Server Tools

What are Server Tools?#

Server tools are functions that run on your backend that the Built-in Agent can invoke. They're defined using defineTool() with Zod schemas for type-safe parameters.

When should I use this?#

Your agent needs to access databases, APIs, or other backend services
You want type-safe tool parameters with validation
The tool logic requires server-side secrets or resources

The tool runs on the server; the UI still runs in the browser

execute runs on your server, so it can reach secrets, a database, or an internal service — and it cannot touch the DOM, component state, or anything else in the page. When the action itself belongs in the browser, register it as a frontend tool instead.

Rendering is separate from executing. A server tool's call still streams to the browser, so you can give it UI with a render-only registration under the same tool name while execute keeps running on the server.

Use registerFrontendTool for the browser-side action and registerRenderToolCall for the render-only registration.

Defining a tool#

src/copilotkit.ts

import { BuiltInAgent, defineTool } from "@copilotkit/runtime/v2";
import { z } from "zod";

const getWeather = defineTool({
  name: "getWeather",
  description: "Get the current weather for a location",
  parameters: z.object({
    location: z.string().describe("The location's name"),
  }),
  execute: async ({ location }) => {
    // Your implementation here
    return { temperature: 72, condition: "sunny", location };
  },
});

const builtInAgent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  tools: [getWeather],
  maxSteps: 2                   //Important for tool calls
});

`parameters` takes a schema validator, not JSON Schema

parameters must be a Standard Schema v1 validator — Zod, Valibot, ArkType and others qualify. Handing it a plain JSON Schema object throws while the tool's schema is being converted. The validator is also what gives execute its inferred argument type, so a hand-written JSON Schema loses that too.

Tool response#

Tools can return any JSON-serializable value. The agent uses the response to continue the conversation.

Multiple tools#

Pass an array of tools — the agent chooses which to call based on the user's request:

Two names to avoid

AGUISendStateSnapshot and AGUISendStateDelta are injected for you whenever the Built-in Agent makes the model call itself. Defining your own tool under either name replaces the built-in one and breaks shared state.

A server tool also wins any name collision with a frontend tool: define getWeather on both sides and the model only ever sees the server one, so the frontend handler never fires. Give each side a distinct name when both need their own.

src/copilotkit.ts

const searchDocs = defineTool({
  name: "searchDocs",
  description: "Search the documentation for relevant articles",
  parameters: z.object({
    query: z.string().describe("The search query"),
  }),
  execute: async ({ query }) => {
    const results = await search(query);
    return { results, count: results.length };
  },
});

const createTicket = defineTool({
  name: "createTicket",
  description: "Create a support ticket",
  parameters: z.object({
    title: z.string().describe("Ticket title"),
    priority: z.enum(["low", "medium", "high"]).describe("Ticket priority"),
    description: z.string().describe("Detailed description of the issue"),
  }),
  execute: async ({ title, priority, description }) => {
    const ticket = await db.tickets.create({ title, priority, description });
    return { ticketId: ticket.id, status: "created" };
  },
});

const builtInAgent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  tools: [searchDocs, createTicket], 
  maxSteps: 2                        
});

Complex Zod schemas#

Use nested objects, arrays, enums, and optional fields for sophisticated tool parameters:

const bookFlight = defineTool({
  name: "bookFlight",
  description: "Search for and book flights",
  parameters: z.object({
    trip: z.object({
      origin: z.string().describe("Origin airport code (e.g., SFO)"),
      destination: z.string().describe("Destination airport code (e.g., JFK)"),
      date: z.string().describe("Departure date in YYYY-MM-DD format"),
    }),
    passengers: z.array(
      z.object({
        name: z.string(),
        seatPreference: z.enum(["window", "middle", "aisle"]).optional(),
      })
    ).describe("List of passengers"),
    class: z.enum(["economy", "business", "first"]).default("economy"),
  }),
  execute: async ({ trip, passengers, class: seatClass }) => {
    const flights = await searchFlights(trip, seatClass);
    return { flights, passengerCount: passengers.length };
  },
});

Error handling#

Throw errors or return error objects from your tool — the agent will see the error and can inform the user or try a different approach:

const getUser = defineTool({
  name: "getUser",
  description: "Look up a user by email",
  parameters: z.object({
    email: z.string().email().describe("The user's email address"),
  }),
  execute: async ({ email }) => {
    const user = await db.users.findByEmail(email);
    if (!user) {
      throw new Error(`No user found with email: ${email}`); 
    }
    return { id: user.id, name: user.name, role: user.role };
  },
});

Return plain data

Whatever a tool returns is serialized before the model sees it. A class instance, a circular reference, or anything else JSON.stringify refuses is replaced by a placeholder string naming the tool, and the model reasons over that instead of your data. Return plain objects, arrays and primitives — including for error shapes.

Multi-step tool calling#

By default, the agent performs a single step. If your agent needs to chain tool calls (e.g., search first, then create a ticket), set maxSteps:

const builtInAgent = new BuiltInAgent({
  model: "openai:gpt-5.4-mini",
  maxSteps: 5, 
  tools: [searchDocs, createTicket, getUser],
});

With maxSteps: 5, the agent can:

Call searchDocs to find relevant info
Process the result
Call createTicket with details from the search
Continue until done (up to 5 iterations)

See Advanced Configuration for more options like toolChoice, temperature, and providerOptions.

A factory ignores the `tools` array

Everything on this page describes a Built-in Agent constructed with model and tools. If you build one from a factory instead, the tools array is not read — the factory owns the model call, so it has to pass the tools itself. Convert them with convertToolDefinitionsToVercelAITools for an AI SDK factory, or the equivalent converter for your runner, and pass the result into the call the factory makes.

Using a custom backend?

If you're using the Custom Agent instead of BuiltInAgent, see the "With Tools" examples for how to wire tools with AI SDK, TanStack AI, or custom backends.