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

Annotations already declare readOnlyHint, idempotentHint and non-destructive behavior, so the tool's safety profile is largely covered. The description adds genuinely useful context beyond that: the actual payment happens on an external secure checkout page and 'this tool never charges anything itself,' which resolves the ambiguity of a tool named 'buy'.

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?

Three tight sentences, front-loaded with what the tool returns, then when to use it, then the payment-safety caveat. No sentence is filler.

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-param, no-output-schema tool with annotations present, this covers the essentials — purpose, trigger, and the fact that charging happens elsewhere. It could go slightly further on what the returned link looks like or whether it expires, but nothing critical 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?

Zero parameters, so there is nothing for the description to disambiguate and no schema gap to compensate for. The baseline for a no-param tool applies cleanly.

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?

A specific verb and resource — returns a link to a credit pack purchase — with the key distinguishing detail that it's a one-time pack, not a subscription. An agent can immediately tell this apart from account/quote siblings like get_account or quote_job.

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?

Explicitly states trigger conditions: 'when the account is out of credits or the person asks to top up.' Clear context for invocation, though it doesn't name an alternative (e.g., get_account for checking balance) or state when not to use it.

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