Where's My DB?

A Claude skill + MCP server that tells you how Deutsche Bahn delays will affect your trip — not just whether your train is late.

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What it does

Knowing a train is "5 min late" isn't useful. Knowing that the 12-minute delay three stops up the line will eat your 9-minute transfer in Berlin — that's useful.

Where's My DB? turns "is my train on time?" into a real answer. You ask Claude in plain language and it walks the upstream stops, traces the connection chain, and tells you which delays actually affect your journey:

Likely 15–18 minutes late.

• ICE 597 was already 18 min late at Solingen Hbf (three stops upstream) at 15:42.
• No scheduled padding before your boarding station at 16:02.
• Your Frankfurt connection (ICE 599) is currently on time — you'd lose ~14 min of a 16-min transfer.

Caveat: a single 5-min recovery en route can change this. The official board hasn't updated yet.

See examples/ for three full transcripts.

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How it works

┌──────────────────────────────────────────────────────────┐
│  Claude (Code or Desktop)                                │
│  ┌────────────────────────────────────────────────────┐  │
│  │  Skill: db-delay-predictor                         │  │
│  │  Reasoning playbook: which tool, how to combine    │  │
│  │  signals, how to express confidence honestly.      │  │
│  └────────────────────────────────────────────────────┘  │
└──────────────────────────────────────────────────────────┘
                          │  MCP (stdio, JSON-RPC)
                          ▼
┌──────────────────────────────────────────────────────────┐
│  MCP server: db-mcp (TypeScript)                         │
│  Typed data access. No reasoning.                        │
│  ┌─────────────────────┐    ┌─────────────────────┐      │
│  │ marudor (primary)   │ →  │ db-rest (fallback)  │      │
│  │ tRPC + devalue      │    │ plain REST          │      │
│  └─────────────────────┘    └─────────────────────┘      │
│  + 5-min in-memory LRU cache                             │
└──────────────────────────────────────────────────────────┘

Two artifacts. Clean split:

• The MCP server knows nothing about reasoning. It's typed data access with transparent provider fallback.
• The skill knows nothing about API endpoints. It's a reasoning playbook in markdown.

Both are independently testable. Both ship in this repo.

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What's interesting under the hood

• The primary data source isn't a public REST API. Marudor (the community DB tracker) runs internal tRPC with custom devalue encoding. The MCP includes a hand-rolled tRPC transport that speaks it, because no public alternative exposes per-stop connection-chain data ("will my Anschluss be held?") — which is the whole point.
• Transparent fallback to a stable REST API. If marudor fails (it's an unofficial endpoint), the MCP transparently falls back to db-rest, and the skill tells the user the answer is degraded. Belt-and-suspenders engineering, not a hedge.
• The skill is honest about uncertainty. Delay predictions are rounded to 5-minute buckets ("about 10–15 minutes late") because precision would be theater. The skill explicitly enumerates which signals fired and what could change.
• 5 atomic MCP tools, no reasoning baked in. resolve_station, find_train, get_train_status, plan_journey, get_disruptions. The skill composes them.
• 52 tests, mostly with mocked HTTP. Includes failure modes: timeout → fallback, both providers down → clean error, partial failure on a single stop's connection chain doesn't break the whole call.

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Quick start

Build it:

git clone https://github.com/<you>/wheres-my-db.git
cd wheres-my-db
npm install
npm run build

Wire into Claude Code (terminal)

claude mcp add db-mcp -s user -- node "$PWD/packages/db-mcp/dist/index.js"
ln -s "$PWD/packages/skill-db-delay" ~/.claude/skills/db-delay-predictor

Open a claude session, run /mcp to confirm db-mcp is connected with 5 tools, then ask:

Is ICE 597 on time today?

Wire into Claude Desktop

Edit ~/Library/Application Support/Claude/claude_desktop_config.json:

{
  "mcpServers": {
    "db-mcp": {
      "command": "node",
      "args": ["/absolute/path/to/wheres-my-db/packages/db-mcp/dist/index.js"]
    }
  }
}

Then symlink the skill the same way and fully quit + relaunch Claude Desktop.

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Repo layout

packages/
├── db-mcp/                  TypeScript MCP server
│   ├── src/
│   │   ├── server.ts        wiring + cache integration
│   │   ├── tools/           one file per MCP tool
│   │   ├── clients/         marudor (tRPC) + db-rest + provider router
│   │   ├── cache.ts         LRU
│   │   └── types.ts         normalized domain types
│   └── tests/               vitest + msw
└── skill-db-delay/
    └── SKILL.md             the reasoning playbook
examples/                    saved demo transcripts

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Test it

npm test

52 tests across cache, provider clients, fallback routing, tool wrappers, and server wiring. Network calls are mocked via msw; failure modes (timeout, 5xx, partial collapse) are part of the test surface, not afterthoughts.

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Status

v0.1 — usable. Connection-chain prediction works end-to-end against live marudor data.

Known limitations:

• Marudor's internal tRPC isn't a public contract; if they change it, the MCP will fall back to db-rest automatically with degraded data and the skill will tell you so.
• bahn.routing (the journey-search procedure inside marudor) occasionally returns 5xx upstream from DB itself; this is reflected as a transparent fallback to db-rest's /journeys.

Stretch goals (not in v0.1)

• Loop skill for monitoring a specific train at an interval.
• Hosted HTTP MCP with SSE transport, so it can be added by URL instead of cloned.
• Historical baseline collector — per-train on-time stats over weeks, used as a fifth prediction signal.

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Credits

• marudor / bahn.expert — the data source that makes the connection-chain feature possible. Without it this project doesn't exist.
• db-rest by Jannis R — the stable REST fallback.

License

MIT