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

Tool	Tier	What it does
`convert`	free	Unit conversion (length/mass/temperature/volume/data/time/area/speed/angle/pressure/energy).
`text`	free	slugify · case-convert (camel/pascal/snake/kebab/constant/…) · `{{template}}` fill.
`hash`	free	sha256/sha512/sha384/sha1/md5/sha3-256/keccak256/ripemd160, optional HMAC, hex/base64 out.
`encode`	free	base64 / base64url / hex / url / html encode & decode.
`id`	free	uuid v4/v7, ULID, nanoid, random hex/base64url/int (count up to 1000).
`regex`	free	test/match/extract/replace with catastrophic-backtracking protection (killable worker + timeout).
`timezone`	$0.001	Instant → IANA zone with historically-correct DST; offsets, abbreviations, DST flags.
`cron`	$0.001	Cron expression → next N run times + plain-English description.
`rrule`	$0.001	iCal RRULE (RFC 5545) → next N occurrences.
`currency`	$0.001	Live currency conversion at reference/mid-market rates (keyless ECB feed, cached).
`diff`	$0.001	Text diff: unified patch + structured op-list + add/remove/hunk stats.
`jsonschema`	$0.001	Validate against JSON Schema (draft 2020-12) and explain every error in plain English.
`datemath`	$0.001	Add/subtract ISO-8601 durations, diff into chosen units, business-day add/diff.

Install / run

stdio (Claude Desktop, Cursor, any MCP client)

jsonc

// 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:

config.json

{ "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.