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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered; the description goes beyond them by disclosing the source (public FIPE API), the per-call price of $0.002 USDC via x402 on Base/Solana/Arc, and that failed calls are not charged. The caveat that reference prices are not a sale offer is useful framing. It does not discuss rate limits or data freshness (reference month is only implied).

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?

Core purpose and required codes come first, then sibling routing, then pricing. Dense and slightly run-on in places (multiple clauses per sentence), but nearly every sentence carries information an agent needs, with no filler.

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?

An output schema exists, so return values need not be explained. For a 3-parameter read-only lookup, the description covers source, prerequisites, alternatives and payment mechanics; only data freshness/update cadence and failure modes are unaddressed.

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 meaning the schema lacks: it flags that the year parameter is a composite 'year-fuel' code and tells the agent where to obtain valid brand/model codes (/cars/brands). That is genuine actionable guidance beyond the three property descriptions.

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 verb+resource: retrieving the Brazilian FIPE reference price for one vehicle, enumerated by brand, model, model year, fuel, FIPE code and reference month. This is clearly distinguishable from generic US used-car listing siblings such as travel_carsdirect_vehicle or travel_carfax.

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?

Explicitly explains the prerequisite FIPE brand/model/year codes and points to the lookup list at /cars/brands, and it names two alternative tools with the exact condition that selects each (US ZIP listings via CarsDirect, US complaints via NHTSA). The when-not-to-use case is thereby handled by contrast.

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