fiat-usdc-rate

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 cost ($0.020 per call), the payment mechanism (x402/USDC), that it is read-only ('spot rate only, no execution'), and the shape of the response. It omits auth/rate-limit details, which keeps it just 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?

Front-loads purpose, then usage, exclusion, and price in tight sentences with no filler. Slightly dense with four distinct clauses, but each clause carries decision-relevant 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?

No output schema exists, but the description enumerates the returned fields (spot, spread, fees, verdict), so an agent knows what comes back. Combined with the rail enumeration and cost, the definition is complete enough to call correctly, with only minor gaps around error/empty-currency behavior.

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 baseline is 3. The description confirms the query form ('GET ?currency=EUR') and adds a concrete currency example, but the schema's other properties (best_rail, coinbase_spot, rail_fees_usd) look like response fields rather than inputs and are not clarified beyond the schema text.

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 ('best fiat↔USDC conversion rate per payment rail') and enumerates the return content (Coinbase spot, spread, network fees, best-rail verdict). It explicitly distinguishes itself from the sibling crypto-price tool, so an agent can route correctly 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?

Gives explicit when-to-use ('when settling x402 payments in a chosen rail'), an alternative ('For the price of a crypto asset use crypto-price instead'), and a when-not ('Not a currency-exchange service — spot rate only, no execution'). All routing conditions are stated, not inferred.

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