@jsonfabrica/mcp-server

A local Model Context Protocol (MCP) server that lets an AI coding agent — Claude Desktop, Cursor, or anything else that speaks MCP — generate realistic, schema-conformant JSON test data mid-session by calling JsonFabrica's REST API as MCP tools.

It runs over stdio transport only: your AI client launches it as a subprocess, so there's no network service to host and no port to open. It never talks to anything except the JsonFabrica gateway you configure.

New to JsonFabrica? It's an API-first service for generating synthetic JSON test data from reusable templates — deterministic, seed-reproducible, with referential integrity across related records. See jsonfabrica.com and the API docs.

Install / run

Add it to your MCP client's config so the client launches it via npx:

{
  "mcpServers": {
    "jsonfabrica": {
      "command": "npx",
      "args": ["-y", "@jsonfabrica/mcp-server"],
      "env": {
        "JSONFABRICA_API_KEY": "sk_live_...",
        "JSONFABRICA_API_URL": "https://api.jsonfabrica.com"
      }
    }
  }
}

• Claude Desktop — add the block above to claude_desktop_config.json (Settings → Developer → Edit Config), then restart.
• Cursor — Settings → MCP → Add new MCP server, or add the block to ~/.cursor/mcp.json.

Get an API key by signing up at jsonfabrica.com — Settings → API Keys.

Or run it directly for local testing:

npm install
npm run build
JSONFABRICA_API_KEY=sk_live_... node dist/index.js

What it looks like in a session

Once connected, an agent can do things like:

You: Generate 20 realistic customer records and 60 orders linked to them, and drop them into fixtures/seed.json.

Agent: calls jsonfabrica_create_template for the customer and order shapes, then jsonfabrica_create_batch with a relations map so each order references a generated customer id, then writes the result to the file.

No tab-switching to a dashboard, no hand-written fixtures.

Configuration

Tools

Every tool's description states, verbatim, which REST endpoint it calls. Tool names are prefixed jsonfabrica_ to avoid collisions with other MCP servers your client may have loaded.

Health / Auth

Templates

Template body strings use JsonFabrica's function-call placeholder syntax with angle brackets, e.g. <getRandomFullName()>, <getRandomEmail()>, <createSeq('orderNo')>. The full catalogue of built-in functions is documented at jsonfabrica.com/docs/functions — it's authoring reference, not something this server exposes as tools.

Sequences

Batches

Usage

Admin — function weights

Requires an API key with role=admin; the gateway returns 403 otherwise.

Explicitly out of scope

• POST /v1/signup and PATCH /v1/billing/tier — unauthenticated account-creation / billing-tier-change endpoints. Wrapping these would let a model create real paid subscriptions or change billing tiers on the user's behalf; excluded by design (generation API surface only).
• POST /v1/webhooks/stripe — Stripe-only webhook ingestion, not a developer-facing capability.

Error handling

Every tool catches errors internally and returns an MCP isError: true result with a readable message — it never throws out of the handler or crashes the host process. Common cases:

• 401 → "Invalid or missing API key — check JSONFABRICA_API_KEY."
• 403 (admin tools) → "This API key does not have admin role."
• 402 (blocked account) → the upstream blockReason is passed through.
• Network failure (gateway unreachable) → a message naming the configured JSONFABRICA_API_URL.
• Anything else → JsonFabrica API error [CODE] (HTTP status): message.

Troubleshooting

• 401 / "Invalid or missing API key" → check JSONFABRICA_API_KEY is set and valid.
• Connection refused / UPSTREAM_UNREACHABLE → check JSONFABRICA_API_URL and that the gateway is actually running and reachable from wherever this process runs.
• 403 on admin tools → your API key's tenant doesn't have role=admin.

Development

npm install
npm run build   # tsc -p tsconfig.json
npm test        # build + node --test dist/
npm start       # node dist/index.js (requires JSONFABRICA_API_KEY)

Source layout mirrors the OpenAPI spec's tags: one file per tag under src/tools/. src/client.ts is the only place that knows about fetch, the base URL, and the Authorization header.

License

MIT — see LICENSE.