Skilljack Docs

Skilljack MCP Documentation

An MCP server that jacks Agent Skills directly into your LLM's brain.

Recommended: For best results, use an MCP client that supports tools/listChanged notifications (e.g., Claude Code). This enables dynamic skill discovery - when skills are added or modified, the client automatically refreshes its understanding of available skills. Alternatively, use --static mode for predictable behavior with a fixed skill set.

Tool search / deferred tools. By default, skilljack delivers its skill catalog via MCP server instructions, which arrive in the initialize handshake and reach the model even when tool search / deferred tool loading is enabled (the default on modern Claude Code, ~2.1.x) — automatic skill activation works out of the box. The legacy --catalog=tool-description mode (not recommended) delivers the catalog inside the load-skill tool description instead, but tool-search clients defer MCP tool descriptions out of context, so that mode requires disabling tool search (e.g. ENABLE_TOOL_SEARCH=false) for auto-activation. See Catalog Delivery.

Features

• Dynamic Skill Discovery - Watches skill directories and automatically refreshes when skills change
• Multiple Sources - Local directories, GitHub repositories, and .well-known/agent-skills/ publishers (both allowlisted)
• Tool List Changed Notifications - Sends tools/listChanged so clients can refresh available skills
• load-skill Tool - Load full skill content on demand (progressive disclosure)
• MCP Prompts - Load skills via /skill prompt with auto-completion or per-skill prompts
• MCP Resources - Access skills via skill:// URIs aligned with SEP-2640
• Resource Subscriptions - Real-time file watching with notifications/resources/updated
• Configuration UI - Manage skill directories through an interactive UI in supported clients

Motivation

This server demonstrates a way to approach integrating skills using existing MCP primitives.

MCP already has the building blocks:

• Tools for on-demand skill loading (the load-skill tool)
• Server instructions for surfacing the skill catalog at initialize (the default catalog channel)
• Resources for explicit skill access (skill:// URIs)
• Notifications for real-time updates (tools/listChanged, resources/updated)
• Prompts for explicitly invoking skills by name (/my-server-skill)

This approach provides separation of concerns. Rather than every MCP server needing to embed skill handling, the server acts as a dedicated 'skill gateway'. Server authors can bundle skills alongside their MCP servers without modifying the servers themselves. If MCP registries support robust tool discovery, skill tools become discoverable like any other tool.

Usage

Configure one or more skills directories containing your Agent Skills:

# Single directory
skilljack-mcp /path/to/skills

# Multiple directories (separate args or comma-separated)
skilljack-mcp /path/to/skills /path/to/more/skills
skilljack-mcp /path/to/skills,/path/to/more/skills

# Using environment variable (comma-separated for multiple)
SKILLS_DIR=/path/to/skills skilljack-mcp
SKILLS_DIR=/path/to/skills,/path/to/more/skills skilljack-mcp

Each directory is scanned along with its .claude/skills/ and skills/ subdirectories for skills. Duplicate skill names are handled by keeping the first occurrence.

Remote Sources

In addition to local directories, skills can be pulled from two kinds of remote sources. Both are default-deny — a remote source is only synced if its origin is on the corresponding allowlist.

# GitHub repository (allowlisted via GITHUB_ALLOWED_ORGS / GITHUB_ALLOWED_USERS)
GITHUB_ALLOWED_ORGS=acme skilljack-mcp github.com/acme/skills

# Well-known publisher serving /.well-known/agent-skills/index.json
# (allowlisted via WELL_KNOWN_ALLOWED_ORIGINS)
WELL_KNOWN_ALLOWED_ORIGINS=https://example.com \
  skilljack-mcp https://example.com/.well-known/agent-skills/

• GitHub repos are cloned into a local cache and polled for updates; skills are namespaced with an owner-repo prefix.
• Well-known publishers serve a discovery index (per the agent-skills discovery RFC). Every artifact is SHA-256-verified against the index before it is written, archives (.tar.gz/.zip) are extracted with path-traversal/symlink/size protections, and every fetched URL — index, artifact, and any redirect target — must be on WELL_KNOWN_ALLOWED_ORIGINS (so a separate CDN origin must be allowlisted too). See the Developer Guide for the full list of WELL_KNOWN_* tuning env vars.

Sources can be mixed freely on one command line (local dirs, GitHub URLs, and well-known URLs together).

Static Mode

By default, Skilljack MCP watches skill directories for changes and notifies clients when skills are added, modified, or removed.

Enable static mode to freeze the skills list at startup:

skilljack-mcp --static /path/to/skills
# or
SKILLJACK_STATIC=true skilljack-mcp /path/to/skills

In static mode:

• Skills are discovered once at startup and never refreshed
• No file watchers are set up for skill directories
• tools.listChanged and prompts.listChanged capabilities are false
• Resource subscriptions remain fully dynamic (individual skill files can still be watched)

Use static mode when you need predictable behavior or have a fixed set of skills that won't change during the session.

Windows note: Use forward slashes in paths when using with MCP Inspector:

skilljack-mcp "C:/Users/you/skills"

Catalog Delivery

The <available_skills> catalog is delivered through exactly one channel, selected with --catalog=<mode> or SKILLJACK_CATALOG — never both:

# Default: catalog in server instructions (survives tool search)
skilljack-mcp /path/to/skills

# Legacy escape hatch, not recommended: catalog in the load-skill tool description
skilljack-mcp --catalog=tool-description /path/to/skills

• instructions (default) — the catalog (plus load-skill usage guidance) is sent as MCP server instructions in the initialize handshake. It is visible to the model even under tool search / deferred tool loading. Trade-off: on stdio, the MCP SDK sets instructions once at construction, so the catalog is frozen until restart — live skill add/remove won't reach connected clients. (Over HTTP, instructions are regenerated per request from the watched skill state, so newly connecting clients always get a current catalog; already-connected clients receive instructions only at initialize and see changes after reconnecting.)
• tool-description (not recommended — legacy escape hatch, also used as the control condition in the evals) — the catalog lives in the load-skill tool description and refreshes live via tools/listChanged when skills change. Why it's not recommended: (1) clients with tool search enabled (the modern Claude Code default) defer MCP tool descriptions out of context, so the model never sees the catalog and auto-activation silently breaks unless the client sets ENABLE_TOOL_SEARCH=false; (2) its one advantage — live refresh — is a prompt-cache trap: tool definitions sit at the top of the cached prompt prefix, so every tools/listChanged refresh invalidates the entire prompt cache (tools + system prompt + conversation history) and re-writes it at cache-write prices; (3) measured 2026-07-06 (issue #78), it has no steady-state cost advantage over instructions either.

HTTP Transport

By default skilljack communicates over stdio. To serve over stateless Streamable HTTP instead, use --http (or SKILLJACK_HTTP_PORT):

skilljack-mcp --http=3000 /path/to/skills

Clients connect at POST /mcp. HTTP mode serves the core skill surface — load-skill, skill-resource, skill:// resources, and /skill prompts. Discovery-on-change works the same as stdio: file watchers and remote-source polling keep the skill state fresh, and every request reads it — so newly connecting clients get a current catalog, and load-skill/tools/list reflect changes immediately. Because the transport is stateless (no session, no server→client stream), it does not push listChanged / resources/updated notifications — already-connected clients see changes on their next request (for the instructions catalog, on their next reconnect). The UI config/display tools are stdio-only.

Configuration UI

In MCP clients that support MCP Apps (like Claude Desktop), you can manage skill directories through an interactive UI.

To open the configuration UI, ask your assistant to show the skilljack config:

"show me the skilljack config"

The UI displays:

• Current skill directories with skill counts
• Status indicators showing which directories are from config vs command-line
• Options to add new directories or remove existing ones

Changes made through the UI are persisted to the server's configuration. Clients that support tools/listChanged notifications will see updates immediately; others may require reconnection.

Skill Display UI

View all available skills and customize their invocation settings through the skill display UI.

To open the skill display UI, ask your assistant:

"what skills are configured in skilljack?"

The UI displays:

• All discovered skills with name, description, and file path
• Source indicators showing whether each skill is from a local directory, a GitHub repository, or a well-known publisher
• Invocation toggles to enable/disable Assistant (model auto-invoke) and User (prompts menu) visibility
• Customized badge when settings differ from frontmatter defaults

Skills from GitHub repositories show the org/repo name (e.g., modelcontextprotocol/ext-apps), and well-known skills show the publisher origin, making it easy to identify where each skill originates.

How It Works

The server implements the Agent Skills progressive disclosure pattern with dynamic updates:

1. At startup: Discovers skills from configured directories and starts file watchers
2. On connection: the skill catalog is delivered via server instructions (default) or the load-skill tool description (--catalog=tool-description)
3. On file change: Re-discovers skills, updates the tool description and prompts, sends tools/listChanged (in tool-description mode this refreshes the catalog; in instructions mode the catalog is frozen until restart on stdio)
4. On tool call: Agent calls load-skill tool to load full SKILL.md content
5. As needed: Agent calls skill-resource to load additional files

┌─────────────────────────────────────────────────────────┐
│ Server starts                                            │
│   • Discovers skills from configured directories         │
│   • Starts watching for SKILL.md changes                 │
│   ↓                                                      │
│ MCP Client connects                                      │
│   • Skill catalog delivered via server instructions      │
│     (default) or load-skill tool description             │
│   • Prompts registered for each skill                    │
│   ↓                                                      │
│ LLM sees skill metadata in the catalog                   │
│   ↓                                                      │
│ SKILL.md added/modified/removed                          │
│   • Server re-discovers skills                           │
│   • Updates load-skill tool description                  │
│   • Updates prompt list (add/remove/modify)              │
│   • Sends tools/listChanged notification                 │
│   • Sends prompts/listChanged notification               │
│   • Client refreshes tool and prompt definitions         │
│   ↓                                                      │
│ User invokes /skill prompt or /skill-name prompt         │
│   OR LLM calls "load-skill" tool with skill name         │
│   ↓                                                      │
│ Server returns full SKILL.md content                     │
│   ↓                                                      │
│ LLM calls "skill-resource" for additional files          │
│   • Scripts, snippets, references, assets, etc.          │
└─────────────────────────────────────────────────────────┘

Tools vs Resources vs Prompts

This server exposes skills via tools, resources, and prompts:

• Tools (load-skill, skill-resource) - For your agent to use autonomously. The LLM sees available skills in the skill catalog (server instructions by default, or the load-skill tool description) and calls them as needed.
• Prompts (/skill, /skill-name) - For explicit user invocation. Use /skill with auto-completion or select a skill directly by name.
• Resources (skill:// URIs) - For manual selection in apps that support it (e.g., Claude Desktop's resource picker). Useful when you want to explicitly attach a skill to the conversation.

Skills are context delivered through MCP primitives. Tools enable autonomous activation by the agent. Prompts enable user-initiated loading with auto-completion. Resources provide explicit access for manual control. Each mechanism suits different workflows — skills aren't tied to any single delivery path.

Progressive Disclosure Design

This server implements the Agent Skills progressive disclosure pattern, which structures skills for efficient context usage:

How it works

1. Discovery - Server loads metadata from all skills into the skill catalog (server instructions by default, or the load-skill tool description)
2. Activation - When a skill is loaded (via tool, prompt, or resource), only the SKILL.md content is returned
3. Execution - SKILL.md references additional files; agent fetches them with skill-resource as needed

Why SKILL.md documents its own resources

The skill catalog and the loaded SKILL.md body don't enumerate every file in a skill directory. Instead, skill authors document available resources directly in their SKILL.md (e.g., "Copy the template from templates/server.ts"). This design choice follows the spec because:

• Skill authors know best - They decide which files are relevant and when to use them
• Context efficiency - Loading everything upfront wastes tokens on files the agent may not need
• Natural flow - SKILL.md guides the agent through resources in a logical order

(Supporting files are enumerated in resources/list for clients that browse resources — see Resources — but at lower priority than SKILL.md, and the progressive-disclosure tool/prompt path above still loads only what the SKILL.md points to.)

For skill authors: Reference files using relative paths from the skill root (e.g., snippets/tool.ts, references/api.md). Keep your main SKILL.md under 500 lines; move detailed reference material to separate files. See the Agent Skills specification for complete authoring guidelines.

Tools

load-skill

Load and activate an Agent Skill by name. Returns the full SKILL.md content.

Named load-skill (not skill) to avoid colliding with clients that ship a built-in Skill tool (e.g. Claude Code); a distinct verb-first name removes the routing ambiguity. The user-facing prompt is still /skill.

Input:

{
  "name": "skill-name"
}

Output: Full SKILL.md content including frontmatter and instructions.

skill-resource

Read files within a skill's directory (scripts/, references/, assets/, snippets/, etc.).

This follows the Agent Skills spec's progressive disclosure pattern - resources are loaded only when needed.

Read a single file:

{
  "skill": "mcp-server-ts",
  "path": "snippets/tools/echo.ts"
}

Read all files in a directory:

{
  "skill": "algorithmic-art",
  "path": "templates"
}

Returns all files in the directory as multiple content items.

List available files (pass empty path):

{
  "skill": "mcp-server-ts",
  "path": ""
}

Security: Path traversal is prevented - only files within the skill directory can be accessed.

Prompts

Skills can be loaded via MCP Prompts for explicit user invocation.

/skill Prompt

Load a skill by name with auto-completion support.

Arguments:

• name (string, required) - Skill name with auto-completion

The prompt description includes all available skills for discoverability. As you type the skill name, matching skills are suggested.

Per-Skill Prompts

Each discovered skill is also registered as its own prompt (e.g., /mcp-server-ts, /algorithmic-art).

• No arguments needed - just select and invoke
• Description shows the skill's own description
• List updates dynamically as skills change

Example: If you have a skill named mcp-server-ts, you can invoke it directly as /mcp-server-ts.

Content Annotations

Prompt responses include MCP content annotations for proper handling:

• audience: ["assistant"] - Content is intended for the LLM, not the user
• priority: 1.0 - High priority content that should be included in context

Prompts return embedded resources with the skill's skill:// URI, allowing clients to track the content source.

Resources

Skills are also accessible via MCP Resources using skill:// URIs, following the SEP-2640 Skills Extension convention. The server advertises support via the extensions["io.modelcontextprotocol/skills"] capability key in its initialize response.

URI Patterns

<skill-path> is <prefix>/<baseName> for prefixed skills (the prefix segments come from the skill's source — e.g., a local directory basename or owner-repo for GitHub-sourced skills) or just <baseName> for bundled skills. The final <skill-path> segment always matches the name field in the skill's frontmatter, per SEP.

Resource Subscriptions

Clients can subscribe to resources for real-time updates when files change.

Capability: resources: { subscribe: true, listChanged: true }

Subscribe to a resource:

→ resources/subscribe { uri: "skill://skilljack-docs/SKILL.md" }
← {} (success)

Receive notifications when files change:

← notifications/resources/updated { uri: "skill://skilljack-docs/SKILL.md" }

Unsubscribe:

→ resources/unsubscribe { uri: "skill://skilljack-docs/SKILL.md" }
← {} (success)

Subscribing to skill://index.json watches every SKILL.md, so any skill change re-fires that subscription as well as notifications/resources/updated for skill://index.json directly when skills are added or removed.

How it works:

1. Client subscribes to a skill:// URI
2. Server resolves URI to file path(s) and starts watching with chokidar
3. When files change, server debounces (100ms) and sends notification
4. Client can re-read the resource to get updated content

Security

Skills are treated as trusted content. This server reads and serves skill files directly to clients without sanitization. Only configure skills directories containing content you trust.

Protections in place:

• Path traversal prevention (symlink-aware)
• File size limits (1MB default, configurable via MAX_FILE_SIZE_MB env var)
• Directory depth limits
• Skill content is confined to configured directories

Not protected against:

• Malicious content within trusted skill directories
• Prompt injection via skill instructions (skills can influence LLM behavior by design)

Dynamic Skill Discovery

The server watches skill directories for changes. When SKILL.md files are added, modified, or removed:

1. Skills are re-discovered from all configured directories
2. The load-skill tool's description is updated with current skill names and metadata (in --catalog=tool-description mode; in the default instructions mode the catalog itself is frozen until restart on stdio)
3. Per-skill prompts are added, removed, or updated accordingly
4. tools/listChanged and prompts/listChanged notifications are sent to connected clients
5. Clients that support these notifications will refresh tool and prompt definitions

Skill Metadata Format

The skill catalog (server instructions by default, or the load-skill tool description in --catalog=tool-description mode) includes metadata for all available skills in XML format:

# Skills

When a user's task matches a skill description below: 1) activate it, 2) follow its instructions completely.

<available_skills>
<skill>
<name>mcp-server-ts</name>
<description>Build TypeScript MCP servers with composable code snippets...</description>
<location>C:/path/to/mcp-server-ts/SKILL.md</location>
</skill>
</available_skills>

In tool-description mode this metadata is dynamically updated when skills change — clients supporting tools/listChanged will automatically refresh. In the default instructions mode it is generated at startup (stdio) or per request (HTTP).

Skill Discovery

Skills are discovered at startup from the configured directories. For each directory, the server checks:

• The directory itself for skill subdirectories
• .claude/skills/ subdirectory
• skills/ subdirectory

Each skill subdirectory must contain a SKILL.md file with YAML frontmatter including name and description fields.

Skill Visibility Control

Control which skills appear in tools vs prompts using optional frontmatter fields:

Example: User-Only Skill

Hide from model auto-discovery, require explicit user invocation via /skill-name prompt:

---
name: deploy
description: Deploy to production
disable-model-invocation: true
---

Example: Model-Only Skill

Hide from prompts menu, model uses automatically when relevant:

---
name: codebase-context
description: Background information about this codebase
user-invocable: false
---

Note: Resources (skill:// URIs) always include all skills regardless of visibility settings, allowing explicit access when needed.

Testing

Manual Testing with MCP Inspector

npm run build
npm run inspector -- /path/to/skills

Automated Evals (Development Only)

The evals/ directory contains an evaluation framework for testing skill activation across different delivery modes. Evals are only available when developing from source (not included in the npm package).

# Clone the repo first
git clone https://github.com/olaservo/skilljack-mcp.git
cd skilljack-mcp

# Install dev dependencies (includes claude-agent-sdk for evals)
npm install

# Build and run evals
npm run build
npm run eval                              # Default: greeting task, MCP mode
npm run eval -- --task=xlsx-openpyxl      # Specific task
npm run eval -- --mode=local              # Local skill mode
npm run eval -- --mode=mcp+local          # Both MCP and local enabled

See evals/README.md for details on available tasks, modes, and findings about activation behavior differences.

Related

• Agent Skills Specification
• MCP TypeScript SDK
• Example MCP Clients