C4 modelling · MCP server · self-hosted
A C4 model your team maintains and your agents can read
Draw system context, containers, components and code. Attach the docs, sequence diagrams and API contracts to the elements they belong to. Serve the whole thing to Cursor or Claude Code over MCP.
free · no account · runs in this browser · export any time
01
Levels
Four levels, one model
Open a system to see its containers, a container to see its components. Every level is a view of the same model, so a rename or a new dependency shows up everywhere it belongs.
02
Notation
The technology picks the colour
Pick a technology and the element takes its colour and logo. Datastores are drawn as cylinders, people as actors — so a diagram is scannable before anyone reads a label.
03
What an element carries
Docs, diagrams and schemas live on the element
Every element carries its own Markdown pages — a design note on a service, a runbook on a worker, an ADR on a system. Write in the editor with a live preview, insert a stored sequence diagram or an API endpoint as a block, and the page keeps rendering the current version instead of a screenshot that rotted last quarter.
04
Testimonials
What teams say once the model is theirs
The tool earns its place when the diagram stops being a screenshot someone has to remember to update.
I built Viaduct because I kept redrawing the same diagram to answer the same question. Now the model answers it, and my agent reads it too.
Igor Golovko
Founder, Quiet Grid Labs
The C4 levels finally match how we actually talk about the system. Drilling from context into containers takes one click instead of three diagrams.
Marta Kowalczyk
Backend lead, fintech
Our architecture diagram used to be a screenshot in Confluence, six months stale. Now it is the thing we open during incident review.
Daniel Osei
SRE, marketplace platform
The MCP server is the part that sold the team. Cursor answers questions about our services without anyone pasting a diagram into the chat.
Priya Raghunathan
Staff engineer, logistics
Change sets let us review an architecture change the way we review code. The diff is the conversation.
Tomas Lindqvist
Head of platform, SaaS
Sequence diagrams next to the model, not in a separate tool that nobody updates. That alone saved us the weekly sync.
Elena Duarte
Tech lead, healthtech
We imported an OpenAPI spec and had the endpoints on the container in a minute. No one had to draw a box by hand.
Ahmed Barakat
Integration engineer, telecom
New joiners read the model on day one and ask better questions on day two.
Hannah Brecht
Engineering manager, e-commerce
05
Magic flows
A request, played back across the canvas
A Magic flow is an ordered walk through the model: each step names a hop, the elements it goes from and to, and the connection it uses. Steps that happen at the same time share a stage, so a fan-out reads as a fan-out.
- Build a flow from the elements you already modelled — no second diagram to maintain.
- Play it on the canvas: everything dims except the current hop, and the camera follows the request.
- Generate a sequence diagram from the flow, and attach the docs and diagrams that explain each step.
Playing “Payment leaves the bank”, step by step.
06
Contracts
Endpoints are stored as OpenAPI
Describe an endpoint in the editor — typed parameters, request body, a card per status — and the container holds the OpenAPI document for the whole service. Import a spec to fill the canvas from it; export it back as a file.
{
"openapi": "3.1.0",
"info": { "title": "Accounts API", "version": "1.4.0" },
"paths": {
"/api/accounts/{id}": {
"get": {
"operationId": "getAccount",
"summary": "Get account",
"parameters": [
{ "name": "id", "in": "path", "required": true,
"schema": { "type": "string" }, "example": "acc_42" }
],
"responses": {
"200": { "description": "OK", "content": { "application/json": {
"example": { "id": "acc_42", "currency": "EUR" } } } },
"404": { "description": "No account with that id" }
}
}
}
}
}accounts-api-openapi.json — exported from a container
07
MCP
One server, one token, the whole architecture
Cursor, Claude Code, Claude Desktop and any other MCP client can read the architecture straight from Viaduct: systems and services, API endpoints with their contracts, WebSocket channels, documentation and sequence diagrams. One hosted MCP server, one personal token, no repository to clone.
- Your agent answers “what talks to what, and over which contract?” from the model instead of guessing from the codebase.
- It can write back too — new services, endpoints, connections and docs land in the same model your team reads.
- Hand it a change set: a piece of work pinned to a fixed version, with the constraints and acceptance criteria it has to prove.
Work an agent can finish, and you can verify
A change set is scoped to one pinned version, so nothing shifts under the agent while it builds. It reports each finished piece with the criterion it proves and the commit that proves it — the status moves itself from draft to done, live on your screen.
08
Your data
Your architecture, on terms you can check
An architecture model is a map of everything you run, which makes “where does it live?” the first fair question about a tool like this. Three answers, all of them verifiable.
It stays where you put it
Without an account the model never leaves your browser. With one it lives on servers in the Netherlands, inside the EU, and so do the backups.
You can take it with you
The whole model exports as JSON, and a container’s endpoints as an OpenAPI document — any time, without asking us. Nothing here is a one-way door.
The agent gets in only if you let it
MCP access needs a personal token you issue yourself, scoped to your projects. Revoke it and the access ends that moment.
09
Questions
Answered before you have to ask
What does it cost?
Nothing. Viaduct is free — every level, the MCP server, collaboration and change sets included. There is 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.
Do I need an account?
Not to draw. The editor opens with a sample model and works entirely in your browser, where the model stays on your machine and never reaches our servers. An account is for the things a browser cannot do alone: keeping a project across devices, sharing it with your team, and serving it over MCP.
Where does my architecture live?
On servers inside the EU, along with the backups. Without an account it lives in your browser and nowhere else. Either way you can export the whole model as JSON, and any container’s contracts as an OpenAPI document, whenever you like.
What can an AI agent do with it?
Read the model and write to it: Cursor, Claude Code, Claude Desktop or any other MCP client can ask what talks to what and over which contract, and can add services, endpoints, connections and documentation back. Access needs a personal token you issue yourself, scoped to your projects, and revoking it ends the access immediately.
How is this different from a diagramming tool?
A diagram is a picture; this is a model. An element exists once and appears on every level it belongs to, so a rename or a new dependency shows up everywhere at once. The endpoints on a container are a real OpenAPI document, not labels on a box — which is what makes the whole thing readable by something other than a person.
Can I bring what I already have?
Import an OpenAPI specification and the endpoints land on the container with their parameters, request bodies and responses. Models export and import as JSON, so moving a workspace — or keeping a copy outside this service — is a file.
10
Start here
The sample model is already loaded
Open the editor and an Internet Banking system is waiting, modelled across every level, with docs, flows and contracts attached. Clear it when you want to start your own.