Shayan Rais
e6bfcf2217
Adds the time trio (agent, command, skill) demonstrating all three extension mechanisms for the same task, plus a comprehensive report comparing when to use each pattern with worked examples and screenshots. Co-Authored-By: Claude <noreply@anthropic.com>
213 lines
8.6 KiB
Markdown
213 lines
8.6 KiB
Markdown
# Agents vs Commands vs Skills — When to Use What
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A comparison of the three extension mechanisms in Claude Code: subagents, commands, and skills.
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<table width="100%">
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<tr>
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<td><a href="../">← Back to Claude Code Best Practice</a></td>
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<td align="right"><img src="../!/claude-jumping.svg" alt="Claude" width="60" /></td>
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</tr>
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</table>
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---
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## At a Glance
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| | Agent | Command | Skill |
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|---|---|---|---|
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| **Location** | `.claude/agents/<name>.md` | `.claude/commands/<name>.md` | `.claude/skills/<name>/SKILL.md` |
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| **Context** | Separate subagent process | Inline (main conversation) | Inline (main conversation) |
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| **User-invocable** | No `/` menu — invoked by Claude or via Agent tool | Yes — `/command-name` | Yes — `/skill-name` (unless `user-invocable: false`) |
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| **Auto-invoked by Claude** | Yes — via `description` field | No | Yes — via `description` field (unless `disable-model-invocation: true`) |
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| **Accepts arguments** | Via `prompt` parameter | `$ARGUMENTS`, `$0`, `$1` | `$ARGUMENTS`, `$0`, `$1` |
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| **Dynamic context injection** | No | Yes — `` !`command` `` | Yes — `` !`command` `` |
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| **Own context window** | Yes — isolated | No — shares main | No — shares main (unless `context: fork`) |
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| **Model override** | `model:` frontmatter | `model:` frontmatter | `model:` frontmatter |
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| **Tool restrictions** | `tools:` / `disallowedTools:` | `allowed-tools:` | `allowed-tools:` |
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| **Hooks** | `hooks:` frontmatter | — | `hooks:` frontmatter |
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| **Memory** | `memory:` frontmatter (user/project/local) | — | — |
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| **Can preload skills** | Yes — `skills:` frontmatter | — | — |
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| **MCP servers** | `mcpServers:` frontmatter | — | — |
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---
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## When to Use Each
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### Use an Agent when:
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- The task is **autonomous and multi-step** — the agent needs to explore, decide, and act without constant guidance
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- You need **context isolation** — the work shouldn't pollute the main conversation window
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- The agent needs **persistent memory** across sessions (e.g., a code reviewer that learns patterns)
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- You want to **preload domain knowledge** via skills without cluttering the main context
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- The task benefits from **running in the background** or in a **git worktree**
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- You need **tool restrictions** or a **different permission mode** (e.g., `acceptEdits`, `plan`)
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**Example**: `weather-agent` — autonomously fetches weather data using its preloaded `weather-fetcher` skill, runs in a separate context with restricted tools.
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### Use a Command when:
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- You need a **user-initiated entry point** — a workflow the user explicitly triggers
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- The workflow involves **orchestrating** other agents or skills
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- You want **dynamic context injection** (`` !`git diff` ``, `` !`gh issue view $0` ``) to pull live data into the prompt
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- The task should run **inline** in the main conversation so the user sees everything
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**Example**: `weather-orchestrator` — the user triggers it, it asks for C/F preference, invokes the agent, then invokes the SVG skill.
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### Use a Skill when:
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- You want **Claude to auto-invoke** based on user intent (semantic matching via `description`)
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- The task is a **reusable procedure** that can be invoked from multiple places (commands, agents, or Claude itself)
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- You need **agent preloading** — baking domain knowledge into a specific agent at startup
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- The task is **lightweight** and doesn't need a separate context window
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**Example**: `weather-svg-creator` — Claude auto-invokes it when the user asks for a weather card; also callable from commands.
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---
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## The Command → Agent → Skill Architecture
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This repository demonstrates a layered orchestration pattern:
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```
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User triggers /command
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↓
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Command orchestrates the workflow
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↓
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Command invokes Agent (separate context, autonomous)
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↓
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Agent uses preloaded Skill (domain knowledge)
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↓
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Command invokes Skill (inline, for output generation)
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```
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**Concrete example** — the weather system:
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```
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/weather-orchestrator (command — entry point, asks C/F)
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↓
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weather-agent (agent — fetches temperature autonomously)
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├── weather-fetcher (agent skill — preloaded API instructions)
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↓
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weather-svg-creator (skill — creates SVG inline)
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```
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---
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## Frontmatter Comparison
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### Agent Frontmatter
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```yaml
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---
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name: my-agent
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description: Use this agent PROACTIVELY when...
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tools: Read, Write, Edit, Bash
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model: sonnet
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maxTurns: 10
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permissionMode: acceptEdits
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memory: user
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skills:
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- my-skill
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---
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```
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### Command Frontmatter
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```yaml
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---
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description: Do something useful
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argument-hint: [issue-number]
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allowed-tools: Read, Edit, Bash(gh *)
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model: sonnet
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---
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```
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### Skill Frontmatter
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```yaml
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---
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name: my-skill
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description: Do something when the user asks for...
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argument-hint: [file-path]
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disable-model-invocation: false
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user-invocable: true
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allowed-tools: Read, Grep, Glob
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model: sonnet
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context: fork
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agent: general-purpose
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---
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```
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---
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## Key Distinctions
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### Auto-invocation
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| Mechanism | Can Claude auto-invoke? | How to prevent |
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|-----------|------------------------|----------------|
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| Agent | Yes — via `description` (use "PROACTIVELY" to encourage it) | Remove or soften the description |
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| Command | No — always user-initiated via `/` | N/A |
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| Skill | Yes — via `description` | Set `disable-model-invocation: true` |
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### Visibility in `/` menu
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| Mechanism | Appears in `/` menu? | How to hide |
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|-----------|---------------------|-------------|
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| Agent | No | N/A |
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| Command | Yes — always | Cannot be hidden |
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| Skill | Yes — by default | Set `user-invocable: false` |
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### Context isolation
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| Mechanism | Runs in own context? | How to configure |
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|-----------|---------------------|-----------------|
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| Agent | Always | Built-in behavior |
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| Command | Never | N/A |
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| Skill | Optional | Set `context: fork` |
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---
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## Worked Example: "What is the current time?"
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This repository has all three mechanisms defined for the same task — displaying the current time in PKT. Here's what happens when a user types **"What is the current time?"** without explicitly invoking any `/` command:
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| Mechanism | Will it fire? | Why / Why not |
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|-----------|--------------|---------------|
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| `time-command` | No | Commands are **never auto-invoked**. The user would need to explicitly type `/time-command` for it to run. Commands have no auto-discovery pathway — they are strictly user-initiated. |
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| `time-agent` | **Yes** (possible) | The agent's `description` says *"Use this agent to display the current time in Pakistan Standard Time"*. Claude matches this against the user's intent and may spawn it via the Agent tool. However, agents run in a **separate context window**, making them heavier than necessary for this simple task. |
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| `time-skill` | **Yes** (most likely) | The skill's `description` says *"Display the current time in Pakistan Standard Time (PKT, UTC+5). Use when the user asks for the current time, Pakistan time, or PKT."* Claude matches this and invokes it via the Skill tool. Since it runs **inline** with no context overhead, it's the most efficient match. |
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### Resolution order
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When multiple mechanisms match the same intent, Claude prefers the **lightest-weight option** that satisfies the request:
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```
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1. Skill (inline, no context overhead) ← preferred
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2. Agent (separate context, autonomous) ← used if skill is unavailable or task is complex
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3. Command (never — requires explicit /) ← only if user types /time-command
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```
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### What if `disable-model-invocation: true` were set on the skill?
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Then Claude **cannot** auto-invoke the skill. The agent becomes the only auto-invocable option, so Claude would spawn `time-agent` instead — at the cost of a separate context window for a one-liner bash command.
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### What if both skill and agent had auto-invocation disabled?
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Then **nothing fires automatically**. Claude would fall back to its own general knowledge and likely just run `TZ='Asia/Karachi' date` directly — no extension mechanism involved. The user would need to explicitly type `/time-command` or `/time-skill` to use one.
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---
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## Sources
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- [Claude Code Skills — Docs](https://code.claude.com/docs/en/skills)
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- [Claude Code Sub-agents — Docs](https://code.claude.com/docs/en/sub-agents)
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- [Claude Code Slash Commands — Docs](https://code.claude.com/docs/en/slash-commands)
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- [Skills Best Practice](../best-practice/claude-skills.md)
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- [Commands Best Practice](../best-practice/claude-commands.md)
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- [Sub-agents Best Practice](../best-practice/claude-subagents.md)
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