Understanding Agent Skills Architecture: A Comprehensive Guide
Learn how Agent Skills leverage Claude's VM environment with progressive disclosure to provide specialized capabilities beyond what's possible with prompts alone.
Agent Skills are modular capabilities that extend Claude's functionality, providing domain-specific expertise through a sophisticated filesystem-based architecture. This comprehensive guide explores how Skills work, their three-level loading model, and the benefits they offer.
What Are Agent Skills?
Agent Skills are reusable, filesystem-based resources that provide Claude with domain-specific expertise: workflows, context, and best practices that transform general-purpose agents into specialists. Unlike prompts (conversation-level instructions for one-off tasks), Skills load on-demand and eliminate the need to repeatedly provide the same guidance across multiple conversations.
Key Benefits
- Specialize Claude: Tailor capabilities for domain-specific tasks
- Reduce repetition: Create once, use automatically
- Compose capabilities: Combine Skills to build complex workflows
The Three-Level Architecture
Skills leverage Claude's VM environment to provide capabilities beyond what's possible with prompts alone. This filesystem-based architecture enables progressive disclosure: Claude loads information in stages as needed, rather than consuming context upfront.
Level 1: Metadata (Always Loaded)
The Skill's YAML frontmatter provides discovery information:
---
name: pdf-processing
description: Extract text and tables from PDF files, fill forms, merge documents. Use when working with PDF files or when the user mentions PDFs, forms, or document extraction.
---
Claude loads this metadata at startup and includes it in the system prompt. This lightweight approach means you can install many Skills without context penalty; Claude only knows each Skill exists and when to use it.
Level 2: Instructions (Loaded When Triggered)
The main body of SKILL.md contains procedural knowledge: workflows, best practices, and guidance. When you request something that matches a Skill's description, Claude reads SKILL.md from the filesystem via bash. Only then does this content enter the context window.
Level 3+: Resources and Code (Loaded As Needed)
Skills can bundle additional materials:
- Instructions: Additional markdown files containing specialized guidance
- Code: Executable scripts that Claude runs via bash
- Resources: Reference materials like database schemas, API documentation, templates
Claude accesses these files only when referenced. The filesystem model means each content type has different strengths: instructions for flexible guidance, code for reliability, resources for factual lookup.
Progressive Disclosure in Action
Progressive disclosure ensures only relevant content occupies the context window at any given time. Here's how it works:
- Default state: System prompt and skill metadata are pre-loaded
- Skill triggered: Claude reads SKILL.md via bash
- Additional files: Claude optionally reads bundled files as needed
- Task completion: Claude proceeds with the task using loaded information
This dynamic loading ensures only relevant skill content occupies the context window.
Where Skills Work
Skills are available across Claude's agent products:
Claude API
Supports both pre-built Agent Skills and custom Skills. Use pre-built Skills by referencing their skill_id (e.g., pptx, xlsx), or create and upload your own via the Skills API.
Claude Code
Supports only Custom Skills, which are filesystem-based and don't require API uploads.
Claude.ai
Supports both pre-built Agent Skills (already working behind the scenes) and custom Skills (uploaded as zip files through Settings > Features).
Claude Agent SDK
Supports custom Skills through filesystem-based configuration in .claude/skills/.
Pre-built Agent Skills
Anthropic provides the following pre-built Agent Skills:
- PowerPoint (pptx): Create presentations, edit slides, analyze presentation content
- Excel (xlsx): Create spreadsheets, analyze data, generate reports with charts
- Word (docx): Create documents, edit content, format text
- PDF (pdf): Generate formatted PDF documents and reports
Security Considerations
Skills provide Claude with new capabilities through instructions and code. We strongly recommend using Skills only from trusted sources: those you created yourself or obtained from Anthropic.
Key security considerations:
- Audit thoroughly: Review all files bundled in the Skill
- External sources are risky: Skills that fetch data from external URLs pose particular risk
- Tool misuse: Malicious Skills can invoke tools in harmful ways
- Data exposure: Skills with access to sensitive data could leak information
Limitations and Constraints
Cross-surface Availability
Custom Skills do not sync across surfaces. Skills uploaded to one surface are not automatically available on others.
Sharing Scope
Skills have different sharing models:
- Claude.ai: Individual user only
- Claude API: Workspace-wide
- Claude Code: Personal or project-based
Runtime Environment Constraints
Skills run in the code execution container with these limitations:
- No network access: Cannot make external API calls
- No runtime package installation: Only pre-installed packages are available
- Pre-configured dependencies only: Check available packages in the code execution tool documentation
Next Steps
Ready to start using Agent Skills? Check out these resources:
- Quickstart Tutorial: Learn how to use pre-built Agent Skills in the API
- Authoring Guide: Create your own custom Skills with domain-specific expertise
- Skills Cookbook: Explore example Skills and implementation patterns
- API Reference: Use Skills programmatically with the Claude API
For more information about Agent Skills, explore our comprehensive documentation and tutorials.