thinking type enabled with budget_tokens returns a 400 on Claude Haiku 5.5

Q: How do I fix this when moving to claude-haiku-5-5?

Remove the budget and set an effort level in output_config.effort.

Returns 400 Applies to claude-haiku-5-5 on the Anthropic Messages API.

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

How to fix it

Remove the budget and set an effort level in output_config.effort.

Before

{
  "max_tokens": 1024,
  "messages": [
    {
      "content": "Summarize this ticket.",
      "role": "user"
    }
  ],
  "model": "claude-haiku-4-5-20251001",
  "thinking": {
    "budget_tokens": 4000,
    "type": "enabled"
  }
}

After (Claude Haiku 5.5)

{
  "max_tokens": 1024,
  "messages": [
    {
      "content": "Summarize this ticket.",
      "role": "user"
    }
  ],
  "model": "claude-haiku-5-5",
  "output_config": {
    "effort": "high"
  },
  "thinking": {
    "type": "adaptive"
  }
}

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