Anthropic API Guide 2026

The Anthropic API gives you programmatic access to Claude—Anthropic's family of large language models. This guide covers everything you need: available models, authentication, endpoints, pricing, and how to get access outside the US.

What Is the Anthropic API?

The Anthropic API is Anthropic's official interface for Claude models. It uses a Messages format (distinct from OpenAI's Chat Completions), though most third-party providers—including APIMaster.ai—also expose Claude via an OpenAI-compatible endpoint.

Two ways to call Claude:

Method	Endpoint	SDK
Anthropic Messages (native)	/messages	anthropic Python package
OpenAI-compatible	/v1/chat/completions	openai Python package

APIMaster supports both.

Available Claude Models

Model	Parameters	Context	Strengths
claude-sonnet-4-6	—	200K	Best value: coding, analysis, writing
claude-opus-4-8	—	200K	Most capable: research, complex tasks
claude-haiku-4-5	—	200K	Fastest, cheapest: real-time apps
claude-fable-5	—	200K	Creative, long-form content

Anthropic API Authentication

Anthropic uses API keys passed in the x-api-key header (native SDK) or Authorization: Bearer header (OpenAI-compatible mode).

import anthropic

# Native Anthropic SDK
client = anthropic.Anthropic(
    api_key="YOUR_API_KEY",
    base_url="https://apimaster.ai",  # APIMaster endpoint
)

from openai import OpenAI

# OpenAI-compatible (easier for existing projects)
client = OpenAI(
    api_key="YOUR_API_KEY",
    base_url="https://apimaster.ai/v1",
)

Core Anthropic API Endpoints

Endpoint	Method	Purpose
/messages	POST	Create a Claude conversation turn
/v1/chat/completions	POST	OpenAI-compatible chat (APIMaster)
/v1/models	GET	List available models

Messages API Example

response = client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=1024,
    system="You are a helpful assistant.",
    messages=[
        {"role": "user", "content": "What is the Anthropic API?"}
    ],
)
print(response.content[0].text)

Streaming

with client.messages.stream(
    model="claude-sonnet-4-6",
    max_tokens=512,
    messages=[{"role": "user", "content": "Write a haiku about APIs."}],
) as stream:
    for text in stream.text_stream:
        print(text, end="", flush=True)

Anthropic API Pricing

Anthropic charges per million tokens (input + output separately). Current list prices:

Model	Input	Output
Claude Haiku 4.5	$0.80/M	$4.00/M
Claude Sonnet 4.6	$3.00/M	$15.00/M
Claude Opus 4.8	$15.00/M	$75.00/M

APIMaster offers the same models at discounted rates. See the live marketplace for current prices.

Accessing the Anthropic API Without a VPN

Anthropic's official API requires a US/EU billing address and may be blocked in some regions. APIMaster.ai removes these barriers:

• No geographic restrictions: works in China, Russia, Southeast Asia, and anywhere else
• Local payment methods: Alipay, WeChat Pay, USDT, credit card
• OpenAI-compatible: drop into existing code with two-line change
• Verified Claude: weekly fingerprint testing confirms model authenticity

Setting Up Anthropic API Access via APIMaster

1. Register — takes 30 seconds
2. Top up balance (minimum $1)
3. Create an API key in the console
4. Update your code:

# Before (official Anthropic)
client = anthropic.Anthropic(api_key="sk-ant-...")

# After (APIMaster, 2 lines changed)
client = anthropic.Anthropic(
    api_key="YOUR_APIMASTER_KEY",
    base_url="https://apimaster.ai",
)

Rate Limits and Error Handling

APIMaster mirrors Anthropic's rate limit tiers. For production use, implement exponential backoff:

import time
import anthropic

def call_with_retry(client, **kwargs):
    for attempt in range(3):
        try:
            return client.messages.create(**kwargs)
        except anthropic.RateLimitError:
            time.sleep(2 ** attempt)
    raise RuntimeError("Max retries exceeded")

Is APIMaster a Real Anthropic API?

APIMaster routes requests to real Anthropic infrastructure and runs live fingerprint verification to confirm Claude responses are authentic. Detection results are published publicly at https://apimaster.ai/detect—you can verify yourself.

Start using the Anthropic API →