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Anthropic · AIintermediate

Anthropic Messages API Request

A request body for the Anthropic Messages API (POST /v1/messages) with adaptive thinking, a system prompt, tools and a multi-turn conversation.

anthropicclaudellmaimessages-apitools

Preview

{
  "model": "claude-opus-4-8",
  "max_tokens": 1024,
  "system": "You are a concise assistant. Answer directly.",
  "thinking": {
    "type": "adaptive"
  },
  "messages": [
    {
      "role": "user",
      "content": "What is the capital of France?"
    },
    {
      "role": "assistant",
      "content": "Paris."
    },
    {
      "role": "user",
      "content": "And its population, roughly?"
    }
  ],
  "tools": [
    {
      "name": "get_population",
      "description": "Get the approximate population of a city.",
      "input_schema": {
        "type": "object",
        "properties": {
          "city": {
            "type": "string",
            "description": "City name"
          }
        },
        "required": [
          "city"
        ]
      }
    }
  ]
}

AI actions

Documentation

Purpose

Call Claude via the Messages API — the single endpoint for chat, tool use and structured output.

When to use

Building an application on Claude: pass this JSON as the body of POST https://api.anthropic.com/v1/messages.

Required fields

  • model — the Claude model id, e.g. claude-opus-4-8
  • max_tokens — the hard cap on output tokens
  • messages — the conversation turns (user/assistant)

Optional fields

  • system — a top-level system prompt (string or blocks)
  • thinking — {"type":"adaptive"} lets Claude decide how much to think
  • tools — function definitions Claude may call
  • stream — set true to receive server-sent events

Best practices

  • Default to model "claude-opus-4-8" unless you need a smaller tier.
  • Use thinking {"type":"adaptive"} rather than a fixed token budget.
  • Stream when max_tokens is large to avoid HTTP timeouts.

Security considerations

  • Send your key in the x-api-key header, never in this body.
  • Keep API keys server-side; never ship them to a browser.
  • Do not put secrets in the prompt — they are sent to the API and logged app-side.

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