Using the Messages API

Anthropic offers two ways to build with Claude, each suited to different use cases:

This guide covers common patterns for working with the Messages API, including basic requests, multi-turn conversations, prefill techniques, and vision capabilities. For complete API specifications, see the Messages API reference. For the managed agent harness instead, see the Claude Managed Agents overview.

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Basic request and response

message = anthropic.Anthropic().messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Hello, Claude"}],
)
print(message)

{
  "id": "msg_01XFDUDYJgAACzvnptvVoYEL",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Hello!"
    }
  ],
  "model": "claude-opus-5",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 12,
    "output_tokens": 6
  }
}

Refusal responses (stop_reason: "refusal") also include a stop_details object identifying the policy category that triggered the refusal, on every model. See Handling stop reasons for the field reference and example handling code.

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Multiple conversational turns

The Messages API is stateless, which means that you always send the full conversational history to the API. You can use this pattern to build up a conversation over time. Earlier conversational turns don't necessarily need to actually originate from Claude. You can use synthetic assistant messages.

message = anthropic.Anthropic().messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[
        {"role": "user", "content": "Hello, Claude"},
        {"role": "assistant", "content": "Hello!"},
        {"role": "user", "content": "Can you describe LLMs to me?"},
    ],
)
print(message)

{
  "id": "msg_018gCsTGsXkYJVqYPxTgDHBU",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "Sure, I'd be happy to provide..."
    }
  ],
  "model": "claude-opus-5",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 30,
    "output_tokens": 309
  }
}

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System role in messages

On Claude Fable 5, Claude Mythos 5, Claude Opus 4.8, and Claude Opus 5, you can include messages with "role": "system" after a user turn (subject to placement rules) to add a new system instruction partway through a conversation. A system message cannot be the first entry in messages; use the top-level system field for instructions that apply from the start.

A mid-conversation system message has the same authority as the top-level system field, but because it is appended to the end of the message history, it does not invalidate any cached prefix that came before it. Use the top-level system field for instructions that should apply from the very first turn, and a mid-conversation system message for instructions that only become relevant later.

See Mid-conversation system messages for the complete guide, including how to combine it with prompt caching.

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Prefilling Claude's response

You can pre-fill part of Claude's response in the last position of the input messages list. Use this technique to shape Claude's response. The following example uses "max_tokens": 1 to get a single multiple choice answer from Claude.

message = anthropic.Anthropic().messages.create(
    model="claude-sonnet-4-5",
    max_tokens=1,
    messages=[
        {
            "role": "user",
            "content": "What is latin for Ant? (A) Apoidea, (B) Rhopalocera, (C) Formicidae",
        },
        {"role": "assistant", "content": "The answer is ("},
    ],
)
print(message)

{
  "id": "msg_01Q8Faay6S7QPTvEUUQARt7h",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "C"
    }
  ],
  "model": "claude-sonnet-4-5",
  "stop_reason": "max_tokens",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 42,
    "output_tokens": 1
  }
}

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Vision

Claude can read both text and images in requests. You can supply images using the base64, url, or file source types. The file source type references an image uploaded through the Files API. Supported media types are image/jpeg, image/png, image/gif, and image/webp. See the vision guide for more details.

import base64
import httpx

# Option 1: Base64-encoded image
image_url = "https://platform.claude.com/docs/images/vision-example.jpg"
image_media_type = "image/jpeg"
image_data = base64.standard_b64encode(httpx.get(image_url).content).decode("utf-8")

message = anthropic.Anthropic().messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[
        {
            "role": "user",
            "content": [
                {
                    "type": "image",
                    "source": {
                        "type": "base64",
                        "media_type": image_media_type,
                        "data": image_data,
                    },
                },
                {"type": "text", "text": "What is in the above image?"},
            ],
        }
    ],
)
print(message)

# Option 2: URL-referenced image
message_from_url = anthropic.Anthropic().messages.create(
    model="claude-opus-5",
    max_tokens=1024,
    messages=[
        {
            "role": "user",
            "content": [
                {
                    "type": "image",
                    "source": {
                        "type": "url",
                        "url": "https://platform.claude.com/docs/images/vision-example.jpg",
                    },
                },
                {"type": "text", "text": "What is in the above image?"},
            ],
        }
    ],
)
print(message_from_url)

{
  "id": "msg_011CdKmWtV3oFx1C5yUbf5CY",
  "type": "message",
  "role": "assistant",
  "content": [
    {
      "type": "text",
      "text": "This image is a beautiful minimalist/flat-design illustration of a sunset landscape. Here's what it contains:\n\n**Sky & Sun:**\n- A warm gradient sky transitioning from golden-yellow at the top to deep orange toward the horizon\n- A large pale yellow sun positioned in the upper-right area\n\n**Birds:**\n- Three small silhouetted birds flying in the upper-left portion of the sky, depicted as simple \"M\" or \"v\" shapes\n\n**Mountains:**\n- Multiple layered mountain peaks in purple and maroon tones\n- The mountains overlap to create depth, with varying shades of dusty purple and deep burgundy\n\n**Water:**\n- A dark purple body of water at the bottom of the image\n- A reflection of the sun shown as horizontal cream/peach colored lines in the center-bottom area\n\nThe overall style is clean, geometric, and uses a warm sunset color palette (oranges, yellows, purples, and maroons), giving it a peaceful, serene aesthetic typical of modern vector/flat design artwork."
    }
  ],
  "model": "claude-opus-5",
  "stop_reason": "end_turn",
  "stop_sequence": null,
  "usage": {
    "input_tokens": 1030,
    "output_tokens": 350
  }
}

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Next steps