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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds real value beyond that by disclosing what content the lookup covers (rates, fees, caps, payout behavior), which matters since there is no output schema. Minor gap: 'and more' leaves the card set partially open, and there is no statement about data freshness or currency of rates.

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?

Two sentences, front-loaded with the resource and its returned fields, followed by the usage rule. The long parenthetical card list is informative but slightly bloats the first sentence; otherwise there is little waste.

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 no output schema, the description must convey the shape of the result, and it does so reasonably by naming the rate categories, fees, caps, and payout mechanics. For a single-parameter read-only lookup this is close to complete; only data currency and full card coverage remain unstated.

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% and the single parameter already documents itself, including the 'empty array = all cards' semantics. The description only implicitly reinforces this by saying 'one of these cards'; it adds no slug format or filtering detail beyond the schema. Baseline 3 is appropriate when the schema does the work.

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 resource (earn rates for crypto-cashback and flat-cashback cards) and enumerates the exact data points returned: airfare/hotel/travel-portal/other rates, fees, caps, and payout mechanics. It also lists concrete card examples (Coinbase One Card, Citi Double Cash, etc.), so an agent can tell this is a card-economics lookup rather than an award chart or transfer lookup.

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?

It states a clear trigger: 'Use when the traveler pays cash with one of these cards, to value the cash option.' It also names the downstream consumer ('pass cash_back_pct to points_value'), which helps sequence the call correctly. It stops short of a 5 because there is no explicit when-not condition or named alternative for the award/points path.

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