A document block in an assistant message returns a 400 in API version 2026-10-21

Returns 400 Applies to claude-* on the Anthropic Messages API with anthropic-version: 2026-10-21.

Anthropic's docs describe this error but don't quote the exact message, so we don't either.

How to fix it

Move the document into a user message.

Before

{
  "max_tokens": 1024,
  "messages": [
    {
      "content": "Read this.",
      "role": "user"
    },
    {
      "content": [
        {
          "source": {
            "data": "...",
            "media_type": "text/plain",
            "type": "text"
          },
          "type": "document"
        }
      ],
      "role": "assistant"
    },
    {
      "content": "Summarize it.",
      "role": "user"
    }
  ],
  "model": "claude-opus-4-8"
}

After (Claude Opus 4.8 (API version rules only))

{
  "max_tokens": 1024,
  "messages": [
    {
      "content": [
        {
          "source": {
            "data": "...",
            "media_type": "text/plain",
            "type": "text"
          },
          "type": "document"
        },
        {
          "text": "Summarize it.",
          "type": "text"
        }
      ],
      "role": "user"
    }
  ],
  "model": "claude-opus-4-8"
}

Check your whole request in the linter