.batches.create(
    requests=[
        {
            "custom_id": f"request-{i}",
            "params": {
                "model": "claude-sonnet-4-0",
                "max_tokens": 1024,
                "messages": [{"role": "user", "content": prompt}]
            }
        }
        for i, prompt in enumerate(prompts)
    ]
)

# Poll for completion
while True:
    status = client.messages.batches.retrieve(batch.id)
    if status.processing_status == "ended":
        break
    time.sleep(30)

# Get results
for result in client.messages.batches.results(batch.id):
    print(result.result.message.content[0].text)
```

## Claude Agent SDK

Build multi-step agents:

```python
# Note: Agent SDK API surface may change — check official docs
import anthropic

# Define tools as functions
tools = [{
    "name": "search_codebase",
    "description": "Search the codebase for relevant code",
    "input_schema": {
        "type": "object",
        "properties": {"query": {"type": "string"}},
        "required": ["query"]
    }
}]

# Run an agentic loop with tool use
client = anthropic.Anthropic()
messages = [{"role": "user", "content": "Review the auth module for security issues"}]

while True:
    response = client.messages.create(
        model="claude-sonnet-4-0",
        max_tokens=4096,
        tools=tools,
        messages=messages,
    )
    if response.stop_reason == "end_turn":
        break
    # Handle tool calls and continue the loop
    messages.append({"role": "assistant", "content": response.content})
    # ... execute tools and append tool_result messages
```

## Cost Optimization

| Strategy | Savings | When to Use |
|----------|---------|-------------|
| Prompt caching | Up to 90% on cached tokens | Repeated system prompts or context |
| Batches API | 50% | Non-time-sensitive bulk processing |
| Haiku instead of Sonnet | ~75% | Simple tasks, classification, extraction |
| Shorter max_tokens | Variable | When you know output will be short |
| Streaming | None (same cost) | Better UX, same price |

## Error Handling

```python
import time

from anthropic import APIError, RateLimitError, APIConnectionError

try:
    message = client.messages.create(...)
except RateLimitError:
    # Back off and retry
    time.sleep(60)
except APIConnectionError:
    # Network issue, retry with backoff
    pass
except APIError as e:
    print(f"API error {e.status_code}: {e.message}")
```

## Environment Setup

```bash
# Required
export ANTHROPIC_API_KEY="your-api-key-here"

# Optional: set default model
export ANTHROPIC_MODEL="claude-sonnet-4-0"
```

Never hardcode API keys. Always use environment variables.