# Viaduct

> A browser-based C4 architecture modeling tool that attaches documentation, sequence diagrams, API contracts, and MCP-powered AI agent context directly to your system model.

Viaduct is a browser-based architecture modeling editor built by Quiet Grid Labs, founded by Igor Golovko. It uses the C4 model as a navigation layer and attaches documentation, PlantUML sequence diagrams, ER diagrams, and API endpoint details directly to model elements — keeping architectural knowledge connected to the systems it describes. The editor works locally in the browser without an account, and authenticated users can save projects in the cloud, collaborate in real time, and serve architecture context to AI agents via a hosted MCP server.

## What It Is

Viaduct is a C4-model-based architecture documentation tool that treats the model as the single entry point for all architectural knowledge. Rather than scattering diagrams across Miro boards, wikis, and database tools, it lets teams attach Markdown documentation, PlantUML sequence diagrams, ER schemas, and OpenAPI-style endpoint definitions directly to the elements they describe. The four C4 levels — System Context, Containers, Components, and Code — form the navigation structure, and relationships between elements can be traced across levels to answer questions like "which endpoint and internal component actually handle this interaction?"

## How the Model Works

Every element in Viaduct exists once and appears on every level it belongs to, so a rename or new dependency propagates everywhere automatically. Key capabilities include:

- **Relationship tracing** — a container-level arrow can be associated with the specific components in the target container, letting reviewers follow a path from SPA → API → Auth Service → endpoint without searching multiple diagrams.
- **Magic flows** — an ordered walk through existing model elements and connections that plays back on the diagram itself, dims everything except the current hop, and can generate a PlantUML sequence diagram from its steps.
- **ER editor** — database containers open a table/column/foreign-key editor with per-table documentation for ownership, retention, and migration rules.
- **API endpoint elements** — a dedicated component type that holds request, response, and header definitions and can be imported from an OpenAPI specification.
- **Change sets** — a piece of work written against a pinned model version, with intent, scope, constraints, and acceptance criteria, designed to be handed off to an AI agent via MCP.

## MCP Server and Agent Integration

Viaduct ships a hosted MCP server at `mcp.quietgridlabs.com/mcp` that requires no installation — authenticated users point Cursor, Claude Code, Claude Desktop, or any MCP client at the URL with a personal access token. The server exposes tools for reading the model (`c4_project_context`, `c4_get_element`, `c4_search`), writing architecture (`c4_create_element`, `c4_update_element`, `c4_upsert_connection`, `c4_upsert_doc`), and managing change sets (`c4_get_implementation_bundle`, `c4_report_progress`). A compact one-line-per-element notation reduces token usage by roughly a third compared to Markdown output. Tokens are personal, scoped to projects, and never stored on the server; revoking one cuts off all clients immediately.

## AI Assistant

An in-editor AI assistant handles two tasks the docs describe as "dull and mechanical": writing descriptions for undescribed elements and reviewing the whole model for structural issues. The assistant runs on Quiet Grid Labs' own provider — no API key required — and every account receives a monthly allowance measured in units (one per input token, eight per output token). Suggestions are never applied automatically; a description appears as a preview and lands in the model only when the user confirms it. The review panel names real elements, links directly to them on the canvas, and discards findings that reference elements not in the model. Users can also point the assistant at a self-hosted endpoint (Ollama, vLLM, or any OpenAI-compatible `/chat/completions`) to use their own model outside the allowance.

## Collaboration, API, and Webhooks

Authenticated users can share projects with view or edit access, see collaborators' positions on the canvas in real time, and export models as JSON or individual elements as ZIP archives. A REST API under `/api/v1` uses the same personal access tokens as the MCP server, supports read-only scoped tokens for CI pipelines, and enforces a rate limit of 600 requests per minute per token. Webhooks can post element, connection, and change-set events to an HTTPS endpoint, with HMAC-SHA256 signatures and five retries over an hour on failure.

## Deployment Model and Access

The editor opens without an account and stores the model in the browser only. Cloud persistence, team sharing, MCP access, and change sets require a signed-in account. According to the homepage, Viaduct is free — every level, the MCP server, collaboration, and change sets included — with no trial period and no features held behind a paid plan. The project is operated by Quiet Grid Labs, a solo-founder company based on EU servers, with Igor Golovko serving as Founder and CTO.

## Features
- C4 model editor (Context, Containers, Components, Code levels)
- Markdown documentation attached to model elements
- PlantUML sequence diagram editor with C4-level participant validation
- ER diagram editor for database containers
- API endpoint elements with request/response/header definitions
- OpenAPI specification import
- Magic flows — ordered walkthroughs that play back on the diagram
- Relationship tracing across C4 levels
- MCP server for AI agent architecture context
- Change sets with pinned versions, scope, constraints, and acceptance criteria
- In-editor AI assistant for element descriptions and model review
- Self-hosted LLM endpoint support for AI assistant
- Real-time collaboration with collaborator position tracking
- Project sharing with view or edit access
- JSON export/import for full models
- REST API under /api/v1 with scoped personal access tokens
- Webhooks for element, connection, and change-set events
- Branch-based change proposals with merge review
- Local browser-only mode (no account required)
- EU-hosted cloud storage with backups

## Integrations
Cursor, Claude Code, Claude Desktop, Any MCP client, OpenAPI specifications, PlantUML, Ollama, vLLM, OpenAI-compatible /chat/completions endpoints, RunPod

## Platforms
WINDOWS, MACOS, WEB, API

## Pricing
Free

## Links
- Website: https://c4.quietgridlabs.com
- Documentation: https://c4.quietgridlabs.com/docs
- EveryDev.ai: https://www.everydev.ai/tools/viaduct