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

Annotations already declare readOnly/idempotent/non-destructive, and the description layers on genuinely additive behavior: rates are variable and change every block, unreadable venues are excluded from 'best' rather than guessed, funds are never moved on an unconfirmed rate, and the access model ($0.01 per call over x402, prepaid x-credit-token header, 1 free call/day, no API key). That freshness, failure-handling, and billing detail is exactly what annotations cannot convey.

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-loaded with the scoping question, then data-source, freshness, parameter, and pricing facts in a sensible order; nothing critical is buried. It is on the verbose side, with a rhetorical opening and a 'Not financial advice' tail that are soft marketing rather than invocation-critical, costing it the top score.

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?

With an output schema present, return formatting need not be described, and the description still covers decision context, data provenance, freshness caveats, failure semantics, the single optional parameter, and the payment/auth path. Nothing an agent needs to call it correctly is missing.

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% and there is a single parameter, so the schema already carries the semantics. 'Pass amount= for the yield projection' only restates the schema's 'Idle USDC balance (optional, for projection)' without adding format, units, or precision guidance — the baseline 3 is appropriate.

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 a specific action (read the live USDC supply rate), the exact venue set (Base money markets: Aave v3, Moonwell), and the output shape (best venue plus annual yield on your balance). It explicitly distinguishes itself from data-feed-style siblings by stressing 'straight from each protocol — not a feed,' and from chain-specific yield tools like near_staking_yields by scoping to Base USDC.

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?

Opens with the decision it supports — where idle USDC in a pre-funded agent wallet should sit — and frames the trade-off ('weigh moving against the gas'), which tells the agent when this is the right call. However, it never names an alternative tool or states an explicit when-not-to-use condition, so it stops short of full routing guidance.

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