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Claude Native Format

POST /v1/messages
This page only documents Claude Messages behavior that was revalidated against Crazyrouter production on 2026-03-22. Current primary example models:
  • claude-sonnet-4-6
  • claude-opus-4-6-thinking

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Authentication

This recheck confirmed that both of the following auth styles work: 
x-api-key: YOUR_API_KEY

Authorization: Bearer YOUR_API_KEY
Keep: 
anthropic-version: 2023-06-01

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Basic conversation

cURL
curl https://crazyrouter.com/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: YOUR_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-sonnet-4-6",
    "max_tokens": 128,
    "messages": [
      {
        "role": "user",
        "content": "Explain quantum computing in one sentence."
      }
    ]
  }'
Verified response shape: 
{
  "type": "message",
  "content": [
    {
      "type": "text",
      "text": "..."
    }
  ]
}

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Function calling

In the current production environment, Claude 4.6 tool calling is reproducible through /v1/messages and returns tool_use. For first-pass validation, use:
  • an explicit tool_choice
  • a prompt that clearly says Claude must call the tool and not answer directly
cURL
curl https://crazyrouter.com/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: YOUR_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-sonnet-4-6",
    "max_tokens": 256,
    "tools": [
      {
        "name": "get_time",
        "description": "Get the current time for a timezone",
        "input_schema": {
          "type": "object",
          "properties": {
            "timezone": {
              "type": "string"
            }
          },
          "required": ["timezone"]
        }
      }
    ],
    "tool_choice": {
      "type": "any"
    },
    "messages": [
      {
        "role": "user",
        "content": "Use the get_time tool for Asia/Shanghai. Do not answer directly."
      }
    ]
  }'
Verified response shape: 
{
  "stop_reason": "tool_use",
  "content": [
    {
      "type": "tool_use",
      "name": "get_time",
      "input": {
        "timezone": "Asia/Shanghai"
      }
    }
  ]
}

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Extended thinking

In this production recheck, the combination that clearly returned a thinking block was:
  • claude-opus-4-6-thinking
Base claude-opus-4-6 did not produce an equally explicit thinking block in this round, so the docs prioritize the thinking variant here.
cURL
curl https://crazyrouter.com/v1/messages \
  -H "Content-Type: application/json" \
  -H "x-api-key: YOUR_API_KEY" \
  -H "anthropic-version: 2023-06-01" \
  -d '{
    "model": "claude-opus-4-6-thinking",
    "max_tokens": 320,
    "thinking": {
      "type": "enabled",
      "budget_tokens": 128
    },
    "messages": [
      {
        "role": "user",
        "content": "Which is larger, 9.11 or 9.9? Explain briefly."
      }
    ]
  }'
Verified response shape: 
{
  "content": [
    {
      "type": "thinking",
      "thinking": "..."
    },
    {
      "type": "text",
      "text": "..."
    }
  ]
}
When using thinking, max_tokens must be greater than budget_tokens. Thinking tokens are billed as well.

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Current recommendation

  • Standard text chat: use claude-sonnet-4-6
  • Tool calling: use claude-sonnet-4-6, and include tool_choice in first-pass validation
  • Explicit thinking blocks: use claude-opus-4-6-thinking
Related pages: