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

Annotations already declare readOnly/idempotent/non-destructive, and the description goes well beyond them: rates are variable and change every block, an unread venue is excluded from 'best' rather than acted on, no funds are ever moved on an unconfirmed rate, and it discloses full payment mechanics ($0.01/call over x402, prepaid header, 1 free call/day, no wallet/API key). These are exactly the operational traits an agent needs before calling.

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?

It is front-loaded with the use-case question and then moves through data sourcing, failure behavior, and pricing in a mostly necessary order. It is on the long side for a one-parameter tool and carries some rhetorical padding ('Not financial advice.'), but each block conveys actionable information rather than 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 no explanation, and the annotations cover the safety profile. The description still supplies everything else relevant: data freshness, exclusion-on-failure semantics, payment/access model, and what the optional parameter produces.

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?

Only one optional parameter, and schema coverage is 100% — the schema already documents 'amount' as the optional USDC balance for projection. 'Pass amount= for the yield projection' restates that rather than adding units, format, or behavior, so the 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?

States a precise verb+resource: it reads live USDC supply rates across Base money markets (Aave v3, Moonwell) directly from each protocol and returns the best with an annual yield projection. The scope ('idle USDC on Base') and the 'not a feed' distinction make it unmistakable against generic rate or price tools in the sibling list. An agent can identify this without opening the 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?

The opening question ('Where should an agent's idle USDC sit on Base right now?') plus 'idle USDC that earns nothing' clearly frames when to invoke it, and it explains that amount= produces a projection. However, it names no alternative tool (e.g. near_staking_yields, morpho_vault, exit_liquidity) and gives no conditions under which not to use it, so the when-not half is missing.

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