Viaduct
Compared
Structurizr is the reference implementation of C4: you write a DSL, it draws the views, and the model lives in git next to the code. Viaduct is a visual model where documentation, contracts and flows hang off the elements, served to coding agents over MCP. The real choice is whether your model is written or drawn.
01What it is
Structurizr was created by Simon Brown, who created the C4 model, and its own site calls it the reference implementation — which makes it the safest choice when strict C4 fidelity matters. You describe the model in Structurizr DSL and generate every view from that one description. It is open source, runs on your own machine, and the text diffs in a pull request like any other file.
02Side by side
| Viaduct | Structurizr | |
|---|---|---|
| How the model is authored | A canvas: drill into a system to see its containers, into a container to see its components. | Structurizr DSL — text you write, from which the views are generated. |
| Where it lives | A hosted workspace, or JSON files on your machine with the desktop app. | Files in your repository, rendered locally, on-premises or by the hosted service. |
| Review in a pull request | Branches and change sets inside the model, with a merge view; the desktop JSON also diffs in git. | Native — the DSL is a text file, so the diff is the review. |
| Documentation and contracts | Markdown, PlantUML sequences, ER tables, OpenAPI and Protobuf contracts attached to the element they describe. | Markdown and ADRs attached to the workspace, rendered alongside the diagrams. |
| AI coding agents | A hosted MCP server: agents read the model and write back to it with scoped tokens. | The DSL is text an agent can edit; the site describes it as AI friendly. |
| Audience outside engineering | A link opens the diagram; viewers need no tooling and no DSL. | Readers need the rendered views published somewhere for them. |
| Licence and price | Free in the browser with an account. Desktop is €10 a month after 7 days; the Community edition is open source. | Open source and free to run yourself; the hosted service is a separate product. |
Plans and prices move. Before deciding, read them at structurizr.com and on our own pages.
03The decision
04Questions
They are separate tools and there is no direct import today. Both describe the same four C4 levels, so a model can be rebuilt either way, and Viaduct exports JSON, PlantUML, OpenAPI and Protobuf for whatever has to leave it. If a Structurizr DSL import would decide it for you, say so — it is a fair thing to ask for.
Structurizr is open source and free to run on your own machine; its hosted service is a separate, paid product. Viaduct is free in the browser with an account, with every level, the MCP server and collaboration included, and the desktop app is the paid part.
Yes, with the desktop app: projects are plain JSON files in a folder you choose, so they commit and diff like any other file, and the MCP server runs on localhost beside them. The browser edition keeps the model in a hosted workspace instead, with branches and change sets for review.
The browser edition is free with an account — all four levels, the documentation, the flows and the MCP server, with nothing held back for a paid plan.
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