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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false, so safety and repeat-safety are fully covered structurally. The description adds the useful nuance that pack prices are fixed while model prices vary, but says nothing about return shape or currency/regional behavior — acceptable given annotation coverage, but not rich.

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 with zero filler. The content enumeration comes first and the sibling hand-off is last, which is the correct front-loading for a routing decision.

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?

No output schema exists, so the description carries the return-value burden — and it does name the four content categories an agent will get back. It stops short of describing structure or freshness of pricing data, which for a static info tool is a minor gap rather than a blocking one.

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 there is nothing for the description to disambiguate; baseline 4 applies. No parameter text is needed and none is misleadingly implied.

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?

The description enumerates exactly what the tool surfaces — credit packs with fixed prices, sign-up credits, payment methods, and the definition of a credit — which is effectively a specific resource statement for a no-argument info tool. It also separates itself from the sibling that covers per-model pricing, so an agent can tell the two apart without opening a schema.

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?

'For per-model prices use list_models' gives an explicit alternative and the condition that selects it, which is strong routing. What's missing is any statement of when this tool itself is the right call (e.g., before purchase, when quoting costs), but the boundary case is covered.

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