Repro: Vercel AI Gateway rejects Kimi K3 documented builtin_function $web_search tool

kimi_k3_vercel_web_search_repro.mjs

// npm install openai
import OpenAI from "openai";

const client = new OpenAI({
  apiKey: process.env.VERCEL_API_KEY,
  baseURL: "https://ai-gateway.vercel.sh/v1",
});

const completion = await client.chat.completions.create({
  model: "moonshotai/kimi-k3",
  messages: [
    { role: "system", content: "You are Kimi." },
    {
      role: "user",
      content:
        "Please search for Moonshot AI Context Caching technology and tell me what it is.",
    },
  ],
  tools: [
    {
      type: "builtin_function",
      function: { name: "$web_search" },
    },
  ],
});

console.log(JSON.stringify(completion, null, 2));

Result

BadRequestError: 400 Invalid input: expected "function"

status: 400
type: invalid_request_error
param: tools.0.type
code: invalid_request_error

Kimi documentation: https://platform.kimi.ai/docs/guide/use-web-search