> ## Documentation Index
> Fetch the complete documentation index at: https://docs.open-harness.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom Tools

> Create your own tools using the AI SDK tool() helper

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

## Creating a Custom Tool

```typescript theme={"dark"}
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 },
});
```

## Combining Built-in and Custom Tools

Mix and match built-in tools with your own:

```typescript theme={"dark"}
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 },
});
```

## 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