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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered. The description adds genuinely useful context beyond the annotations: the operation is free, the auth requirement (connected account or dv_sk_ key), the pricing convention (1 credit = $1.00), and the underlying endpoint.

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 compact sentences with the core purpose front-loaded; the parenthetical endpoint/auth note is dense but earns its place. No filler or redundancy.

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 read-only list tool with no output schema, the description covers purpose, cost, auth prerequisites, and endpoint, which is sufficient for an agent to call it correctly. Only the relationship to credits_checkout is unaddressed.

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 and the schema is empty, so the baseline of 4 applies. No parameter detail is needed and none is missing.

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 and resource: lists the prepaid credit packs the current key can buy, with a clear scoping clause ('this key can buy'). It does not explicitly contrast itself with the sibling credits_checkout, so differentiation is implied rather than stated, keeping it just under a 5.

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 implied (browse available packs, likely before checkout) and the auth prerequisite is noted, but there is no explicit when-to-use or when-not-to-use guidance and no reference to credits_checkout as the alternative. Adequate but with a clear gap in routing.

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