5.3a Skill Basics

A Skill is an on-demand loaded domain knowledge package, defined through SKILL.md, that Agents automatically discover and load based on task semantics.

📝 Course Notes

Key concepts from this lesson:

What You'll Learn

Understand the core design philosophy of Skills
Create SKILL.md files that follow conventions
Configure Skill permission controls
Distinguish between Skill and CLAUDE.md use cases

Why Skills Are Needed

Your Current Problem

User: Help me query revenue data from the data warehouse

Claude: I suggest using this SQL query...
SELECT * FROM revenue WHERE date > '2024-01-01'

Claude provides a generic SQL pattern, but it doesn't know:

What your table structure looks like
How "revenue" is defined in your company
Test accounts must be excluded
Which summary table should be used

Root Cause: Claude starts fresh with every conversation, lacking your team's domain knowledge.

How Skills Solve This

User: Help me query revenue data from the data warehouse

Claude: [Automatically loads sql-analysis Skill]
I'll help you query revenue data. Based on your data conventions:
- Use the monthly_revenue summary table
- Exclude test accounts (account != 'Test')
- Revenue is calculated as ARR (monthly * 12)

SELECT ...

Skills encapsulate your domain knowledge into resources that Claude can load on demand.

Core Design Philosophy of Skills

Progressive Disclosure

Skills don't stuff everything into context; they use layered loading:

┌─────────────────────────────────────────────────────────┐
│ Layer 1: name + description (~100 words)               │
│ → Always visible, used to determine if loading is needed│
├─────────────────────────────────────────────────────────┤
│ Layer 2: SKILL.md body content                         │
│ → Loaded when task matches, contains main instructions  │
├─────────────────────────────────────────────────────────┤
│ Layer 3: Detailed documents in references/ directory    │
│ → Loaded only when specific details are needed          │
└─────────────────────────────────────────────────────────┘

Why This Design?

Context window is a precious resource. Stuffing all domain knowledge causes:

Excessive token consumption
Distracted model attention
Irrelevant content interfering with output

Progressive disclosure lets Claude load only what's needed for the current task.

Skill vs CLAUDE.md

Feature	CLAUDE.md	Skill
Loading Timing	Always loaded to context	Only loaded when task matches
Scope	Current project	Reusable across projects
Content Type	Pure Markdown	Markdown + code + resource files
Platform Support	Claude Code / OpenCode only	Claude.ai / Code / API
Typical Use	Project coding conventions, local commands	Domain knowledge, workflows

Selection Principle:

Project-specific conventions (code style, branch naming) → CLAUDE.md
Reusable domain knowledge (data analysis workflows, audit standards) → Skill

Skill Directory Structure

Basic Structure

.opencode/
└── skill/
    └── code-review/
        └── SKILL.md      # Skill definition file (must be uppercase)

Recommended Complete Structure

.opencode/
└── skill/
    └── sql-analysis/
        ├── SKILL.md              # Main file: workflow and key logic
        └── references/           # Detailed documents (loaded on demand)
            ├── finance.md        # Finance table structures
            ├── product.md        # Product table structures
            └── examples.md       # Query examples

Principle: Keep SKILL.md concise, put detailed content in references/, let Claude read it when needed.

OpenCode Search Locations

Location	Scope	Description
`.opencode/skill/<name>/SKILL.md`	Current project	Project-specific skills
`~/.config/opencode/skill/<name>/SKILL.md`	Global	Available for all projects
`.claude/skills/<name>/SKILL.md`	Current project	Claude compatible format
`~/.claude/skills/<name>/SKILL.md`	Global	Claude compatible format

Project paths are traversed from current directory up to git root.

Custom Configuration Directory

You can specify additional Skill search paths via the OPENCODE_CONFIG_DIR environment variable:

bash

export OPENCODE_CONFIG_DIR="/path/to/custom/config"

OpenCode will scan both:

Default config directory: ~/.config/opencode/skill/
Custom config directory: $OPENCODE_CONFIG_DIR/skill/

This is useful for team-shared Skills or using different Skill sets in different environments.

Source evidence: skill/skill.ts:82-85

typescript

const configDirs = process.env.OPENCODE_CONFIG_DIR
  ? [process.env.OPENCODE_CONFIG_DIR, ...defaultConfigDirs]
  : defaultConfigDirs

Nested Directory Support

OpenCode supports nested Skill directories:

.opencode/
└── skill/
    └── audit/
        └── security/
            └── SKILL.md    # Skill name determined by frontmatter name field

Source evidence: skill/skill.ts:38

typescript

const OPENCODE_SKILL_GLOB = new Bun.Glob("{skill,skills}/**/SKILL.md")

The ** means matching subdirectories at any depth.

SKILL.md Format

Required Frontmatter

yaml

---
name: sql-analysis
description: Used for analyzing business data: revenue, ARR, customer segmentation, product usage, sales pipeline. Provides table structures, metric definitions, required filters, and query patterns.
---

Field	Required	Description
`name`	Yes	Skill identifier, used for invocation
`description`	Yes	Trigger condition description (most important!)
`license`	No	License information
`compatibility`	No	Compatibility markers
`metadata`	No	Custom key-value pairs

name Naming Convention

Official recommendations (source doesn't enforce, but following ensures compatibility):

✓ code-review
✓ sql-analysis
✓ git-release
✗ Code_Review    ← Don't use uppercase
✗ sql--analysis  ← Don't use consecutive hyphens
✗ -review        ← Don't start with hyphen

Regex reference: ^[a-z0-9]+(-[a-z0-9]+)*$

description Writing (Key to Triggering)

description is the sole factor determining whether a Skill triggers. Claude uses semantic understanding (not keyword matching) to judge if a task needs a certain Skill.

Poor Example:

yaml

description: Help with documents

Problem: Too vague, AI can't determine when to trigger.

Good Example:

yaml

description: |
  Extract tables from PDFs and convert to CSV format for data analysis workflows.
  Suitable for: filling PDF forms, batch processing PDF documents, extracting embedded PDF data.
  Not suitable for: simple PDF viewing, basic format conversion, PDF editing.

description Writing Template:

yaml

description: |
  [One sentence explaining core capability]
  Provides: [resources this Skill contains, like table structures, formulas, templates]
  Suitable for: [trigger scenario 1], [trigger scenario 2], [trigger scenario 3]
  Not suitable for: [boundary scenario 1], [boundary scenario 2]

description Elements:

Specific Capability: What it can do (extract tables, convert formats)
Provided Resources: What it contains (table structures, formulas, templates)
Trigger Scenarios: When to use (batch processing, form filling)
Boundary Constraints: When not to use (simple viewing)

Complete Example

markdown

---
name: sql-analysis
description: |
  Used for analyzing business data: revenue trends, ARR calculation, customer segmentation, product usage, sales pipeline.
  Provides: company table structures, metric definition formulas, standard filters, common query templates.
  Suitable for: writing SQL to analyze business data, understanding company metric definitions, querying data warehouse.
  Not suitable for: database administration, DDL operations, performance tuning, general SQL education.
---

# SQL Analysis Skill

## Quick Workflow

When user requests data analysis:

1. **Clarify Requirements**
   - What time range? (Default: current year)
   - Which customer segment?
   - What decision is this analysis for?

2. **Check Existing Dashboards**
   - See if `references/dashboards.md` has ready-made reports
   - If yes, prioritize guiding user to use them

3. **Determine Data Source**
   - Prefer summary tables over raw event data
   - Confirm table has required fields before querying

4. **Execute Analysis**
   - Apply required filters (exclude test accounts, etc.)
   - Validate results against known baselines

## Standard Query Filters

All revenue queries must:
- Exclude test accounts: `WHERE account != 'Test'`
- Use only complete periods: `WHERE month <= DATE_TRUNC(CURRENT_DATE(), MONTH)`

## ARR Calculation Method

- Monthly to ARR: `monthly_revenue * 12`
- 7-day run rate: `rolling_7d * 52`

## Detailed Documentation

When table structures and query patterns are needed, refer to:
- **Revenue & Finance** → `references/finance.md`
- **Product Usage** → `references/product.md`
- **Sales Pipeline** → `references/sales.md`

Note: SKILL.md only contains workflow and key logic; detailed table structures go in the references/ directory.

How Skills Are Discovered and Loaded

Discovery Mechanism

At startup, OpenCode scans all Skills and aggregates name and description into the skill tool description:

xml

<available_skills>
  <skill>
    <name>sql-analysis</name>
    <description>Used for analyzing business data: revenue, ARR, customer segmentation...</description>
  </skill>
  <skill>
    <name>code-review</name>
    <description>Perform code review, check conventions, bugs, performance and security</description>
  </skill>
</available_skills>

Loading Mechanism

When user sends a message, Claude determines whether to load a Skill based on semantics:

User message: Help me analyze last quarter's revenue data

Claude determines: This is a data analysis task, matches sql-analysis Skill

Claude invokes: skill({ name: "sql-analysis" })

Result: SKILL.md content loaded to context

Output After Loading

## Skill: sql-analysis

**Base directory**: /path/to/.opencode/skill/sql-analysis

[SKILL.md content]

The Base directory information tells Claude how to access relative path files in references/.

Permission Configuration

Global Permissions

Configure in opencode.json:

jsonc

{
  "permission": {
    "skill": {
      "pr-review": "allow",        // Load immediately
      "internal-*": "deny",        // Hidden, access denied
      "experimental-*": "ask",     // Ask user before loading
      "*": "allow"                 // Default allow others
    }
  }
}

Permission Value	Behavior
`allow`	Skill loads immediately
`deny`	Skill hidden from Agent, access denied
`ask`	Prompt user confirmation before loading

Wildcards supported: internal-* matches internal-docs, internal-tools, etc.

Override Permissions per Agent

In Markdown Agent:

yaml

---
permission:
  skill:
    "documents-*": "allow"
---

In opencode.json:

jsonc

{
  "agent": {
    "plan": {
      "permission": {
        "skill": {
          "internal-*": "allow"
        }
      }
    }
  }
}

Disable Skill Tool

For Agents that don't need Skills, you can completely disable:

Markdown Method:

yaml

---
tools:
  skill: false
---

JSON Method:

jsonc

{
  "agent": {
    "plan": {
      "tools": {
        "skill": false
      }
    }
  }
}

When disabled, <available_skills> section won't appear in that Agent's tool description at all.

Simple Examples

Translation Skill

markdown

---
name: translate
description: Professional translation, preserving format and terminology. Used for translating technical documentation, API docs, code comments.
---

# Translation Skill

## Translation Standards

1. Preserve original format and paragraph structure
2. Keep proper nouns in original with annotation
3. Check terminology table for technical terms
4. Proofread after translation

## Output Format

Wrap translation result in code block:


For uncertain translations, annotate original in parentheses.

Brand Guidelines Skill

markdown

---
name: brand-guidelines
description: Apply company official brand colors and typography standards. Used for creating documents, presentations, interface designs requiring company visual style.
---

# Brand Guidelines Skill

## Colors

**Primary Colors**:
- Dark: `#141413` - Main text and dark backgrounds
- Light: `#faf9f5` - Light backgrounds and text on dark
- Medium Gray: `#b0aea5` - Secondary elements

**Accent Colors**:
- Orange: `#d97757` - Primary accent
- Blue: `#6a9bcc` - Secondary accent
- Green: `#788c5d` - Tertiary accent

## Typography

- **Headings**: Poppins (fallback Arial)
- **Body**: Lora (fallback Georgia)

## Application Rules

- Use Poppins for headings (24pt and above)
- Use Lora for body text
- Intelligently select text color based on background

Common Pitfalls

Issue	Cause	Solution
Skill won't load	SKILL.md case incorrect	Must be uppercase `SKILL.md`
Skill not showing	Missing frontmatter fields	Must include `name` and `description`
Task matches but doesn't trigger	description too vague	Add specific capabilities, scenarios, boundaries
Same-name Skill conflict	Same name defined in multiple places	Later loaded overwrites earlier, check log warnings
Access denied	Permission set to deny	Check permission configuration
Skill directory not recognized	Directory spelling issue	Both `skill/` and `skills/` supported

Lesson Summary

You learned:

Skill Core Concept: Progressive disclosure, on-demand loading
Directory Structure: skill/<name>/SKILL.md + references/
SKILL.md Format: name and description required
description Writing: Specific capability + trigger scenarios + boundary constraints
Permission Control: allow/ask/deny + wildcards
Skill vs CLAUDE.md: On-demand loading vs always loaded

Next Lesson Preview

Next lesson dives into advanced Skill usage: progressive disclosure three-layer structure, executable scripts, creation workflow, testing validation, and real-world examples.

Continue Learning: 5.3b Skill Advanced

5.3a Skill Basics ​

📝 Course Notes ​

What You'll Learn ​

Why Skills Are Needed ​

Your Current Problem ​

How Skills Solve This ​

Core Design Philosophy of Skills ​

Progressive Disclosure ​

Skill vs CLAUDE.md ​

Skill Directory Structure ​

Basic Structure ​

Recommended Complete Structure ​

OpenCode Search Locations ​

Nested Directory Support ​

SKILL.md Format ​

Required Frontmatter ​

name Naming Convention ​

description Writing (Key to Triggering) ​

Complete Example ​

How Skills Are Discovered and Loaded ​

Discovery Mechanism ​

Loading Mechanism ​

Output After Loading ​

Permission Configuration ​

Global Permissions ​

Override Permissions per Agent ​

Disable Skill Tool ​

Simple Examples ​

Translation Skill ​

Brand Guidelines Skill ​

Common Pitfalls ​

Lesson Summary ​

Next Lesson Preview ​

5.3a Skill Basics

📝 Course Notes

What You'll Learn

Why Skills Are Needed

Your Current Problem

How Skills Solve This

Core Design Philosophy of Skills

Progressive Disclosure

Skill vs CLAUDE.md

Skill Directory Structure

Basic Structure

Recommended Complete Structure

OpenCode Search Locations

Nested Directory Support

SKILL.md Format

Required Frontmatter

name Naming Convention

description Writing (Key to Triggering)

Complete Example

How Skills Are Discovered and Loaded

Discovery Mechanism

Loading Mechanism

Output After Loading

Permission Configuration

Global Permissions

Override Permissions per Agent

Disable Skill Tool

Simple Examples

Translation Skill

Brand Guidelines Skill

Common Pitfalls

Lesson Summary

Next Lesson Preview