Simon Willison’s Weblog

21st March 2025 - Link Blog

The "think" tool: Enabling Claude to stop and think in complex tool use situations (via) Fascinating new prompt engineering trick from Anthropic. They use their standard tool calling mechanism to define a tool called "think" that looks something like this:

{
  "name": "think",
  "description": "Use the tool to think about something. It will not obtain new information or change the database, but just append the thought to the log. Use it when complex reasoning or some cache memory is needed.",
  "input_schema": {
    "type": "object",
    "properties": {
      "thought": {
        "type": "string",
        "description": "A thought to think about."
      }
    },
    "required": ["thought"]
  }
}

This tool does nothing at all.

LLM tools (like web_search) usually involve some kind of implementation - the model requests a tool execution, then an external harness goes away and executes the specified tool and feeds the result back into the conversation.

The "think" tool is a no-op - there is no implementation, it just allows the model to use its existing training in terms of when-to-use-a-tool to stop and dump some additional thoughts into the context.

This works completely independently of the new "thinking" mechanism introduced in Claude 3.7 Sonnet.

Anthropic's benchmarks show impressive improvements from enabling this tool. I fully anticipate that models from other providers would benefit from the same trick.