Claude Sonnet 4.5

Text Generation • Anthropic

anthropic/claude-sonnet-4.5

• Third-party
• Zero data retention

Claude Sonnet 4.5 is the best coding model to date, with significant improvements across the entire development lifecycle.

Usage

const response = await env.AI.run(
  'anthropic/claude-sonnet-4.5',
  {
    max_tokens: 1024,
    messages: [{ content: 'What are the three laws of thermodynamics?', role: 'user' }],
  },
)
console.log(response)

curl https://api.cloudflare.com/client/v4/accounts/$CLOUDFLARE_ACCOUNT_ID/ai/v1/messages \
  --header "Authorization: Bearer $CLOUDFLARE_API_TOKEN" \
  --header "Content-Type: application/json" \
  --data '{
  "model": "anthropic/claude-sonnet-4.5",
  "max_tokens": 1024,
  "messages": [
    {
      "content": "What are the three laws of thermodynamics?",
      "role": "user"
    }
  ]
}'

# The Three Laws of Thermodynamics

## **First Law: Conservation of Energy**
Energy cannot be created or destroyed, only converted from one form to another. The total energy of an isolated system remains constant.
- Also stated as: ΔU = Q - W (change in internal energy equals heat added minus work done)

## **Second Law: Entropy**
The entropy of an isolated system always increases over time. Heat naturally flows from hot to cold, and processes tend toward disorder.
- This law explains why certain processes are irreversible and why we can't have 100% efficient heat engines

## **Third Law: Absolute Zero**
As temperature approaches absolute zero (0 Kelvin or -273.15°C), the entropy of a perfect crystal approaches zero.
- Practically, this means absolute zero cannot be reached through any finite number of processes

**Note:** There's also a "Zeroth Law" (named later): If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This establishes the concept of temperature.

{
  "content": [
    {
      "text": "# The Three Laws of Thermodynamics\n\n## **First Law: Conservation of Energy**\nEnergy cannot be created or destroyed, only converted from one form to another. The total energy of an isolated system remains constant.\n- Also stated as: ΔU = Q - W (change in internal energy equals heat added minus work done)\n\n## **Second Law: Entropy**\nThe entropy of an isolated system always increases over time. Heat naturally flows from hot to cold, and processes tend toward disorder.\n- This law explains why certain processes are irreversible and why we can't have 100% efficient heat engines\n\n## **Third Law: Absolute Zero**\nAs temperature approaches absolute zero (0 Kelvin or -273.15°C), the entropy of a perfect crystal approaches zero.\n- Practically, this means absolute zero cannot be reached through any finite number of processes\n\n**Note:** There's also a \"Zeroth Law\" (named later): If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This establishes the concept of temperature.",
      "type": "text"
    }
  ],
  "gatewayMetadata": {
    "keySource": "BYOK"
  },
  "id": "msg_01GZMN4RKsBb5RHXUJYBgBdG",
  "model": "claude-sonnet-4-5-20250929",
  "role": "assistant",
  "stop_details": null,
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "type": "message",
  "usage": {
    "input_tokens": 17,
    "output_tokens": 240
  }
}

Examples

<details>

<summary>With System Message — Using a system message to set context</summary>

const response = await env.AI.run(
  'anthropic/claude-sonnet-4.5',
  {
    max_tokens: 1024,
    messages: [{ content: 'How do I read a JSON file in Python?', role: 'user' }],
    system: 'You are a helpful coding assistant specializing in Python.',
    temperature: 0.3,
  },
)
console.log(response)

curl https://api.cloudflare.com/client/v4/accounts/$CLOUDFLARE_ACCOUNT_ID/ai/v1/messages \
  --header "Authorization: Bearer $CLOUDFLARE_API_TOKEN" \
  --header "Content-Type: application/json" \
  --data '{
  "model": "anthropic/claude-sonnet-4.5",
  "max_tokens": 1024,
  "messages": [
    {
      "content": "How do I read a JSON file in Python?",
      "role": "user"
    }
  ],
  "system": "You are a helpful coding assistant specializing in Python.",
  "temperature": 0.3
}'

````

Reading a JSON File in Python

You can read a JSON file using Python's built-in json module. Here are the common methods:

Basic Method

```python import json