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Any AI SDK-compatible tool works with OpenHarness. Define tools using the tool() function from the ai package with Zod schemas for input validation.

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Creating a Custom Tool

import { tool } from "ai";
import { z } from "zod";

const myTool = tool({
  description: "Do something useful",
  inputSchema: z.object({ query: z.string() }),
  execute: async ({ query }) => {
    return { result: `You asked: ${query}` };
  },
});

const agent = new Agent({
  name: "my-agent",
  model: openai("gpt-5.4"),
  tools: { myTool },
});

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Combining Built-in and Custom Tools

Mix and match built-in tools with your own: 
import { createFsTools, createBashTool, NodeFsProvider, NodeShellProvider } from "@openharness/core";
import { tool } from "ai";
import { z } from "zod";

const fsTools = createFsTools(new NodeFsProvider());
const { bash } = createBashTool(new NodeShellProvider());

const deployTool = tool({
  description: "Deploy the application to production",
  inputSchema: z.object({
    environment: z.enum(["staging", "production"]),
    version: z.string(),
  }),
  execute: async ({ environment, version }) => {
    // your deployment logic
    return { status: "deployed", environment, version };
  },
});

const agent = new Agent({
  name: "dev",
  model: openai("gpt-5.4"),
  tools: { ...fsTools, bash, deploy: deployTool },
});

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Tool Best Practices

  • Clear descriptions help the model understand when and how to use each tool
  • Zod schemas provide both validation and type safety
  • Return structured data so the model can reason about results
  • Handle errors gracefully — thrown errors are surfaced to the model as tool errors, allowing it to adjust its approach