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

Annotations already declare the tool as mutating, destructive, and idempotent. The description adds useful behavior beyond that: the POST endpoint, the SQL new-eSIM path triggered by omitting iccid, the ownership precondition for a set iccid, and the 'Never Stars' prohibition. The cryptic 'Never Stars' is a minor weakness but does not contradict the annotations.

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?

The description is only a few sentences, front-loaded with the endpoint and core action, and packs the key usage constraints into compact language. Every sentence contributes actionable information without padding.

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 mutating purchase tool with no output schema and no parameter descriptions, more context would help: how to obtain plan_id, what payment_source accepts, and what the response contains. That said, the critical iccid special-case and ownership precondition are covered, and annotations handle the mutation/idempotence/destruction profile.

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 0%, so the description must compensate. It meaningfully explains iccid's optional new-eSIM behavior and the ownership requirement, and it notes ext_id is required. However, plan_id and payment_source are left undefined, and 'Never Stars' is too vague to serve as clear payment_source guidance.

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 states a specific verb ('Buy') and a specific resource ('travel/data eSIM plan'), and even names the exact endpoint. It is clearly distinguishable from siblings like list_esim_plans and purchase_gift.

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?

It gives actionable usage guidance: omit blank/null iccid for the new-eSIM path, only set iccid if it already belongs to the agent, and ext_id is required. It does not explicitly name alternatives, but the context is clear enough from the sibling list and the 'Buy...eSIM plan' framing.

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