> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fallom.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Anthropic (Python)

> Trace Claude API calls in Python

## Installation

```bash theme={null}
pip install fallom anthropic
```

## Quick Start

<Warning>
  **Import order matters!** Initialize Fallom **before** importing Anthropic.
</Warning>

```python theme={null}
import os

# Import and initialize Fallom FIRST
import fallom
fallom.init(api_key=os.environ["FALLOM_API_KEY"])

# NOW import Anthropic
from anthropic import Anthropic
client = Anthropic()

# Set session context (config_key, session_id, customer_id)
fallom.trace.set_session("my-app", "session-123", customer_id="user-456")

# Use as normal - automatically traced!
response = client.messages.create(
    model="claude-sonnet-4-20250514",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello!"}]
)

print(response.content[0].text)
```

## With System Prompt

```python theme={null}
import fallom
fallom.init(api_key="your-fallom-api-key")

from anthropic import Anthropic
client = Anthropic()

fallom.trace.set_session("my-agent", "session-123")

response = client.messages.create(
    model="claude-sonnet-4-20250514",
    max_tokens=1024,
    system="You are a helpful assistant who speaks like a pirate.",
    messages=[{"role": "user", "content": "Tell me about the weather."}]
)
```

## Streaming

```python theme={null}
import fallom
fallom.init(api_key="your-fallom-api-key")

from anthropic import Anthropic
client = Anthropic()

fallom.trace.set_session("my-agent", "session-123")

with client.messages.stream(
    model="claude-sonnet-4-20250514",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Write a poem."}]
) as stream:
    for text in stream.text_stream:
        print(text, end="")
```

## Model A/B Testing

```python theme={null}
import fallom
from fallom import models

fallom.init(api_key="your-fallom-api-key")

from anthropic import Anthropic
client = Anthropic()

# Get assigned model
model_id = models.get("my-experiment", "session-123", fallback="claude-sonnet-4-20250514")

fallom.trace.set_session("my-experiment", "session-123")

response = client.messages.create(
    model=model_id,
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello!"}]
)
```

## What Gets Traced

| Field       | Description                      |
| ----------- | -------------------------------- |
| Model       | `claude-sonnet-4-20250514`, etc. |
| Duration    | Total request time (ms)          |
| Tokens      | Input, output tokens             |
| Cost        | Calculated from token usage      |
| Prompts     | System + user messages           |
| Completions | Model response                   |
| Session     | Your config key + session ID     |

## Next Steps

<CardGroup cols={2}>
  <Card title="Model A/B Testing" icon="flask" href="/model-testing">
    Compare Claude models in production.
  </Card>

  <Card title="OpenAI" icon="microchip" href="/integrations/python/openai">
    Also using OpenAI? See that guide.
  </Card>
</CardGroup>