Plugin vs skill (Cowork)
What you'll learn
Plugins add capabilities (what the agent CAN do). Skills add instructions (HOW the agent should work). Both attach to a Cowork task.
- Plugin = capability (what the agent can do).
- Skill = instructions (how the agent works on a task).
- Without the plugin, the capability isn't present.
Cowork is the persistent, multi-step task workspace — a managed agent that keeps state across hours or days, runs file work, and exposes plugins and skills. Two extension surfaces: **plugins** add *capabilities* (file system, web access, integrations); **skills** add *instructions* (markdown bundles steering how the agent works). The distinction is the same as elsewhere in the Anthropic ecosystem — instructions vs. capabilities — but it matters specifically inside Cowork because both are first-class menu surfaces.
Plugins are *what the agent can do*. Installing a filesystem plugin means the agent can now read and write files; installing a web plugin means it can now fetch URLs. Plugins introduce tools (sometimes resources or prompts via MCP shape) and side-effecting capabilities. Without the plugin, the capability simply isn't there.
Skills are *how the agent should work* on a given task. A `weekly-report` skill steers the agent's approach to producing weekly reports; it doesn't add any capability the agent didn't have. Skills load on user invocation; their body is instructions the agent follows during that task.
The miswire: putting capabilities in a skill (it's just instructions; the underlying tool doesn't exist) or putting instructions-only behaviour in a plugin (the agent gets no new capability and the menu is cluttered with what should be a Skill). The decision rule: 'does this add a new ability the agent didn't have, or does it steer how the agent uses existing abilities?' Ability → plugin. Steering → skill.
Key points
- Plugin = capability (what the agent can do).
- Skill = instructions (how the agent works on a task).
- Without the plugin, the capability isn't present.
- Skills steer existing capabilities; they don't add new ones.
Examples
Installs read/write file tools. The agent can now interact with the local file system. Capability is now present.
Markdown body that says 'Group by component, list breaking changes first, etc.' Doesn't add any tool; uses whatever the agent already has (filesystem plugin, web plugin) to do the work.
Pitfalls
- Trying to ship a 'plugin' that's just instructions. It's a Skill.
- Trying to encode a capability in a Skill. The instructions can't conjure a tool that doesn't exist; install the plugin.
- Treating plugins and skills as substitutes. They're complementary — most useful tasks need both.
Source notes: 00-academy-basics/notes/09-cowork.md
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