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

Annotations already cover read-only, idempotent, open-world and non-destructive traits, so the bar is lower. The description still adds real value beyond them: prices are quoted 'at today's prices' (so results are time-sensitive despite idempotentHint) and the bracketed note says it works without payment and without a tier, clarifying the auth/payment model.

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?

The core sentence is front-loaded and dense with the useful facts (which APIs, which packs, where to buy). The trailing bracket block is slightly padded – 'Free' appears twice and 'without payment' duplicates 'without tier' – but the free-tier disclosure is worth its space.

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 single-optional-parameter, no-output-schema tool with full annotations, the description covers what is returned (per-API call counts) and the purchase path. It stops short of describing response ordering or whether all nine APIs are always listed, but nothing essential 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 coverage is 100% and the parameter carries its own enum description, so the schema does the heavy lifting. The description mentions the $1 and $5 packs but omits the 'sample' ($0.005) enum value, adding nothing beyond the schema.

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 a precisely scoped resource: how many calls of each named paid API a $1 or $5 pack covers, plus where to purchase via x402. An agent can distinguish it from credits_balance by the 'quote for a pack' framing, though the description never says so explicitly.

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?

Usage is only implied – the reader infers you call this to preview pack value and find the purchase path. There is no explicit when-to-use, no exclusion, and no named alternative such as credits_balance or find_paid_api.

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