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

With no annotations, the description carries the full burden and does well: it discloses the holiday/weekend fallback behavior, the authoritative source, and a concrete payment requirement ($0.005 USDC per call via x402), which an agent must know before calling. It omits any error or rate-limit behavior and how the value is returned, which keeps it short of a 5.

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?

A single dense sentence carries the resource, unit convention, source, and fallback rule, followed by a short cost sentence. Everything is front-loaded and nothing is wasted.

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 two-parameter, zero-required scalar lookup with no output schema, the description covers the essential context: what the rate means, where it comes from, edge-case date behavior, and the per-call cost. The only meaningful gap is the shape of the returned value, which is minor for such a simple tool.

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 coverage is 100%, so the baseline is 3, but the description adds real meaning beyond the schema by defining the quoting convention ('BRL per unit of foreign currency') and by tying the fallback rule to the date parameter's semantics. It does not add format or validation detail beyond what the schema already states.

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?

States a specific resource (official PTAX exchange rate), its exact semantics ('BRL per unit of foreign currency'), and the publishing authority (Banco Central do Brasil). This is trivially distinguishable from siblings like selic_rate or ipca_inflation, which are different indicators entirely.

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?

Usage is implied by the resource itself (fetch the rate for a given date), and the weekend/holiday fallback hints at acceptable date inputs. But there is no explicit when-to-use/when-not guidance or mention of which sibling to prefer for other Brazilian financial data.

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