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 fully covered by structured data. The description adds real value beyond that with 'Costs nothing' (no billing impact) and the fact that it returns the full catalogue rather than a page, but it is silent on ordering and pagination, so it is an increment rather than a deep disclosure.

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?

One dense, front-loaded sentence with no filler; the scope statement comes first and the most agent-relevant detail (the id that buy_credits consumes) is emphasised inline rather than buried.

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?

There is no output schema, so the description must carry the return shape, and it does: it names every field an agent needs, including the awkward but important detail that prices are in minor units ex VAT. Nothing needed to call this tool or interpret its result 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 the single enum parameter is documented in detail in the schema itself, so the baseline is 3. The description only repeats 'Optionally filter by kind' and adds no semantics the schema does not already carry.

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?

Names a specific verb and resource (list prepaid packs) and enumerates exactly what each record contains: id, name, what it buys, price in minor units ex VAT, currency, and credit duration. It also ties the returned id to the sibling buy_credits, making the tool distinguishable from every other sibling at a glance.

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 parenthetical 'the one buy_credits takes' implies the intended workflow (look up pack ids before purchasing), and 'Optionally filter by kind' plus 'Costs nothing' tell the agent it is a free, filterable read. It stops short of an explicit when-not statement or a named alternative for the purchase step.

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