Example Custom Agent

Example custom agent

In Plain, Ask Sidekick lets a teammate pick which agent answers a thread.

Pick a custom agent and power the Ask Sidekick surface with your own agent.

How it works

Plain fires a webhook at your server and waits for you to write the reply back. It answers by running the `claude` CLI on whatever machine it is running on, then posts the answer into the discussion.

┌─────────────────────┐                              ┌─────────────────────┐
│        Plain        │  discussion.message_created  │     this process    │
│     Ask Sidekick    │ ───────────────────────────> │    bun run serve    │
│                     │                              │                     │
│                     │ <─────────────────────────── │                     │
└─────────────────────┘    sendDiscussionMessage     └──────────┬──────────┘
                                                                │
                                                                v
                                                     ┌─────────────────────┐
                                                     │     Claude Code     │
                                                     │      claude -p      │
                                                     └─────────────────────┘

Claude runs with `--permission-mode auto` and no sandbox. The prompt is whatever someone typed into a discussion, which means anyone in your workspace effectively has a shell on this machine.

It's recommended to run this in a sandbox when deployed in a production environment.

Written in TypeScript, run with [Bun](https://bun.sh).

Setting it up

1. Create a machine user under [Settings → Machine users](https://app.plain.com/~/settings/machine-users/) and give it an API key.

   Make sure you also toggle the "Custom agent" toggle on the machine user so it's available as a target when running a Sidekick conversation.

   Minimum permissions required:

   • `threadDiscussionMess