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Your first plugin

This tutorial creates a minimal Harness plugin and loads it into the Web UI. Start from a repository checkout that has completed the run-from-source path.

Create a local project

From the repository root, create a scratch project for the tutorial:

mkdir -p scratch-plugin/src

What is a plugin?

In Harness, a plugin is a TypeScript module that exports an apply function. The framework calls apply when loading the plugin and passes a ctx context object through which the plugin registers capabilities:

import type { Context } from '@deepseek-ai/cordis'

export const name = 'my-plugin'

export function apply(ctx: Context) {
  // Register capabilities here.
}

That is the complete configuration.

Create the plugin file

Create scratch-plugin/src/my-plugin.ts:

import type { Context } from '@deepseek-ai/cordis'

export const name = 'hello-plugin'

export function apply(ctx: Context) {
  // Required dependencies are ready before apply runs.
  console.log('[hello-plugin] plugin loaded!')
}

Register it in cordis.yml

Run pwd from the repository root, then create scratch-plugin/cordis.yml as a Web overlay that inserts the local plugin. Replace /absolute/path/to/deepseek-harness below with the printed path:

- insert:
    - id: hello
      name: '/absolute/path/to/deepseek-harness/scratch-plugin/src/my-plugin.ts'

The plugin path must be absolute. A patch file contributes configuration but does not change the profile directory from which the loader resolves module paths.

Start the Web UI with that overlay:

pnpm dsh web --patch ./scratch-plugin/cordis.yml --no-open --port 0

Open the launch URL printed in the terminal, including its authentication fragment. The terminal prints [hello-plugin] plugin loaded! during startup.

Edit and stop

Change the greeting in the plugin, stop the development command with Ctrl+C, and run the same command again. Verify the new greeting in the terminal. Use the newly printed launch URL after each restart; port 0 requests an available port from the OS. Stop the command when finished.

Automatic cleanup

Anything registered through ctx—event listeners, tools, or timers—is cleaned up when the plugin unloads. You do not need to call removeListener or clearInterval manually.

For a resource that needs explicit cleanup, such as a network connection, use ctx.effect() to provide its disposer:

import type { Context } from '@deepseek-ai/cordis'

export function apply(ctx: Context) {
  ctx.effect(() => {
    const timer = setInterval(() => {
      console.log('heartbeat')
    }, 5000)

    // The returned function runs when the plugin unloads.
    return () => clearInterval(timer)
  })
}

Declare dependencies

If the plugin consumes another service such as tools or llm, declare it in inject:

import type { Context } from '@deepseek-ai/cordis'

export const name = 'my-tool-plugin'
export const inject = ['tools']

export function apply(ctx: Context) {
  // ctx.tools is ready here.
  ctx.tools.register(/* ... */)
}

The framework waits for every required service before loading the plugin.

Three plugin forms

In addition to a function module, a plugin can use object or class form.

Object form

import type { Context } from '@deepseek-ai/cordis'

export default {
  name: 'my-plugin',
  inject: ['tools'],
  apply(ctx: Context) {
    // ...
  },
}

Class form

import { Service, type Context } from '@deepseek-ai/cordis'

export default class MyService extends Service {
  static inject = ['tools']

  constructor(ctx: Context) {
    super(ctx, 'myService')
    // Perform synchronous initialization in the constructor.
  }
}

Function form is sufficient in most cases. Use class form when the plugin provides a service to other plugins; see services and dependencies.

Next steps