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

With no annotations, the description carries the full burden; it does disclose the auth/cost profile ('Free; no key needed') and the shape of the returned fields, which is more than most. It stops short of stating that the operation is read-only/non-mutating in explicit terms or describing pagination or result ordering.

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 short sentences, zero waste, with the resource-scoping clause front-loaded and the cost/auth qualifier trailing. Nothing needs to be moved or cut.

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 zero-parameter catalog listing with no output schema and no annotations, the description covers purpose, cost, auth, and return fields, which is sufficient to call it correctly. Minor gaps (result ordering, whether the list is exhaustive or paginated) remain, but they are low-stakes for this tool.

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?

The tool takes zero parameters, so the schema has nothing to document and the baseline is 4. The description's mention of returned fields (name, credits, price) is a light bonus rather than a substitute for parameter docs.

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 (List) and resource (credit packs) plus the fields returned (name, credits, price), which distinguishes it from the mutating sibling buy_credits and the account-oriented get_credit_balance. It does not explicitly name an alternative, but the phrase 'packs you can buy' makes the read-only catalog purpose unambiguous.

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?

'Free; no key needed' gives real context about when this call is cheap and unauthenticated to use, and the catalog framing implies it precedes buy_credits. However, no alternative is named and no explicit when-to-use/when-not guidance is given, leaving routing to inference.

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