vocabit-mcp

An MCP server for Vocabit, a flashcard app. It lets an AI assistant write a study set into a real app on a real phone, and then read back how the learner actually did with it.

Most MCP servers read from an API. This one closes a loop:

flowchart LR
    A["Assistant<br/>teaches a topic"] --> B["create_study_set"]
    B --> C["Set appears in the<br/>Vocabit app"]
    C --> D["Learner works<br/>through it"]
    D --> E["get_set_results"]
    E -->|weak cards| A

The interesting tool is not create_study_set — anything can generate flashcards. It is get_set_results: which cards the learner marked hard, which they never reached, how many reviews each one took. The next set is built out of that, not out of a guess.

Try it in 30 seconds

No backend, no account, no API key:

npx -y vocabit-mcp --demo

Demo mode runs the same server against an in-memory Vocabit with two seeded sets. Create a set, ask for results, and a deterministic stand-in learner will have worked through it — flagged in the response as simulated, so it is never mistaken for real data.

To poke at it with a UI:

npx @modelcontextprotocol/inspector npx -y vocabit-mcp --demo

Install

Listed in the MCP Registry as io.github.JohnBilousov/vocabit-mcp, so clients that read the registry can find it on their own.

claude mcp add vocabit -- npx -y vocabit-mcp

{
  "mcpServers": {
    "vocabit": {
      "command": "npx",
      "args": ["-y", "vocabit-mcp"],
      "env": {
        "VOCABIT_BASE_URL": "https://your-vocabit-backend.example.com",
        "VOCABIT_AGENT_KEY": "your-agent-key"
      }
    }
  }
}

Drop the env block to run in demo mode.

Tools

Also exposed: the vocabit://set/{setId} resource (a set as JSON, listable) and a study-session prompt that walks the whole loop.

Card states

Progress comes from the app's spaced-repetition engine, not from the assistant:

A set reports completed: true once no card is left in new.

Live mode

Point the server at a Vocabit backend that has the agent API enabled:

export VOCABIT_BASE_URL=https://your-vocabit-backend.example.com
export VOCABIT_AGENT_KEY=...   # must match one of AGENT_API_KEYS on the backend
npx -y vocabit-mcp

Set neither URL nor key and the server starts in demo mode. Set exactly one and it refuses to start — half a configuration is a mistake, not a hint.

Design notes

Demo mode is a first-class client, not a stub. HttpVocabitClient and DemoVocabitClient implement the same VocabitClient interface, so no tool has a branch for "are we pretending?". A reviewer can run the server before they have credentials, and the test suite exercises the real tool surface over a real MCP transport rather than mocking the SDK.

Errors are recoverable, not fatal. A failed call comes back as isError with the backend's own message plus a hint aimed at the model — 404 says "call list_study_sets to see which sets exist", 401 says "or run with VOCABIT_DEMO=1". Mutually exclusive arguments are rejected with an explanation instead of a guess.

Output schemas stay loose on the edges. Identifying fields are required; everything else is optional, so a backend that grows a field does not turn a working tool into a validation error.

Annotations are honest. delete_study_set is marked destructiveHint, the read tools readOnlyHint. notify_learner messages a real person, and its description says to use it sparingly.

Development

git clone https://github.com/JohnBilousov/vocabit-mcp && cd vocabit-mcp
npm install
npm run build
npm test          # tool surface + full loop over an in-memory MCP transport
npm run inspect   # demo mode in the MCP Inspector

src/
  index.ts        CLI entry, stdio transport
  config.ts       env → Config, demo-mode resolution
  server.ts       tool / resource / prompt registration
  schemas.ts      zod input and output shapes
  format.ts       human-readable summaries next to structuredContent
  client/
    types.ts      wire types + VocabitClient contract
    http.ts       live backend
    mock.ts       in-memory backend for demo mode

Roadmap

• Streamable HTTP transport alongside stdio

• Multi-learner support without a backend default UID

• Audio pronunciation cards

• Publish to the MCP registry

License

MIT © Ivan Bilousov