Custom tools

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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Tool structure

Tools are defined as .ts/.js files in the .opencode/tool/ directory. They can also be defined globally in ~/.config/opencode/tool/.

The easiest way to create tools is using the tool() helper which provides type safety and validation. Use tool.schema (which is just Zod) to define argument types:

.opencode/tool/database.ts

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}`
  },
})

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"
  },
}

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

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Multiple tools per file

You can export multiple tools from a single file. Each export becomes a separate tool with the name <filename>_<exportname>:

.opencode/tool/math.ts

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.

---

Arguments

Use tool.schema (which is just Zod) to define tool arguments with validation and descriptions:

.opencode/tool/calculator.ts

import { tool } from "@opencode-ai/plugin"

export default tool({
  description: "Perform mathematical calculations",
  args: {
    expression: tool.schema.string().describe("Mathematical expression to evaluate"),
    precision: tool.schema.number().optional().describe("Decimal precision"),
  },
  async execute(args) {
    // Your calculation logic here
    return `Result: ${eval(args.expression).toFixed(args.precision || 2)}`
  },
})

---

Context

Tools receive context about the current session:

.opencode/tool/project.ts

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 } = context
    return `Agent: ${agent}, Session: ${sessionID}, Message: ${messageID}`
  },
})