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

Annotations already declare readOnly, idempotent, and closed-world behavior, so the safety profile is covered. The description adds useful context — prices are 'today's', so results are time-sensitive, and purchase happens via x402 — but it does not describe the shape or granularity of the returned data. Moderate added value over annotations.

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?

Single front-loaded sentence with no filler, and the core action and output ('how many calls ... covers') comes first. The nine-item API list is long but genuinely informative; slightly heavier than necessary for a one-parameter tool.

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?

For a trivial read-only tool with one optional enum parameter and no output schema, the description conveys the essential semantics: it aggregates per-API call counts per pack at current prices. Only the return granularity (per-API breakdown vs. table format) is left unspecified, which is a minor gap.

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 enum description already explains "1", "5" (10% bonus) and "sample". The description mentions only the $1 and $5 packs and omits the 'sample' option, so it neither compensates nor adds meaning beyond the schema; 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 specific verb ('lists') and clear resource: how many calls each paid API a $1/$5 credits pack covers, plus where to buy it. The enumerated API names and the pricing framing make it unmistakably distinct from the sibling credits_balance, which is about an account's current balance rather than pack pricing.

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 pricing-vs-balance distinction against credits_balance is implied by the content but never stated explicitly, and there is no guidance on when a caller should use this versus checking a balance. Usage is inferable but left to the agent.

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