InvoiceXML MCP Server

A Model Context Protocol server that exposes the InvoiceXML API to AI agents. Covers Factur-X, ZUGFeRD, XRechnung, UBL / CII, and Peppol BIS Billing 3.0.

The same codebase runs in two deployment shapes, selected at startup by one environment variable:

Both run the same binary; only the configuration differs. The repository is platform-independent — it knows nothing about where or how you host it.

Architecture

+----------------------+   ProjectReference   +----------------------+
|  InvoiceXml.Mcp.Core | -------------------> |  InvoiceXml.Mcp.Host |
|  (SDK: client+tools) |                      |  (the deployable)    |
+----------------------+                      +----------------------+

InvoiceXml.Mcp.Core is a small, transport-agnostic SDK:

• IInvoiceXmlClient — typed client over the public REST API
• HttpInvoiceXmlClient — the only implementation; consumes an HttpClient from IHttpClientFactory
• InvoiceXmlClientOptions — base URL, timeout (no auth)
• AddInvoiceXmlMcpCore(IServiceCollection, IConfiguration) — DI entry point; returns the IHttpClientBuilder so the host attaches auth as a DelegatingHandler

The SDK never sees credentials. The host applies them through the HTTP pipeline. That seam is what lets one codebase serve both deployment modes.

InvoiceXml.Mcp.Host is an ASP.NET Core 10 app:

• Reads Mcp:AuthMode (ApiKey or OAuth) at startup
• Wires the matching DelegatingHandler onto the Core HTTP client via AddHostAuth(...)
• Serves the MCP endpoint at POST /, a human-friendly welcome page at GET /, and /health

Adding a new auth mode = one arm in AuthExtensions.cs plus a small folder under Auth/<Mode>/. Adding a new tool = one [McpServerTool] class. Nothing else changes.

Repository layout

invoicexml-mcp/
├── src/
│   ├── InvoiceXml.Mcp.Core/              # the SDK: client, models, tools
│   │   ├── Enums/  Interfaces/  Models/  Options/  Services/  Tools/  Extensions/
│   └── InvoiceXml.Mcp.Host/              # the deployable host
│       ├── Auth/{ApiKey,OAuth}/          # the two auth modes
│       ├── Configuration/
│       ├── Program.cs
│       └── appsettings.json              # safe defaults, no secrets
├── tests/
│   ├── InvoiceXml.Mcp.Core.Tests/
│   └── InvoiceXml.Mcp.Host.Tests/
├── Directory.Build.props                 # repo-wide MSBuild defaults
├── Directory.Packages.props              # Central Package Management
├── global.json                           # pins the .NET SDK
└── InvoiceXml.Mcp.slnx

Running locally

You need a .NET 10 SDK and an InvoiceXML API key.

# 1. Provide your API key (pick one):

# A. dotnet user-secrets (recommended — kept outside the repo)
dotnet user-secrets --project src/InvoiceXml.Mcp.Host set "Mcp:ApiKey:Value" "your-key"

# B. environment variable
$env:INVOICEXML_API_KEY = "your-key"

# 2. Run
dotnet run --project src/InvoiceXml.Mcp.Host

GET http://localhost:5004/ shows a welcome page in a browser; the MCP endpoint is POST http://localhost:5004/; GET /health returns { "status": "ok" }.

Configuration

{
  // Required in OAuth mode. The public origin where this MCP server is reachable.
  // Used in the protected-resource metadata response.
  "McpUri": "https://mcp.example.com",

  "InvoiceXml": {
    "BaseUrl": "https://api.invoicexml.com",   // override for staging / local
    "Timeout": "00:01:40"
  },

  "Mcp": {
    "AuthMode": "ApiKey",                       // "ApiKey" | "OAuth"
    "ApiKey": {
      "Value": ""                               // ApiKey mode: NEVER commit a real key
    },
    "OAuth": {
      "AuthorizationServer": "https://invoicexml.com",
      "ScopesSupported": [ "api_token.read" ]
    },
    "FileInput": {                              // limits for the URL-fetch input mode
      "MaxFileSizeBytes": 5242880,
      "FetchTimeout": "00:00:30"
    }
  }
}

Environment variable equivalents (double underscore = nesting):

OAuth mode

When Mcp:AuthMode=OAuth the host stops accepting a static API key and instead:

1. Returns 401 with WWW-Authenticate: Bearer resource_metadata="…" for any POST / that has no Bearer token.
2. Serves GET /.well-known/oauth-protected-resource pointing MCP clients at invoicexml.com as the authorization server.
3. Forwards the inbound Bearer token verbatim on every outbound call to the InvoiceXML API (the API is the source of truth for token validity; the MCP server does not validate tokens locally).

The dance an MCP client performs:

client → MCP  POST /                           → 401 + resource_metadata
client → /.well-known/oauth-protected-resource → { authorization_servers: [invoicexml.com] }
client → invoicexml.com/.well-known/oauth-authorization-server
                                               → { authorize, token, register endpoints }
client → invoicexml.com/oauth/register         → client_id + client_secret  (DCR)
client → invoicexml.com/oauth/authorize        → user consents, gets code
client → invoicexml.com/oauth/token            → access_token  (= user's API key)
client → MCP  POST /  + Authorization: Bearer  → 200, tool call flows through

Deployment

The host is a standard ASP.NET Core app — run it however you run .NET services (systemd, a container, a PaaS, etc.; the repo doesn't prescribe one):

dotnet publish src/InvoiceXml.Mcp.Host -c Release -o ./publish
# then run ./publish/InvoiceXml.Mcp.Host on your host

Set configuration via environment variables on the host (never commit secrets):

• ASPNETCORE_ENVIRONMENT=Production
• Mcp__AuthMode=ApiKey (or OAuth)
• Mcp__ApiKey__Value=… / INVOICEXML_API_KEY=… (ApiKey mode)
• McpUri=https://your-public-url and Mcp__OAuth__AuthorizationServer=https://invoicexml.com (OAuth mode)

Terminate TLS at your reverse proxy / load balancer and forward to the host's HTTP port. The server is stateless, so you can run multiple instances behind a load balancer.

License

MIT — see LICENSE.