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

Annotations already cover safety (readOnly, idempotent, non-destructive), yet the description adds material behavior: nothing is charged or reserved, no account is required, accepted payment methods, email capture at payment, and eSIM delivery to that email. These are the facts an agent needs before handing a purchaser a link.

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?

Four tightly packed sentences, front-loaded with what the tool returns, followed by buyer-facing behavior and then the alternative-tool routing. No 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?

With an output schema present, return values need not be described; annotations cover the safety profile and the schema covers all three parameters. The description finishes the job by covering buyer experience and the sibling boundary, leaving nothing an agent needs to call this correctly.

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?

Schema description coverage is 100%, so plan_id, source and currency are already documented; the baseline is 3. The description adds only marginal parameter context ('exactly this plan' reinforces plan_id scope, and the note that the page prices in the buyer's own currency complements the currency parameter) but no format or syntax detail beyond the schema.

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 ('get the page where a person buys one specific eSIM plan') and explicitly scopes it to one plan and one buyer. The sibling create-checkout-session is named as the contrasting tool, so an agent can disambiguate without opening either 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?

Gives an explicit when-to-use ('whenever a PERSON is going to pay') and when-to-use-the-alternative ('when the agent is settling the purchase itself, or for more than one plan or unit'). The routing condition is unambiguous.

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