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Novarithm Agent Hub: trust and commerce checks

by com.novarithm.www
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fx_usd_brl

[PAID $0.002 USDC/call, x402, eip155:8453] Official USD/BRL reference rate: latest European Central Bank fixing (or the official peg when BRL 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_brl"
  }
}

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 supplied, the description carries the full burden and does unusually well: it discloses the paid x402 requirement, price ($0.002 USDC), and settlement chain (eip155:8453), plus the source (ECB fixing / official peg), the derivation method, and the 'no estimate' determinism guarantee. It stops short of describing failure behavior, freshness/caching cadence, or what happens outside ECB publication days.

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?

A single dense sentence preceded by a bracketed payment notice; nothing is redundant. The cost/chain metadata is front-loaded ahead of the purpose, which delays the substantive content slightly, but it is a required disclosure for a paid endpoint rather than filler.

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?

With no output schema and no annotations, the description compensates by enumerating the response contents (rate, rate date, method, source attribution) and asserting determinism. The one material omission is the quotation direction — that the rate is BRL per USD rather than the inverse — which an agent must otherwise infer from the tool name.

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 the empty schema is self-explanatory and the baseline of 4 applies. The description correctly makes no parameter claims and instead spends its words on the returned data's provenance.

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 names a specific resource ('Official USD/BRL reference rate') and the exact data lineage (latest ECB fixing, or official peg when BRL is pegged), so an agent knows precisely which quantity it gets. It does not name sibling alternatives (fx_convert, fx_brl_usd, fx_timeseries), but the currency pair is embedded in the tool name, which resolves disambiguation adequately.

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 implied rather than stated: fetch the current USD/BRL reference rate. There is no explicit when-to-use versus fx_convert (for arbitrary conversion math), fx_brl_usd (inverse direction), or fx_timeseries (historical series), and no exclusions or preconditions beyond the payment notice.

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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