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The Messages API (/v1/messages) provides direct compatibility with the Anthropic Messages API format. Use this endpoint when working with Anthropic Claude models and features like interleaved thinking, tool calling, and multi-turn conversations.
The Messages API (/v1/messages) is fully supported on the Enterprise plan using https://enterprise.blackbox.ai. On standard plans (https://api.blackbox.ai), this endpoint may not work as expected. For the best experience, use an Enterprise API key.

​
Basic Request

import os
import requests

response = requests.post(
    'https://enterprise.blackbox.ai/v1/messages',
    headers={
        'Content-Type': 'application/json',
        'Authorization': f"Bearer {os.environ['BLACKBOX_API_KEY']}",
        'anthropic-version': '2023-06-01',
    },
    json={
        'model': 'blackboxai/anthropic/claude-sonnet-4.5',
        'max_tokens': 1024,
        'messages': [
            {'role': 'user', 'content': 'What is the capital of France?'}
        ],
    },
)

result = response.json()
print(result['content'][0]['text'])

​
Response Format

{
  "id": "gen_01KJRNETDECKXVWK0YX46WSHDY",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "The capital of France is Paris."
    }
  ],
  "model": "blackboxai/anthropic/claude-sonnet-4.5",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 19,
    "output_tokens": 10,
    "total_tokens": 29,
    "cache_read_input_tokens": 0,
    "cache_creation_input_tokens": 0
  }
}

​
System Prompt

Use the system parameter to set the model’s behavior for the entire conversation. 
import os
import requests

response = requests.post(
    'https://enterprise.blackbox.ai/v1/messages',
    headers={
        'Content-Type': 'application/json',
        'Authorization': f"Bearer {os.environ['BLACKBOX_API_KEY']}",
        'anthropic-version': '2023-06-01',
    },
    json={
        'model': 'blackboxai/anthropic/claude-sonnet-4.5',
        'max_tokens': 256,
        'system': 'You are a helpful pirate. Always respond in pirate speak.',
        'messages': [
            {'role': 'user', 'content': 'What is 2 + 2?'}
        ],
    },
)

result = response.json()
print(result['content'][0]['text'])
Response
{
  "id": "gen_01KJRNEX4M6JD4GDPF4K5MK2J0",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Arrr, that be an easy bit o' mathematics, matey! 2 + 2 equals 4, as sure as the seven seas be salty!"
    }
  ],
  "model": "blackboxai/anthropic/claude-sonnet-4.5",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 30,
    "output_tokens": 60
  }
}

​
Multi-Turn Conversations

Pass previous messages as an array of alternating user and assistant messages to maintain conversation context. 
import os
import requests

response = requests.post(
    'https://enterprise.blackbox.ai/v1/messages',
    headers={
        'Content-Type': 'application/json',
        'Authorization': f"Bearer {os.environ['BLACKBOX_API_KEY']}",
        'anthropic-version': '2023-06-01',
    },
    json={
        'model': 'blackboxai/anthropic/claude-sonnet-4.5',
        'max_tokens': 256,
        'messages': [
            {'role': 'user', 'content': 'My name is Alice.'},
            {'role': 'assistant', 'content': 'Nice to meet you, Alice! How can I help you today?'},
            {'role': 'user', 'content': "What's my name?"},
        ],
    },
)

result = response.json()
print(result['content'][0]['text'])
Response
{
  "id": "gen_01KJRNF1RP8DWKJ3M59JDAQRHX",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Your name is Alice, as you just told me."
    }
  ],
  "model": "blackboxai/anthropic/claude-sonnet-4.5",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 37,
    "output_tokens": 14
  }
}

​
Request Headers

​
Authorization
string
required
API Key in the format Bearer <api_key>. Get yours from here.
​
anthropic-version
string
required
Anthropic API version. Use 2023-06-01.
​
anthropic-beta
string
Optional beta features. For example, interleaved-thinking-2025-05-14 for legacy interleaved thinking support.

​
Request Parameters

​
model
string
required
The model ID. Supported models include:
  • blackboxai/anthropic/claude-opus-4.6
  • blackboxai/anthropic/claude-opus-4.5
  • blackboxai/anthropic/claude-sonnet-4.6
  • blackboxai/anthropic/claude-sonnet-4.5
​
max_tokens
integer
required
Maximum number of tokens to generate.
​
messages
array
required
Array of message objects with role (user or assistant) and content fields.
​
system
string
System prompt for the conversation.
​
tools
array
Array of tool definitions using the Anthropic input_schema format. See Tool Calling.
​
thinking
object
Configuration for extended / interleaved thinking. See Interleaved Thinking.

​
Response Fields

​
id
string
Unique message identifier.
​
type
string
Always "message".
​
role
string
Always "assistant".
​
content
array
Array of content blocks. Each block has a type field:
  • text — Text response with a text field
  • tool_use — Tool call with id, name, and input fields
  • thinking — Thinking block with a thinking field (when thinking is enabled)
​
stop_reason
string
Why the model stopped generating:
  • end_turn — Natural completion
  • tool_use — Model wants to call a tool
  • max_tokens — Hit the token limit
​
usage
object
Token usage with input_tokens and output_tokens.

​
Supported Models

ModelModel IDTool CallingInterleaved Thinking
Claude Opus 4.6blackboxai/anthropic/claude-opus-4.6✓✓
Claude Opus 4.5blackboxai/anthropic/claude-opus-4.5✓✓
Claude Sonnet 4.6blackboxai/anthropic/claude-sonnet-4.6✓✓
Claude Sonnet 4.5blackboxai/anthropic/claude-sonnet-4.5✓✓

Tool Calling

Define tools and build multi-turn agentic loops

Interleaved Thinking

Enable reasoning between tool calls