Creating Custom Extensions

Build project-specific commands and skills to automate team workflows and extend Claude Code capabilities.

For testing existing prompts or skills, use the /customaize-agent:test-prompt command directly.

When to Use

• Automating repetitive team workflows

• Enforcing project-specific conventions

• Creating reusable knowledge for Claude across sessions

• Building domain-specific capabilities

Plugins needed for this workflow

• Customize Agent

• Docs

Workflow: Creating a Command

How It Works

+---------------------------------------------+
| 1. Create Command                           |
|    (interactive assistant)                  |
+----------------------+----------------------+
                       |
                       | generates command file with proper structure
                       v
+---------------------------------------------+
| 2. Test Command                             |
|    RED-GREEN-REFACTOR cycle                 |
+----------------------+----------------------+
                       |
                       | verify command works as expected
                       v
+---------------------------------------------+
| 3. Document Extension                       |
|    (update project docs)                    |
+---------------------------------------------+

1. Create command

Use the /customaize-agent:create-command command to interactively create a new Claude command. The assistant will guide you through understanding purpose, choosing patterns, and generating the command file.

/customaize-agent:create-command validate API documentation

After LLM completes, you will have a command file with proper frontmatter, structure, and patterns. Review and adjust the generated command as needed.

2. Test command

Use the /customaize-agent:test-prompt command to verify your command works correctly using the RED-GREEN-REFACTOR cycle with subagents.

/customaize-agent:test-prompt

After LLM completes, it will report whether the command handles scenarios correctly. If issues are found, iterate on the command and re-test.

3. Document the extension

Use the /docs:update-docs command to add the new command to project documentation.

/docs:update-docs

After LLM completes, your command will be documented and discoverable by the team.

Workflow: Creating a Skill

How It Works

+---------------------------------------------+
| 1. Create Skill                             |
|    (TDD-based approach)                     |
+----------------------+----------------------+
                       |
                       | understand use cases, plan structure
                       v
+---------------------------------------------+
| 2. Test Skill                               | <--- iterate until bulletproof ---+
|    Pressure scenarios with subagents        |                                   |
+----------------------+----------------------+                                   |
                       |                                                          |
                       | verify skill resists rationalization                     |
                       v                                                          |
+---------------------------------------------+                                   |
| 3. Apply Best Practices                     |-----------------------------------+
|    Anthropic's official guidelines          |
+----------------------+----------------------+
                       |
                       | optimize structure and discoverability
                       v
+---------------------------------------------+
| 4. Document Extension                       |
|    (update project docs)                    |
+---------------------------------------------+

1. Create skill

Use the /customaize-agent:create-skill command to create a new skill. This follows a TDD approach where you first understand concrete use cases before writing the skill.

/customaize-agent:create-skill image-editor

After LLM completes, it will guide you through understanding use cases, planning reusable contents (scripts, references, assets), and generating the SKILL.md file with proper frontmatter and structure.

2. Test skill effectiveness

Use the /customaize-agent:test-skill command to verify skills work under pressure and resist rationalization. This is critical for discipline-enforcing skills.

/customaize-agent:test-skill

After LLM completes, it will run pressure scenarios with subagents, document failures, and help you close loopholes. Continue iterating until the skill is bulletproof.

3. Apply best practices

Use the /customaize-agent:apply-anthropic-skill-best-practices command to review and optimize your skill according to Anthropic's official guidelines.

/customaize-agent:apply-anthropic-skill-best-practices

After LLM completes, your skill will be optimized for discoverability, progressive disclosure, and Claude Search Optimization (CSO).

4. Document the extension

Use the /docs:update-docs command to add the new skill to project documentation.

/docs:update-docs

After LLM completes, your skill will be documented and the team can discover and use it.

Workflow: Creating a Hook

How It Works

+---------------------------------------------+
| 1. Analyze Environment                      |
|    (detect tooling & suggest hooks)         |
+----------------------+----------------------+
                       |
                       | identify relevant hooks for project
                       v
+---------------------------------------------+
| 2. Configure Hook                           |
|    (ask targeted questions)                 |
+----------------------+----------------------+
                       |
                       | understand context and requirements
                       v
+---------------------------------------------+
| 3. Create Hook                              |
|    (generate script & register)             |
+----------------------+----------------------+
                       |
                       | hook is ready to use
                       v
+---------------------------------------------+
| 4. Test & Validate                          | <--- iterate until working ---+
|    (happy path + sad path testing)          |                               |
+----------------------+----------------------+                               |
                       |                                                      |
                       | verify both success and failure scenarios            |
                       v                                                      |
+---------------------------------------------+                               |
| 5. Fix Issues (if needed)                   |-------------------------------+
|    (permissions, registration, logic)       |
+---------------------------------------------+

1. Analyze environment

The /customaize-agent:create-hook command automatically detects your project tooling and suggests relevant hooks:

/customaize-agent:create-hook

The assistant will scan for:

• TypeScript (tsconfig.json) → Type-checking hooks

• Prettier (.prettierrc) → Formatting hooks

• ESLint (.eslintrc.*) → Linting hooks

• Package scripts → Test/build validation hooks

• Git repository → Security scanning hooks

2. Configure hook

The assistant asks targeted questions based on your needs:

1. What should this hook do? (with suggestions from analysis)

2. When should it run? (PreToolUse, PostToolUse, UserPromptSubmit)

3. Which tools trigger it? (Write, Edit, Bash, *)

4. Scope? (global, project, project-local)

5. Should Claude see and fix issues? (integration with additionalContext)

6. Should successful operations be silent? (avoid context pollution)

3. Create hook

After LLM completes, you will have:

• Hook script in ~/.claude/hooks/ or .claude/hooks/

• Proper executable permissions

• Configuration in appropriate settings.json

• Project-specific commands using detected tooling

4. Test and validate

CRITICAL: The assistant tests both happy and sad paths:

• Happy path: Create conditions where hook should pass

• Sad path: Create conditions where hook should fail/warn

• Verification: Check if it blocks/warns/provides context correctly

For example, a hook preventing file deletion will:

1. Create a test file

2. Attempt the protected action

3. Verify the hook prevents it

If issues occur, the assistant will:

• Check hook registration in settings

• Verify script permissions

• Test with simplified version

• Debug hook execution

Extension Types

Commands vs Skills vs Hooks

Aspect	Commands	Skills	Hooks
Purpose	Execute specific workflows	Provide knowledge and patterns	Intercept and validate operations
Location	.claude/commands/	.claude/skills/ or ~/.claude/skills/	.claude/hooks/ or ~/.claude/hooks/
Invocation	/plugin:command-name	Auto-discovered by Claude	Triggered by events (PreToolUse, PostToolUse)
Structure	Markdown with frontmatter	SKILL.md with optional resources	Executable scripts (bash, node, python)
Use when	Automating multi-step tasks	Teaching Claude domain expertise	Quality gates, validation, automation

Command Categories

• Planning: Feature ideation, proposals, PRDs

• Implementation: Technical execution with mode-based variations

• Analysis: Review, audit, generate reports

• Workflow: Orchestrate multiple steps, coordinate areas

• Utility: Simple tools and helpers

Skill Types

• Technique: Concrete methods with steps (condition-based-waiting)

• Pattern: Mental models for problems (flatten-with-flags)

• Reference: API docs, syntax guides, tool documentation

Hook Types

• Code Quality: PostToolUse for feedback and automated fixes (formatting, linting, type-checking)

• Security: PreToolUse to block dangerous operations (secrets detection, unsafe commands)

• Validation: PreToolUse to enforce requirements before operations (tests, builds)

• Development: PostToolUse for automated improvements (documentation, optimization)

Key Concepts

TDD for Documentation

Creating skills follows the same RED-GREEN-REFACTOR cycle as code:

1. RED: Run scenarios WITHOUT the skill, document failures

2. GREEN: Write minimal skill addressing those failures

3. REFACTOR: Close loopholes, optimize structure

Progressive Disclosure

Skills use a three-level loading system:

1. Metadata (name + description) - Always in context

2. SKILL.md body - When skill triggers

3. Bundled resources - As needed by Claude

Claude Search Optimization (CSO)

Make skills discoverable:

• Start descriptions with "Use when..." and specific triggers

• Include error messages and symptoms as keywords

• Name by what you DO, not what you ARE (creating-skills not skill-creation)

Quick Reference

Task	Command
Create command	/customaize-agent:create-command
Create skill	/customaize-agent:create-skill
Test any prompt	/customaize-agent:test-prompt
Test skill under pressure	/customaize-agent:test-skill
Apply Anthropic best practices	/customaize-agent:apply-anthropic-skill-best-practices
Create git hook	/customaize-agent:create-hook
Document changes	/docs:update-docs