Viaduct

Viaduct

Viaduct is a C4 architecture modelling tool by Quiet Grid Labs that keeps a single, editable model across four C4 levels, attaches docs/diagrams/OpenAPI contracts to elements, and serves the architecture to AI coding agents via a hosted MCP server.

Viaduct is developer tools software teams evaluate for creative & design. Use this page to review pricing, integration signals, and the best alternatives before you commit.

Free API 70/100
#119 in Developer Tools (119 tools)
Just launched
Data reviewed Sep 12, 2026

Profile facts come from the vendor source. AiMatch labels unknown pricing or API details instead of estimating them.

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Quick Overview

Best for: Creative & Design

What it does

Developer Tools software for decision-makers comparing workflow fit and alternatives.

Best fit

Creative & Design

Pricing snapshot

Free from Free

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Viaduct

Viaduct is a C4 modelling tool that represents systems at four C4 levels (system context, containers, components and code) as one maintained model. Each element in the model can carry Markdown documentation, sequence diagrams, ER schemas and OpenAPI contracts; edits and renames propagate across all levels so diagrams remain consistent. Viaduct includes a hosted MCP server so AI coding agents (Cursor, Claude Code, Claude Desktop and other MCP clients) can read and write the architecture model, and it offers import/export (OpenAPI and JSON), a Figma integration, change sets for review, and a built-in assistant to help fill in missing documentation.

Viaduct is a C4 modeling tool for software teams, with a hosted MCP server that feeds your architecture to AI development tools. System context, container, component and code diagrams, live collaboration, and change sets your coding agent can implement.

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Key Features

Four C4 levels, one model

A single underlying model presents system context, containers, components and code as four views; changes (renames, dependencies) appear everywhere at once.

Element-attached documentation and diagrams

Each element holds its own Markdown pages, stored sequence diagrams, ER schemas and API endpoint blocks that render the current version rather than static screenshots.

OpenAPI contracts

Containers hold OpenAPI documents for their endpoints; you can author endpoints in the editor, import a spec to populate the canvas, and export an OpenAPI file.

Magic flows

Magic flows are ordered walks through the model that play a request hop-by-hop on the canvas, generate sequence diagrams, and attach explanatory docs to each step.

MCP server for agent access

A hosted MCP server lets MCP clients read systems, services, endpoints and docs directly from Viaduct and optionally write back new services, endpoints, connections and docs.

Change sets

Change sets are scoped to a pinned version and report finished pieces with acceptance criteria and the commit that proves them, enabling architecture changes to be reviewed like code.

Figma integration

Connect a Figma account to reference frames and design tokens (colours, spacing, type) in the model; the integration reads values but does not write back to Figma.

Built-in assistant

A built-in assistant suggests descriptions for elements, names for unnamed flow steps, and a review of the model; suggestions are shown and applied only when the user accepts them.

Import/export and data portability

The entire model exports as JSON and individual container endpoints export as OpenAPI; without an account the model stays in the browser, with an account data is stored on EU servers.

Pricing

Free Tier Available

Viaduct is free; every level, the MCP server, collaboration and change sets are included permanently. Donations are optional to support continued work.

Free

Free
  • All C4 levels and editor features
  • Hosted MCP server
  • Collaboration and change sets
  • Import/export and Figma integration

Use Cases

Living architecture documentation

Replace stale screenshots by keeping a single model that stays consistent across levels and carries documentation, runbooks and ADRs attached to elements.

Agent-assisted coding and automation

Allow MCP-capable agents (Cursor, Claude Code, etc.) to read the architecture and, with a personal token, add services, endpoints and documentation back into the shared model.

Architecture change review

Use change sets to review architecture changes with diffs, acceptance criteria and pinned versions, enabling architecture changes to be discussed like code reviews.

Incident review and onboarding

Teams open the live model during incident review or onboarding so new joiners and SREs see current systems, dependencies and docs rather than outdated diagrams.

Design-implementation alignment

Integrate Figma frames into the model so components point at real design nodes and the palette and tokens are read into the architecture.

Integrations

MCP clients (Cursor, Claude Code, Claude Desktop)

MCP clients can read the architecture model and (with a personal token) write services, endpoints and docs back into Viaduct via the hosted MCP server.

OpenAPI

Import an OpenAPI specification to populate a container's endpoints and export container endpoints as an OpenAPI document.

Figma

Connect a Figma account to link model components to real frames and import design tokens (colours, spacing, type) for consistent visuals.

Benefits

Maintains a single source of truth instead of multiple static diagrams, reducing documentation rot.
Makes architecture readable and actionable by AI agents through MCP, enabling automation and agent-assisted development.
Keeps docs, sequence diagrams and API contracts attached to the elements they belong to, improving discoverability and context.
Supports import/export (OpenAPI, JSON) and data portability so teams retain ownership of their models.
Includes a built-in assistant to reduce the friction of writing routine documentation and model reviews.

Limitations

The built-in assistant requires a signed-in account to apply its monthly allowance; guests can draw but do not get the assistant allowance.
When used without an account the model remains only in the browser (it does not leave the client machine) until you sign in and persist it to the hosted servers.
Data stored with an account is kept on servers in the Netherlands (EU); teams requiring other regional storage are not described on the site.

Frequently Asked Questions

What does it cost?
Nothing. Viaduct is free — every level, the MCP server, collaboration and change sets included. Donations are optional.
What is the AI assistant, and what does it cost me?
The assistant writes missing descriptions, names for flow steps, and performs a model review. It runs on the provider (no key needed), every account gets a monthly allowance, and suggestions are shown for you to accept; nothing is applied automatically.
Do I need an account?
No to draw: the editor opens with a sample model and runs entirely in your browser. An account is required to persist projects, share with a team, use the hosted MCP server and access the assistant fully.
Where does my architecture live?
Without an account the model stays in your browser. With an account it is stored on servers in the Netherlands (inside the EU) along with backups. You can also export the whole model as JSON and OpenAPI files at any time.
What can an AI agent do with it?
MCP clients can read the model (systems, services, API contracts, channels, docs and diagrams) and — with a personal token scoped to projects — write new services, endpoints, connections and documentation back into the model; revoking the token ends access.
How is this different from a diagramming tool?
A diagram is a picture; Viaduct is a model. Elements exist once and appear on every level they belong to; endpoints are real OpenAPI documents, not labels on a box, making the model readable by machines and agents.
Can I bring what I already have?
Yes: import an OpenAPI specification to populate endpoints on a container. Models export and import as JSON, enabling workspace migration or offline copies.

Getting Started

  1. 1 Open the editor: a sample Internet Banking model loads in the browser so you can try Viaduct without signing in.
  2. 2 Edit the model in-browser and attach docs, flows and OpenAPI endpoints to elements; export the model as JSON or endpoints as OpenAPI when needed.
  3. 3 Sign up / log in to persist projects across devices, invite teammates, use the built-in assistant fully, and enable the hosted MCP server for agent access.

Support

email

[email protected] (support & feedback address shown in site footer)

docs

Documentation is linked from the site header under 'Documentation'.

API

Available: Yes

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