transit-mcp

MCP server for Japanese public transit transfer / journey planning, with a map-based iframe UI rendered inside Claude and ChatGPT through the Model Context Protocol Apps SDK.

Built on Cloudflare Workers (Hono) + @modelcontextprotocol/sdk (Streamable HTTP transport) + @modelcontextprotocol/ext-apps + React + MapLibre GL. It wraps the public Transit API (api.transit.ls8h.com) behind the Cloudflare Cache and exposes four tools to MCP clients.

Features

Four tools are exposed over MCP:

• search_places — fuzzy place / station search by free-text query.

• plan_journey — point-to-point journey planning from free-text from / to. Returns a map-based iframe HTML resource with the route drawn over a MapLibre map.

• station_departures — upcoming departures for a station at a given time.

• station_detail — full detail of a single station (lines served, exits, etc.).

Only plan_journey returns an MCP UI resource (_meta.ui.resourceUri); the other three return JSON only.

Related MCP server: ns-bridge

Quick start (clients)

The server speaks the Streamable HTTP MCP transport. Once deployed it is reachable at https://transit-mcp.<your-cf-account>.workers.dev/mcp.

Claude Desktop

Edit your claude_desktop_config.json:

• macOS: ~/Library/Application Support/Claude/claude_desktop_config.json

• Windows: %APPDATA%\Claude\claude_desktop_config.json

• Linux: ~/.config/Claude/claude_desktop_config.json

Add a transit-mcp entry under mcpServers:

{
  "mcpServers": {
    "transit-mcp": {
      "type": "http",
      "url": "https://transit-mcp.<your-cf-account>.workers.dev/mcp"
    }
  }
}

Restart Claude Desktop. The four tools appear in the tool picker. Try:

• 「渋谷から東京駅までの経路を教えて」 → plan_journey runs and an interactive map iframe is embedded directly in the chat.

• 「新宿駅の次の発車は？」 → station_departures returns the next trains.

• "Plan a route from Shibuya to Tokyo Station" → English summary, same map.

• "What's the next train from Shinjuku?" → station_departures.

If you self-host you can point at http://localhost:8787/mcp for local development.

ChatGPT (Apps SDK)

The MCP Apps SDK in ChatGPT discovers servers by URL exactly like Claude Desktop:

1. Open ChatGPT settings → Apps & Connectors → Add MCP Server.

2. Enter the server URL: https://transit-mcp.<your-cf-account>.workers.dev/mcp.

3. Choose transport: Streamable HTTP.

4. Approve the four tools when prompted.

The iframe HTML resource is served from /ui/plan on the same Worker and is rendered inline in ChatGPT just like in Claude Desktop.

Custom GPT (Actions) fallback

If you need to register transit-mcp inside a Custom GPT before Apps SDK is available in your tier, wrap the tools as OpenAI Actions:

1. Create a Custom GPT in the ChatGPT builder.

2. Under Actions → Create new action, import an OpenAPI 3.1 schema that POSTs to /mcp with the standard MCP envelope. (A minimal example schema lives in examples/openai-actions/ — TODO for v0.2.)

3. Note: Custom GPT Actions cannot render the _meta.ui.resourceUri iframe; only the textual summary will be displayed.

For the richest experience, prefer the Apps SDK path.

Local development

pnpm install
pnpm dev          # Wrangler dev server on http://localhost:8787
pnpm inspect      # MCP Inspector against the local server

Useful scripts:

• pnpm test / pnpm test:watch — Vitest (65 tests at time of writing)

• pnpm typecheck — tsc --noEmit

• pnpm lint / pnpm format — Biome

• pnpm build:ui — bundle the iframe UI with esbuild

• pnpm generate:types — regenerate Transit API types from its OpenAPI

To exercise tools locally with the MCP Inspector:

pnpm dev          # terminal 1
pnpm inspect      # terminal 2 → http://localhost:6274
# In the Inspector, click `connect`, then `tools/call`:
#   { "name": "plan_journey", "arguments": { "from": "渋谷", "to": "東京" } }
# Copy the returned _meta.ui.resourceUri into a browser tab — the iframe HTML
# will render the route on a MapLibre map.

Deployment

pnpm build:ui
pnpm dlx wrangler@latest deploy

Pre-flight: the KV namespace UI_CACHE must exist before deploy.

pnpm dlx wrangler@latest kv namespace create UI_CACHE
# Copy the returned id into wrangler.toml under [[kv_namespaces]].

A dry-run is also wired into CI for every PR:

pnpm dlx wrangler@latest deploy --dry-run

Configuration

Attribution

Required attribution is surfaced in the iframe footer:

• Transit data: per-feed credits returned by /api/v1/feeds and /api/v1/operators (cached in KV for 1 hour).

• Map tiles: © OpenStreetMap contributors, served via © OpenFreeMap.

Override MAP_STYLE_URL if you self-host MapTiler / Protomaps and want their attribution shown instead — the iframe reads it from the worker env at render time.

License

MIT.