> We recommend using the word "think" to trigger extended thinking mode, which gives Claude additional computation time to evaluate alternatives more thoroughly. These specific phrases are mapped directly to increasing levels of thinking budget in the system: "think" < "think hard" < "think harder" < "ultrathink." Each level allocates progressively more thinking budget for Claude to use.

I had a poke around and it's not a feature of the Claude model, it's specific to Claude Code. There's a "megathink" option too - it uses code that looks like this:

  let B = W.message.content.toLowerCase();
  if (
    B.includes("think harder") ||
    B.includes("think intensely") ||
    B.includes("think longer") ||
    B.includes("think really hard") ||
    B.includes("think super hard") ||
    B.includes("think very hard") ||
    B.includes("ultrathink")
  )
    return (
      l1("tengu_thinking", { tokenCount: 31999, messageId: Z, provider: G }),
      31999
    );
  if (
    B.includes("think about it") ||
    B.includes("think a lot") ||
    B.includes("think deeply") ||
    B.includes("think hard") ||
    B.includes("think more") ||
    B.includes("megathink")
  )
    return (
      l1("tengu_thinking", { tokenCount: 1e4, messageId: Z, provider: G }), 1e4
    );