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:

Authorization header (recommended for Claude Code):
```
Authorization: Bearer <jwt>
```
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
    participant Auth as JWT Auth
    participant GNS3 as GNS3 REST API

    Note over Client: 1. Connect with JWT
    Client->>MCP: GET /sse (token in header or query)
    MCP->>Auth: Validate Token
    Auth-->>MCP: Token Valid
    MCP-->>Client: event: endpoint /messages/?session_id=xxx

    Note over Client: 2. Initialize
    Client->>MCP: POST /messages/ (initialize)
    MCP-->>Client: event: message (protocolVersion, capabilities)

    Note over Client: 3. List & Call Tools
    Client->>MCP: POST /messages/ (tools/list)
    MCP-->>Client: event: message (tools list)

    Client->>MCP: POST /messages/ (tools/call list_projects)
    MCP->>GNS3: Gns3Connector HTTP request
    GNS3-->>MCP: Projects data
    MCP-->>Client: event: message (tool result)

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()

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