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.
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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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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
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
Limitations
Frequently Asked Questions
What does it cost?
What is the AI assistant, and what does it cost me?
Do I need an account?
Where does my architecture live?
What can an AI agent do with it?
How is this different from a diagramming tool?
Can I bring what I already have?
Getting Started
- 1 Open the editor: a sample Internet Banking model loads in the browser so you can try Viaduct without signing in.
- 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 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 protected] (support & feedback address shown in site footer)
docs
Documentation is linked from the site header under 'Documentation'.
API
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