citations on a user text block return a 400 in API version 2026-10-21

Q: How do I fix this when moving to claude-*?

Remove the citations array from text blocks in user messages, including blocks inside search_result.

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

Remove the citations array from text blocks in user messages, including blocks inside search_result.

Before

{
  "max_tokens": 1024,
  "messages": [
    {
      "content": [
        {
          "citations": [
            {
              "cited_text": "x",
              "document_index": 0,
              "end_char_index": 1,
              "start_char_index": 0,
              "type": "char_location"
            }
          ],
          "text": "As noted earlier.",
          "type": "text"
        }
      ],
      "role": "user"
    }
  ],
  "model": "claude-opus-4-8"
}

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

{
  "max_tokens": 1024,
  "messages": [
    {
      "content": [
        {
          "text": "As noted earlier.",
          "type": "text"
        }
      ],
      "role": "user"
    }
  ],
  "model": "claude-opus-4-8"
}

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