FabTally Utility Belt · deterministic · stateless · sub-100ms
The micro-utilities your agents call a thousand times a day.
Timezone/DST, cron, iCal RRULE, unit & live currency conversion, backtracking-safe regex,
text diff, JSON-Schema validation, hashing/encoding/IDs, business-day date math — one reliable bundle.
Six are free & unlimited; seven are $0.001/call via x402 (USDC on Base) or a prepaid credit key.
Pure functions, strict validation, nothing stored. Also a standalone MCP server.
Paid utilities — $0.001 per call.
Agents pay per call in USDC on Base via x402 (no account — an unpaid call returns a
signed 402 challenge to pay-and-retry). Humans can prepay by card and send an
X-FabTally-Key: fabt_… header instead. Bad input → 400, never charged.
Every paid utility also has a rate-limited free teaser (this page uses them).
HTTP (x402) — POST any utility as JSON:
curl -XPOST https://belt.fabtally.com/v1/timezone \
-d '{"datetime":"2021-03-14T01:30:00","from_zone":"America/New_York","to_zone":"Europe/London"}'
→ 402 with a signed x402 challenge; pay in USDC on Base and retry with the X-PAYMENT header.
Prepaid credit — add X-FabTally-Key: fabt_… to charge a card-funded balance instead.
MCP server — streamable-http: https://belt.fabtally.com/mcp · also stdio (npm: fabtally-belt-mcp).
13 tools (6 free, 7 paid); paid tools relay the x402 challenge to pay-and-retry.
Discovery — /agent/overview · /openapi.json · /.well-known/x402