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Messages API (Native Anthropic)

POST /v1/messages

The Messages API is a native Anthropic endpoint that routes directly to Anthropic-type channels. This endpoint uses the native Anthropic request/response format without any conversion, allowing you to use Anthropic SDKs directly with Apertis.

Native Endpoint

This endpoint exclusively routes to Anthropic-type channels, ensuring full compatibility with Anthropic's API format including streaming, tool use, and all Claude-specific features.

HTTP Request​

curl https://api.apertis.ai/v1/messages \
    -H "Content-Type: application/json" \
    -H "x-api-key: <APERTIS_API_KEY>" \
    -d '{
        "model": "claude-sonnet-4.5",
        "max_tokens": 1024,
        "messages": [
            {"role": "user", "content": "Hello, Claude!"}
        ]
    }'
note

The anthropic-version header is optional and will be ignored. Apertis handles API version compatibility automatically.

Authentication​

The Messages API supports both authentication methods:

HeaderFormatExample
x-api-keyAnthropic stylex-api-key: sk-your-api-key
AuthorizationBearer tokenAuthorization: Bearer sk-your-api-key

Parameters​

Required Parameters​

ParameterTypeDescription
modelstringThe Claude model to use
messagesarrayArray of message objects
max_tokensintegerMaximum tokens in the response

Optional Parameters (Native Anthropic)​

ParameterTypeDescription
systemstringSystem prompt (top-level, not in messages)
temperaturenumberSampling temperature (0-1). Default: 1
top_pnumberNucleus sampling threshold (0-1)
top_kintegerTop-k sampling (Anthropic specific)
streambooleanEnable streaming. Default: false
stop_sequencesarrayCustom stop sequences
toolsarrayTools/functions the model can call
tool_choiceobjectControls tool selection behavior
metadataobjectRequest metadata (e.g., user_id)
thinkingobjectExtended thinking configuration (see below)

Thinking Parameter​

The thinking parameter enables Claude's extended thinking capability for more complex reasoning:

OptionTypeDescription
typestring"enabled" or "disabled"
budget_tokensintegerToken budget for thinking (1024-32768)
# Extended Thinking Example
message = client.messages.create(
    model="claude-sonnet-4.5",
    max_tokens=4096,
    thinking={
        "type": "enabled",
        "budget_tokens": 10240
    },
    messages=[
        {"role": "user", "content": "Solve this complex math problem step by step..."}
    ]
)

Extended Parameters (OpenAI-compatible)​

These additional parameters are supported for compatibility with upstream providers:

ParameterTypeDescription
nintegerNumber of completions to generate. Default: 1
stopstring/arrayUp to 4 sequences where the API will stop generating
presence_penaltynumberPenalize new topics (-2.0 to 2.0). Default: 0
frequency_penaltynumberPenalize repetition (-2.0 to 2.0). Default: 0
logit_biasobjectMap of token IDs to bias values (-100 to 100)
userstringUnique identifier for end-user tracking
response_formatobjectSpecify output format (e.g., JSON mode)
seednumberSeed for deterministic sampling

Example Usage​

Python (Anthropic SDK)​

import anthropic

client = anthropic.Anthropic(
    api_key="sk-your-api-key",
    base_url="https://api.apertis.ai"
)

message = client.messages.create(
    model="claude-sonnet-4.5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "What is the meaning of life?"}
    ]
)

print(message.content[0].text)

JavaScript (Anthropic SDK)​

import Anthropic from '@anthropic-ai/sdk';

const client = new Anthropic({
  apiKey: 'sk-your-api-key',
  baseURL: 'https://api.apertis.ai'
});

const message = await client.messages.create({
  model: 'claude-sonnet-4.5',
  max_tokens: 1024,
  messages: [
    { role: 'user', content: 'What is the meaning of life?' }
  ]
});

console.log(message.content[0].text);

With System Prompt​

message = client.messages.create(
    model="claude-sonnet-4.5",
    max_tokens=1024,
    system="You are a helpful assistant that speaks like a pirate.",
    messages=[
        {"role": "user", "content": "Tell me about the weather."}
    ]
)

Multi-turn Conversation​

message = client.messages.create(
    model="claude-sonnet-4.5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "What is Python?"},
        {"role": "assistant", "content": "Python is a high-level programming language..."},
        {"role": "user", "content": "How do I install it?"}
    ]
)

Streaming​

with client.messages.stream(
    model="claude-sonnet-4.5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Write a poem about coding."}]
) as stream:
    for text in stream.text_stream:
        print(text, end="", flush=True)

Streaming with curl​

curl https://api.apertis.ai/v1/messages \
  -H "x-api-key: <APERTIS_API_KEY>" \
  -H "Content-Type: application/json" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-opus-4-5-20251101",
    "max_tokens": 100,
    "stream": true,
    "messages": [{"role": "user", "content": "Hello!"}]
  }'

SSE events returned:

  • message_start - Initial message metadata
  • content_block_start - Start of content block
  • content_block_delta - Incremental text chunks
  • content_block_stop - End of content block
  • message_delta - Final usage and stop reason
  • message_stop - Stream complete

Vision (Image Input)​

import base64

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

message = client.messages.create(
    model="claude-sonnet-4.5",
    max_tokens=1024,
    messages=[
        {
            "role": "user",
            "content": [
                {
                    "type": "image",
                    "source": {
                        "type": "base64",
                        "media_type": "image/png",
                        "data": image_data
                    }
                },
                {
                    "type": "text",
                    "text": "What do you see in this image?"
                }
            ]
        }
    ]
)

PDF Document Input​

Claude can analyze PDF documents directly. Use the document content type with base64-encoded PDF data:

import base64

with open("document.pdf", "rb") as f:
    pdf_data = base64.standard_b64encode(f.read()).decode("utf-8")

message = client.messages.create(
    model="claude-sonnet-4.5",
    max_tokens=4096,
    messages=[
        {
            "role": "user",
            "content": [
                {
                    "type": "document",
                    "source": {
                        "type": "base64",
                        "media_type": "application/pdf",
                        "data": pdf_data
                    }
                },
                {
                    "type": "text",
                    "text": "Summarize this document."
                }
            ]
        }
    ]
)

Document Source Options​

FieldTypeDescription
typestring"base64" for encoded data
media_typestring"application/pdf" for PDF files
datastringBase64-encoded document content
Supported Formats

Currently, PDF (application/pdf) is the primary supported document format for Claude models.

Response Format​

{
  "id": "msg_abc123",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Hello! How can I help you today?"
    }
  ],
  "model": "claude-sonnet-4.5",
  "stop_reason": "end_turn",
  "usage": {
    "input_tokens": 12,
    "output_tokens": 10
  }
}

Supported Models​

The Messages API supports Claude models via Anthropic-type channels:

ModelDescription
claude-opus-4-5-20251101Claude Opus 4.5 - most capable
claude-sonnet-4.5Claude Sonnet 4.5 - balanced
claude-haiku-4.5Claude Haiku 4.5 - fast and efficient
Anthropic Channels Only

This endpoint routes exclusively to Anthropic-type channels. If you need to access non-Claude models, use the Chat Completions endpoint instead.

Note: Some advanced OpenAI models (like gpt-5-pro, o1-pro, gpt-5-codex-*) only support the Responses API and cannot be used with this endpoint.

Differences from Direct Anthropic API​

FeatureApertisDirect Anthropic
Base URLhttps://api.apertis.aihttps://api.anthropic.com
API KeyApertis API keyAnthropic API key
Request FormatNative Anthropic (no conversion)Native Anthropic
StreamingFull SSE supportFull SSE support
BillingUnified Apertis billingAnthropic billing