FabTally Utility Belt — MCP server

The deterministic micro-utilities AI agents call constantly, as an MCP server (stdio + streamable-HTTP) and a pay-per-call HTTP API. One reliable bundle instead of thirteen flaky npm deps.

• Free tools return real results with no wallet: convert, text, hash, encode, id, regex.
• Paid tools ($0.001/call, x402 · USDC on Base): timezone, cron, rrule, currency, diff, jsonschema, datemath. With no payment a paid tool returns the decoded x402 402 challenge (price / network / asset / payTo) so an agent can pay and retry with x_payment. Humans can instead prepay by card and use an X-FabTally-Key credit key against the HTTP API.

This server holds no private key. It fronts https://belt.fabtally.com. Every utility is a pure, deterministic function with strict input validation; bad input → 400, never charged; nothing is stored.

Tools

Install / run

stdio (Claude Desktop, Cursor, any MCP client)

// claude_desktop_config.json
{
  "mcpServers": {
    "fabtally-belt": { "command": "npx", "args": ["-y", "fabtally-belt-mcp"] }
  }
}

Or clone and run: node index.js (speaks MCP over stdio).

streamable-HTTP (remote)

Hosted: https://belt.fabtally.com/mcp. Or self-host: MCP_HTTP_PORT=8108 node http.js.

Config

• FABTALLY_BELT_BASE — API base (default https://belt.fabtally.com).

Paying for paid tools

A paid tool with no x_payment returns:

{ "payment_required": true, "http_status": 402,
  "challenge": { "x402Version": 2, "accepts": [ { "scheme": "exact", "network": "eip155:8453",
    "amount": "1000", "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", "payTo": "0x…" } ] } }

Build an x402 payment authorization (EIP-3009 transferWithAuthorization to payTo) for the accepts entry and call the tool again with the token in x_payment. amount is atomic USDC (1000 = $0.001).

Guarantees

Deterministic. Stateless — no PII or request data stored. Receive-only wallet (no custody). Currency figures are reference/mid-market rates for informational use, not a dealable quote. MIT licensed.