Look around. The same Claude is behind both. The difference is ownership: ⌘J is yours, a coworker belongs to the workspace.
The personal case → ⌘J
You’re in a channel, you’ve got a question, you want the answer.
Press ⌘J. Your shell opens with the channel’s context and the union of every CLI, every MCP server, and every script the workspace has connected, plus anything installed on your own machine. Ask. Read. Move on.
⌘J is yours. The tool surface is unlimited and shifts with what you and the workspace have wired up. You don’t add anything to the team to use it; you just press the key.
The shared case → AI coworker
A question keeps coming up across the workspace and the answer is always the same kind of work. “Which deals are at risk this week?” “Who’s on call?” “What was the launch metric yesterday?”
Add a coworker. A coworker is workspace-shared: it shows up in the member list like a teammate, anyone in the workspace can @mention it, and it can be added to whichever channels its job applies in.
Give it a narrow purpose and a short toolset. The signal you’ve got a coworker waiting to exist: the same task is happening across people, not just you.
The threshold
A rough rule: if the same task happens across the team’s people, not just one of them, that’s a coworker. If it’s only you (or mostly you), ⌘J is fine. If it happens once a quarter, just do it by hand.
Coworkers are cheap to add but they accumulate in the workspace member list. A workspace with twenty coworkers is harder to read than one with five and a shared ⌘J habit. Add fewer, sharper coworkers and let ⌘J handle the rest.
Two examples
Sales channel. The sales lead asks “which deals slipped this week?” every Monday morning. Third time you answer it, that’s a coworker. Give it HubSpot read access and a narrow purpose (“watch deals that slipped this week, post a list when @mentioned”). The lead @mentions it instead of asking.
Engineering channel. Someone asks “what’s failing in CI right now?” once a day on average. ⌘J is fine here: the question is ad-hoc, the answer is fresh every time. A coworker that polls CI would either fire too often (noise) or too rarely (stale).
The shape question
The simpler question, when in doubt: would this be a job for an extra teammate, or a tool you reach for?
Job for a teammate, recurring across people → coworker. Workspace-shared, named, with a defined responsibility.
Tool you reach for, mostly your own use → ⌘J. Personal shell, unlimited tool surface, no commitment from the rest of the team.
Going deeper
When you’ve decided it’s a coworker, Designing a useful AI coworker covers the choices that make one work. When you’ve decided it’s ⌘J, Press ⌘J: a shell in every channel covers the first things worth trying.