hermes-ice/homelab/raw/articles/opencode/docs/custom-tools.md

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type	agent	source	scraped	content_hash
agent-doc	OpenCode	https://opencode.ai/docs/custom-tools/	2026-04-28T21:02:14.861865+00:00	34c22729

Custom Tools

Create tools the LLM can call in opencode.

Custom tools are functions you create that the LLM can call during conversations. They work alongside opencode’s built-in tools like read, write, and bash.

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Creating a tool

Tools are defined as TypeScript or JavaScript files. However, the tool definition can invoke scripts written in any language — TypeScript or JavaScript is only used for the tool definition itself.

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Location

They can be defined:

• Locally by placing them in the .opencode/tools/ directory of your project.
• Or globally, by placing them in ~/.config/opencode/tools/.

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Structure

The easiest way to create tools is using the tool() helper which provides type-safety and validation.

import { tool } from "@opencode-ai/plugin"
export default tool({  description: "Query the project database",  args: {    query: tool.schema.string().describe("SQL query to execute"),  },  async execute(args) {    // Your database logic here    return `Executed query: ${args.query}`  },})

The filename becomes the tool name. The above creates a database tool.

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Multiple tools per file

You can also export multiple tools from a single file. Each export becomes a separate tool with the name _:

import { tool } from "@opencode-ai/plugin"
export const add = tool({  description: "Add two numbers",  args: {    a: tool.schema.number().describe("First number"),    b: tool.schema.number().describe("Second number"),  },  async execute(args) {    return args.a + args.b  },})
export const multiply = tool({  description: "Multiply two numbers",  args: {    a: tool.schema.number().describe("First number"),    b: tool.schema.number().describe("Second number"),  },  async execute(args) {    return args.a * args.b  },})

This creates two tools: math_add and math_multiply.

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Name collisions with built-in tools

Custom tools are keyed by tool name. If a custom tool uses the same name as a built-in tool, the custom tool takes precedence.

For example, this file replaces the built-in bash tool:

import { tool } from "@opencode-ai/plugin"
export default tool({  description: "Restricted bash wrapper",  args: {    command: tool.schema.string(),  },  async execute(args) {    return `blocked: ${args.command}`  },})

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Arguments

You can use tool.schema, which is just Zod, to define argument types.

args: {  query: tool.schema.string().describe("SQL query to execute")}

You can also import Zod directly and return a plain object:

import { z } from "zod"
export default {  description: "Tool description",  args: {    param: z.string().describe("Parameter description"),  },  async execute(args, context) {    // Tool implementation    return "result"  },}

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Context

Tools receive context about the current session:

import { tool } from "@opencode-ai/plugin"
export default tool({  description: "Get project information",  args: {},  async execute(args, context) {    // Access context information    const { agent, sessionID, messageID, directory, worktree } = context    return `Agent: ${agent}, Session: ${sessionID}, Message: ${messageID}, Directory: ${directory}, Worktree: ${worktree}`  },})

Use context.directory for the session working directory. Use context.worktree for the git worktree root.

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Examples

Write a tool in Python

You can write your tools in any language you want. Here’s an example that adds two numbers using Python.

First, create the tool as a Python script:

import sys
a = int(sys.argv[1])b = int(sys.argv[2])print(a + b)

Then create the tool definition that invokes it:

import { tool } from "@opencode-ai/plugin"import path from "path"
export default tool({  description: "Add two numbers using Python",  args: {    a: tool.schema.number().describe("First number"),    b: tool.schema.number().describe("Second number"),  },  async execute(args, context) {    const script = path.join(context.worktree, ".opencode/tools/add.py")    const result = await Bun.$`python3 ${script} ${args.a} ${args.b}`.text()    return result.trim()  },})

Here we are using the Bun.$ utility to run the Python script.