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C04 Intermediate Design

Skills, Plugins & Agents

Choosing the right extension mechanism for the task

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Comparison Table

MechanismScopeInvocationResourcesUse Case
Skill (user)Codified workflow/name (user)YesRepeatable task triggered manually
Skill (auto)Reusable knowledgeAuto by modelYesShared expertise across agents
PluginMarketplaceGlobalYesThird-party integration
AgentAutonomous specialistTask toolVia memoryComplex delegation

CC 2.1.3: .claude/commands/ is merged into .claude/skills/. Add disable-model-invocation: true for user-triggered-only skills.

User-Invocable Skills (formerly “Commands”)

Since CC 2.1.3, manually triggered workflows live in .claude/skills/ with disable-model-invocation: true.

# .claude/skills/release/SKILL.md
---
name: release
description: Prepare a release (bump version, CHANGELOG, tag)
allowed-tools: [Read, Write, Bash]
disable-model-invocation: true
---
1. Read current version from package.json
2. Bump according to semver (patch/minor/major)
3. Update CHANGELOG.md
4. Create git tag

When to choose: You have a sequence of instructions you trigger intentionally several times a week.

Model-Invocable Skills

A skill without disable-model-invocation is loaded automatically by the model when the description matches the context.

# .claude/skills/security-guardian/SKILL.md
---
name: security-guardian
description: OWASP security review -- use when reviewing auth, routes, or user input
allowed-tools: [Read, Grep, Glob]
---
Analyze code for OWASP Top 10 vulnerabilities...

When to choose: Multiple agents need the same specialized knowledge, or you want Claude to automatically load the right expertise.

Plugins: Third-Party Integrations

Plugins come from the marketplace and add external capabilities. Install via /plugin marketplace add.

When to choose: An existing third-party integration covers exactly your need.

${CLAUDE_PLUGIN_DATA} (v2.1.78+): Persistent directory for storing state between sessions.

Agents: Complex Task Delegation

An agent is a specialized Claude with its own tools and scope. It isolates context, not simulates a human role.

.claude/agents/security-audit.md
---
name: security-audit
model: opus
tools: Read, Grep, Glob
---
Analyze code for OWASP vulnerabilities...

When to choose: A long task that would pollute the main context, or parallelizable work.

Quick Decision Rule

Manually triggered workflow = User-invocable Skill. Knowledge to share across agents = Model-invocable Skill. Third-party integration = Plugin. Task to delegate with isolated context = Agent.

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