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southleftMCP SERVER · OFFICIAL PROJECT

Design Systems MCP Server

395 curated, cited design-system entries — including the 2025-2026 agent-interface specs a frontier model will confidently answer with last year's field names.

by southleft ↗Design Systems & UI Component Libraries✓ Official

LABSCO SUMMARY

It knows it is answering the questions the model gets wrong.

A frontier model already knows Atomic Design and the WCAG criteria; what it cannot know is that MCP Apps moved a field name last quarter. This base is built around that gap — dated, cited, primary-source extracts of the specs that changed recently — and the relevance floor means it returns nothing rather than something adjacent when a topic is genuinely uncovered.

What it is

A hosted MCP server over a curated knowledge base of design systems material: W3C standards, WCAG 2.2, ARIA practices, the major design systems, and the agent-interface protocols that move faster than model training does. It runs on Cloudflare Workers with vector search backed by Supabase pgvector, and a keyword path that keeps working when embeddings are not available.

What you get

  • Semantic search across the whole knowledge base, filterable by category and tags — search_design_knowledge
  • Chunk-level search when the answer is a passage rather than a document — search_chunks
  • Browsing by category across components, tokens, patterns, guidelines, workflows and general — browse_by_category
  • Tag discovery and tag-scoped browsing for exploring what is covered — get_all_tags, browse_by_tag
  • A relevance floor built on IDF-weighted scoring, so a topic the base does not cover returns nothing rather than confident adjacent material
  • Source reliability flags on every answer — Primary, Authoritative, Reference, Example or Community
  • Coverage of the fast-moving specs: A2UI v1.0, MCP Apps / SEP-1865 with its ui:// resources and ui/ method namespace, AG-UI events, the OpenAI Apps SDK to MCP Apps field migration map, component contracts, and the AGENTS.md / SKILL.md layer
  • Depth on the major systems — Carbon, Polaris, Atlassian, Material 3, Fluent, Spectrum, Nord, Mantine, shadcn/ui, Radix, Untitled UI — covering guidance and token architecture rather than prop tables

Requirements

Nothing. It is a public endpoint at https://design-systems-mcp.southleft.com/mcp, added as a custom connector or a one-line claude mcp add --transport http, and works with any MCP client that supports remote servers. Running your own copy means a Cloudflare Workers deployment plus Supabase with pgvector, and either Cloudflare Workers AI for embeddings or an OpenAI key. Licensed MIT.

Setup effort

One command — claude mcp add --transport http design-systems https://design-systems-mcp.southleft.com/mcp

See installation options