Viaduct

What it is

Viaduct is a C4 architecture modeling tool that creates system context, container, component, and code diagrams with live collaboration and AI integration. It includes a hosted MCP server that provides architecture context to AI coding agents like Cursor and Claude Code. The browser version is free with an account and includes all C4 levels, the MCP server, team projects, and change sets. The desktop app for macOS, Windows, and Linux runs locally with projects as JSON files, costs €10 per month after a 7-day free trial, and includes a built-in MCP server for offline use.

Questions from the bench

What is the AI assistant, and what does it cost me?

The AI assistant writes descriptions for blank elements, names for unnamed flow steps, and reviews the whole model pointing at real elements. It runs on Viaduct's provider with no key or configuration needed, and every account gets a monthly allowance included. Nothing it suggests is applied without your approval.

Do I need an account?

Yes, an account is free and created through Google, GitHub, or GitLab. The editor can open with a sample model, but saving projects, sharing with your team, change sets and the MCP server all need an account. Signing up takes no card and no trial clock.

Where does my architecture live?

In the browser version, models are stored on servers inside the EU with backups. You can export the model as JSON and containers' contracts as OpenAPI documents at any time. The desktop app keeps projects as files in a folder you choose with nothing reaching a server.

What does it cost?

Viaduct here, in the browser, is free — every level, the MCP server, collaboration and change sets included. It takes an account, and the account is what costs nothing: no trial that ends and no feature held back behind a plan. If you want the work to continue you can donate, and that buys nothing but the work continuing. The one paid thing is Viaduct Desktop, the app you download and run on your own machine: it is free for 7 days, then €10 a month.

What is Viaduct Desktop, and do I need it?

It is the same editor as a native app for macOS, Windows and Linux, with its own MCP server running on your machine. Projects are plain JSON files in a folder you choose, so nothing reaches a server at all — which is the point for people who cannot put an architecture in a browser tab. You do not need it: the browser edition is free with an account and gives you the whole model. The desktop app is free for 7 days on each machine, then €10 a month, and one licence covers 3 machines.