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

Annotations already declare readOnlyHint=true, non-destructive, and closed-world, so the safety profile is covered. The description adds real behavioral value beyond that by disclosing the exact shape of the result set (which fields come back), which matters since no output schema exists.

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 tight sentences, no filler. The field list is front-loaded and the second sentence delivers the actionable linkage to buy_credits. Every clause earns its place.

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 no-parameter, read-only list tool with no output schema, the description supplies everything an agent needs: what is returned, in what shape, and what to do with the result. No meaningful gap remains.

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 are no semantics to convey and the schema is trivially complete. Baseline for a 0-param tool is 4; nothing in the description could add further parameter meaning.

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?

Specific verb+resource ('List the credit packages available for purchase') followed by an explicit enumeration of the returned fields (id, name, credit amount, price in USD, price per 1k credits). An agent can distinguish this from siblings like get_credits or buy_credits without opening any 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?

States the concrete downstream step — 'Use the returned package id with buy_credits' — which tells the agent exactly when this tool precedes another. It gives clear usage context but no explicit exclusions or when-not-to-use conditions, so it falls short of a 5.

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