gns3-server/docs/features/mcp-service.md

MCP (Model Context Protocol) Service

Overview

GNS3 Server provides a standard Model Context Protocol (MCP) interface, allowing AI assistants like Claude to interact with GNS3 network simulations through SSE (Server-Sent Events) transport.

The MCP service exposes GNS3 project management operations as MCP tools that can be discovered and called by MCP clients.

Endpoints

Path	Method	Description
/v3/mcp/	GET	MCP service metadata
/v3/mcp/transport/sse	GET	SSE stream (MCP connection)
/v3/mcp/transport/messages/	POST	JSON-RPC messages

Authentication

The SSE endpoint requires a valid GNS3 JWT token. It supports two ways to pass the token:

1. Authorization header (recommended for Claude Code):

   Authorization: Bearer <jwt>
   

2. Query parameter (required for Claude Desktop, since EventSource does not support custom headers):

   GET /v3/mcp/transport/sse?token=<jwt>
   

Getting a Token

curl -X POST http://localhost:3080/v3/access/users/authenticate \
  -H "Content-Type: application/json" \
  -d '{"username": "admin", "password": "admin"}'

Token Expiry

Default JWT token lifetime is 1440 minutes (24 hours). This can be configured in gns3_server.conf:

jwt_access_token_expire_minutes = 1440  ; 24 hours

Available Tools

Tool	Description	Required Parameters
list_projects	List all projects	none
get_project	Get project details	project_id
create_project	Create a project	name
delete_project	Delete a project	project_id
open_project	Open a project	project_id
close_project	Close a project	project_id
get_project_stats	Get project statistics	project_id

Configuration

Claude Code (CLI)

# Get a JWT token
TOKEN=$(curl -s -X POST http://localhost:3080/v3/access/users/authenticate \
  -H "Content-Type: application/json" \
  -d '{"username": "admin", "password": "admin"}' | python3 -c \
  "import sys,json; print(json.load(sys.stdin)['access_token'])")

# Add MCP server
claude mcp add --transport sse My_GNS3_Server \
  http://localhost:3080/v3/mcp/transport/sse \
  -H "Authorization: Bearer $TOKEN"

Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "My_GNS3_Server": {
      "url": "http://localhost:3080/v3/mcp/transport/sse?token=your_jwt_token"
    }
  }
}

Architecture

sequenceDiagram
    participant Client as Claude Code / Claude Desktop
    participant MCP as MCP Service<br/>(/v3/mcp/transport)
    participant Auth as JWT Auth
    participant GNS3 as GNS3 REST API
    participant DB as Controller / Database

    Note over Client: Step 1: Connect with JWT
    Client->>MCP: GET /sse?token=&lt;jwt&gt;<br/>or Authorization: Bearer &lt;jwt&gt;

    MCP->>Auth: Validate Token
    Auth-->>MCP: Token Valid
    MCP-->>Client: event: endpoint<br/>data: /messages/?session_id=xxx

    Note over Client: Step 2: Initialize Protocol
    Client->>MCP: POST /messages/?session_id=xxx<br/>{"method":"initialize", ...}
    MCP-->>Client: event: message<br/>{"result": {"protocolVersion": "...", ...}}

    Note over Client: Step 3: List & Call Tools
    Client->>MCP: POST /messages/<br/>{"method":"tools/list"}
    MCP-->>Client: event: message<br/>{"result": {"tools": [...]}}

    Client->>MCP: POST /messages/<br/>{"method":"tools/call",<br/>"params": {"name":"list_projects"}}

    MCP->>GNS3: Gns3Connector (HTTP)
    GNS3->>DB: Query Projects
    DB-->>GNS3: Project Data
    GNS3-->>MCP: JSON Response
    MCP-->>Client: event: message<br/>{"result": {"content": [...]}}

Internal Implementation

• FastMCP (Anthropic MCP SDK) is used for tool registration and SSE transport
• The SSE app is mounted as a Starlette sub-application under /v3/mcp/transport
• JWT tokens are validated using GNS3's existing auth_service
• Tool handlers use Gns3Connector (from custom_gns3fy) to call GNS3's own REST API, keeping the MCP layer decoupled
• The JWT token is stored in a contextvars.ContextVar so it is available within tool handler threads (Python ≥ 3.9 propagates contextvars through asyncio.to_thread)

Source Files

• gns3server/api/routes/mcp/__init__.py — FastMCP server, tool definitions, SSE transport, JWT auth wrapper
• gns3server/api/routes/mcp/projects.py — Project tool handlers using Gns3Connector
• gns3server/api/server.py — Mounts MCP routes via register_starlette_routes()