MCP Inspector as MCP Server

A lean MCP server that enables LLMs to inspect and test other MCP servers. This is a self-contained implementation that uses the MCP SDK directly, without shelling out to external CLIs.

Features

• Direct SDK integration: Uses @modelcontextprotocol/sdk directly for both server and client operations
• All transport types: Supports stdio, SSE, and HTTP (streamable) transports
• Minimal footprint: Single dependency (@modelcontextprotocol/sdk)
• Full MCP inspection: List tools, call tools, list resources, read resources, list prompts, get prompts
• Session management: Persistent connections with automatic garbage collection
• Event buffering: Capture notifications, traffic, and errors for debugging

Installation

npm install
npm run build

Usage

As an MCP Server

Add to your MCP config. While there are slight variances between different harnesses, the general format is the same:

{
  "mcpServers": {
    "mcp-inspector": {
      "command": "node",
      "args": ["/path/to/mcp-inspector-as-mcp-server/dist/server.js"]
    }
  }
}

Available Tools

Session Management (NEW in v2.0)

Tool	Description
insp_connect	Establish a persistent connection to an MCP server. Returns a session_id.
insp_disconnect	Close a persistent session and release resources.
insp_list_sessions	List all active sessions with their status and idle time.
insp_read_events	Read buffered events (notifications, traffic, errors) from a session.

Inspection Tools

Tool	Description
insp_tools_list	List all tools exposed by an MCP server
insp_tools_call	Call a tool on an MCP server
insp_resources_list	List all resources exposed by an MCP server
insp_resources_read	Read a specific resource
insp_resources_templates	List resource templates
insp_prompts_list	List all prompts
insp_prompts_get	Get a specific prompt

Connection Parameters

All tools accept the following connection parameters:

For stdio transport (local commands):

• command: Command to run (e.g., "node", "python")
• args: Array of arguments (e.g., ["path/to/server.js"])

For SSE/HTTP transport (remote servers):

• url: Server URL (e.g., "http://localhost:3000/sse")
• headers: Optional HTTP headers object

Common:

• transport: Force transport type ("stdio", "sse", or "http"). Auto-detected if not specified.
• session_id: (Optional) Use an existing persistent session instead of creating an ephemeral connection.

Session Workflow

For debugging stateful server behavior, use persistent sessions:

1. insp_connect → returns session_id
2. insp_tools_list (with session_id) → uses persistent connection
3. insp_tools_call (with session_id) → state is preserved
4. insp_read_events (with session_id) → see notifications
5. insp_disconnect (with session_id) → cleanup

Sessions auto-close after 30 minutes of inactivity.

Examples

List tools from a local MCP server (ephemeral):

{
  "command": "node",
  "args": ["/path/to/some-mcp-server/dist/server.js"]
}

Create a persistent session:

{
  "command": "node",
  "args": ["/path/to/some-mcp-server/dist/server.js"]
}
// Returns: { "session_id": "sess_abc123", "server_info": {...} }

Call a tool using a session:

{
  "session_id": "sess_abc123",
  "tool_name": "search",
  "tool_args": {"query": "hello"}
}

Architecture

src/
├── server.ts     # MCP server exposing inspector tools
├── client.ts     # Client wrapper (hybrid stateless/session mode)
├── transport.ts  # Transport factory (stdio, SSE, HTTP)
├── session.ts    # SessionRegistry with GC (30-min TTL)
└── events.ts     # EventBuffer (ring buffer for notifications)

Why This Exists

The original MCP Inspector is a web-based UI + CLI combo spread across multiple projects. This consolidates the core functionality into a single, lean MCP server that an LLM can use to:

1. Develop and debug MCP servers iteratively
2. Test MCP server functionality without leaving the conversation
3. Explore what tools/resources/prompts an MCP server exposes
4. Debug stateful behavior with persistent sessions

Changelog

v2.0.0

• Added session management (insp_connect, insp_disconnect, insp_list_sessions)
• Added event buffering (insp_read_events)
• All inspection tools now support optional session_id for persistent connections
• Added automatic garbage collection (30-minute TTL for idle sessions)
• Backward compatible: omit session_id for original ephemeral behavior

v1.0.0

• Initial release with ephemeral connections

License

MIT