Cowork · Lesson 05 Pro+ ~14 min Marketplace + custom

Extending Cowork: plugins, skills, MCP.

Out of the box Cowork is powerful. Its real superpower is extensibility — but there are three mechanisms that sound alike and trip everyone up at first. Plugins bundle. Skills produce files. MCP connectors reach services. Get the distinction once and the whole marketplace makes sense.

The mental model

Three extension types, three jobs.

They sound similar, so people conflate them. The clean split:

Plugins are bundles. A "Sales" plugin might package the Outlook connector + a follow-up-email Skill + prospecting prompt presets. Install one, get the whole workflow.
Skills are file-format experts. The docx Skill knows how to build a real Word file; xlsx knows Excel; pptx knows PowerPoint. Skills are how Cowork produces real files instead of chat text.
MCP connectors reach services. Outlook MCP connects to your email, Slack MCP to Slack, Salesforce MCP to your CRM. They're how Cowork touches systems beyond your local files.

You want Cowork to build a real .pptx deck. Which extension type actually produces the file?

Right. Skills are the engines that produce real-format files. MCP reaches out to services; Plugins just bundle Skills + connectors + prompts together. If a deck prompt isn't working, the first thing to check is whether the pptx Skill is installed. Let's drill the distinction.

Signature move · sort the toolbox

Plugin, Skill, or MCP?

Six things you could add to Cowork. Drop each into the right bucket. The trick: ask "does it bundle, does it build a file, or does it reach a service?"

Sort the capabilities 0 / 6 sorted

Plugin = bundle Skill = builds a file MCP = reaches a service

That's the whole model. Bundle, build, reach. Once you can place any new capability in one of those three buckets, the marketplace stops being confusing — you know exactly what each listing actually does before you install it.

Install vs. build

Check the marketplace before you build.

Most common workflows already have a plugin. Search the marketplace by role before you build anything custom — and vet every install the way you'd vet a browser extension.

Vet before you installFor any plugin you're about to install, check: 1. What MCP connectors it requires (some trigger OAuth) 2. What Skills it adds (confirm they're not doing more than they claim) 3. The publisher + community ratings — these flag the flaky ones

When to build your own

If your workflow is genuinely unique, the skill-creator Skill lets you bundle your templates, voice, and brand into a custom Skill. But a good one takes one to three hours to build — only worth it for a workflow you'll run at least 5 times. Below that bar, use the base Skills with a detailed prompt.

Use read-only credentials for any production database you connect via MCP. Even with permission gates, you don't want Cowork running a DELETE on prod by accident.

Your turn · install judgment

What would you do?

Four real decisions about extending Cowork. Pick the safe, sensible call.

Question 1 of 4

You've got it

A lean, current toolkit.

Audit your extensions

Open your Cowork settings and list every Plugin, Skill, and MCP connector installed. Mark which you've actually used this month. Uninstall the dead weight, install one thing you've been meaning to try. A lean, current toolkit beats a bloated, half-explored one.

Try a prompt in the playground → Back to Claude Mastery

What you can do now

Tell a Plugin (bundle) from a Skill (builds files) from an MCP connector (reaches services)
Search the marketplace by role before building anything custom
Vet installs like browser extensions — publisher, permissions, ratings
Use read-only credentials for any production system
Build a custom Skill only for workflows you'll run 5+ times

Pro+

Up next in Claude Mastery

Lesson 06 · Automation — scheduled tasks and live artifacts

The final piece: automate recurring work with scheduled tasks (daily briefings, weekly reports) and build live artifacts that pull fresh data every time you open them. This is what turns Cowork from a tool into a coworker. Start lesson 06 →

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