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

Annotations already cover readOnly, idempotent and closed-world behavior, so the description's added value is real: it discloses the data provenance (open daily dataset, mid-market, updated daily) and an explicit per-call cost of $0.002 USDC via x402 on Base mainnet, which materially affects whether an agent should call it. It does not cover rate limits, failure modes, or what happens when an unknown currency code is supplied.

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

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

The purpose and scope are front-loaded in the first clause, and the pricing note is placed last where it belongs. The long currency enumeration earns some of its space because it signals African-market coverage, but it is the one portion an editor could trim.

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

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

For a zero-required-parameter read-only lookup with no output schema, the description supplies what matters: currency coverage, data-source caveat, update cadence, and cost. The only material gap is that it does not hint at the response shape (a rate map keyed by symbol) or the default symbol set, though the schema's default note partly covers the latter.

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

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

Schema description coverage is 100%, so all three parameters (base, amount, symbols) are already documented in the input schema. The description restates 'any base currency, optional amount conversion' but adds no syntax, formatting, or default-behavior detail beyond what the schema provides; baseline 3 applies.

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

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

The description gives a specific verb and resource (exchange rates and conversion) with explicit scope: 150+ currencies, any base, optional amount. It also distinguishes itself from a sibling by stating the rates are mid-market reference rates from an open daily dataset, 'not official central-bank fixings' — which separates it from za_sarb_rates without the agent needing to open either schema.

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

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

It names concrete use cases (pricing, invoices, reports) and implicitly excludes official-fixing use cases by noting the source is not central-bank fixings. There is no explicit 'use X instead when Y' routing statement, which keeps it below a 5, but the context is clear enough to select the tool confidently.

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