Viaduct is a browser-based, self-hosted C4 modeling tool designed to keep architecture diagrams, documentation, workflows, and API contracts in one model that both teams and AI coding agents can use. Its four views cover system context, containers, components, and code; each element can carry Markdown documentation, sequence diagrams, ER schemas, and API details, so updates propagate across the views instead of leaving teams with stale screenshots. The tool’s Magic flows describe ordered requests across modeled elements, can play those hops on the canvas, and can generate an attached sequence diagram. Containers store OpenAPI contracts, with support for typed endpoints, parameters, request bodies, responses, importing specifications, and exporting them again. Viaduct exposes the architecture through one MCP server to Cursor, Claude Code, Claude Desktop, and other MCP clients. Agents can inspect services, connections, contracts, WebSocket channels, documentation, and diagrams, and can also write services, endpoints, connections, and documentation back to the model. Change sets pin agent work to a fixed model version and track acceptance criteria alongside the commits reported as evidence. A Figma connection can add frames, components, states, and named design values to the agent-readable model; it does not write back to Figma, and later design edits can mark components as outdated. The built-in assistant can draft missing descriptions, name unnamed flow steps, and review the model for undocumented, disconnected, or possibly duplicated elements, but users must approve each suggestion. Viaduct says the product is free, while assistant usage has a monthly allowance per account. Models remain in the browser without an account; accounts enable cross-device storage, collaboration, and MCP access. Account data and backups are stored on servers in the Netherlands, exports are available as JSON or OpenAPI, and MCP access requires a user-issued, project-scoped token that can be revoked.
