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

Annotations already declare this as a safe, idempotent read. The description goes beyond them usefully by disclosing the data sources/venues (Mercado Bitcoin, BCB PTAX, Coinbase) and the per-call cost ($0.002/call), which affects invocation decisions. It stops short of describing latency or response shape.

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?

Two dense sentences with zero filler; the core comparison is front-loaded and the cost and usage boundary are appended compactly. Every clause carries distinct information.

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 no-parameter, no-output-schema tool the description conveys the sources and the comparison basis well. It never says what the returned value looks like (premium in percent, bps, or a raw spread), which is the one remaining gap an agent might want before calling.

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?

With zero parameters there is nothing to document, so the baseline is 4. The description correctly spends no space on parameter syntax and instead clarifies the comparison basis, which is the only input-like ambiguity.

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 names the exact resource and scope: USDT/USDC-BRL on Mercado Bitcoin vs BCB PTAX, and BTC-BRL vs BTC-USD at both PTAX and the on-exchange USDT rate. This is specific enough that an agent can distinguish it from any sibling FX or pricing tool without opening a 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 states the use case explicitly ("Use for Brazil's crypto-dollar/BTC premium vs the official PTAX") and adds a negative boundary ("not for other FX"), which is genuine routing guidance. It does not name a specific sibling to use instead, so it falls short of a full 5.

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