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Novarithm Agent Hub: trust and commerce checks

by com.novarithm.www
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fx_usd_zar

[PAID $0.002 USDC/call, x402, eip155:8453] Official USD/ZAR reference rate: latest European Central Bank fixing (or the official peg when ZAR is pegged), with the rate date, the method (direct, cross via EUR or peg-derived) and the source attribution. Deterministic, sourced, no estimate.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "fx_usd_zar"
  }
}

Input Schema

NameRequiredDescriptionDefault

No arguments

Schema Changelog

Changes observed during successful MCP inspections.

  1. Added

TDQS

A3.8/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full burden and mostly delivers: it discloses the price ($0.002 USDC/call via x402 on eip155:8453), the data provenance (ECB fixing), the non-estimated/deterministic nature, peg handling, and the returned metadata (rate date, method, source). It stops short of explaining the payment handshake, failure/auth behavior, or rate staleness (e.g., weekend/holiday fixings).

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two dense sentences with no filler, and the essential fact (official USD/ZAR ECB fixing) is front-loaded after the payment tag. The bracketed x402 pricing/chain metadata is useful for a paid tool but is machine-oriented noise in the opening position.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a zero-parameter, no-output-schema lookup, the description covers source, method variants, peg fallback, determinism, and cost — enough to call it correctly. Remaining gaps are the exact value/units formatting and what an agent must do to satisfy the x402 payment.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The tool takes zero parameters, so there is no parameter semantics to add; the description correctly does not invent any. Baseline 4 applies for a no-arg tool.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a concrete resource precisely: the official USD/ZAR reference rate taken from the latest ECB fixing, including the peg fallback case. The specific pair is unambiguous from the name and text, but it never distinguishes itself from siblings like fx_convert, fx_latest, or fx_timeseries, which an agent could plausibly pick for the same pair.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines3/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Usage is only implied: an agent infers you call this when you need an authoritative USD/ZAR reference rate rather than an estimate. There is no explicit when-not guidance, no mention of when to prefer fx_convert (conversion) or fx_timeseries (history), and no prerequisites stated for the paid call.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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