AI EngineerGuide

Forced tool_choice on Claude Haiku 5.5 skips thinking

Behavior change Applies to claude-haiku-5-5 on the Anthropic Messages API.

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

How to fix it

It still works, but the response starts with the tool call and has no thinking block. Use auto with a strict tool if you want the model to reason first.

Before
{
  "max_tokens": 1024,
  "messages": [
    {
      "content": "Summarize this ticket.",
      "role": "user"
    }
  ],
  "model": "claude-haiku-4-5-20251001",
  "tool_choice": {
    "type": "any"
  },
  "tools": [
    {
      "description": "Fetch an invoice",
      "input_schema": {
        "additionalProperties": false,
        "properties": {
          "id": {
            "type": "string"
          }
        },
        "required": [
          "id"
        ],
        "type": "object"
      },
      "name": "get_invoice"
    }
  ]
}
After (Claude Haiku 5.5)
{
  "max_tokens": 1024,
  "messages": [
    {
      "content": "Summarize this ticket.",
      "role": "user"
    }
  ],
  "model": "claude-haiku-5-5",
  "tool_choice": {
    "type": "auto"
  },
  "tools": [
    {
      "description": "Fetch an invoice",
      "input_schema": {
        "additionalProperties": false,
        "properties": {
          "id": {
            "type": "string"
          }
        },
        "required": [
          "id"
        ],
        "type": "object"
      },
      "name": "get_invoice",
      "strict": true
    }
  ]
}
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Official source

Claude Haiku 5.5 accepts a forced tool_choice (any or a named tool), but the response starts with the tool call and has no thinking block.

https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide (last checked 2026-11-02)

Other changes for the same models

All migration errors

FAQ

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

It still works, but the response starts with the tool call and has no thinking block. Use auto with a strict tool if you want the model to reason first.

Where is this documented?

In Anthropic’s official docs: https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide, last checked 2026-11-02.