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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds that the call returns both an eligibility determination and a plan list, which is genuinely useful context, but it says nothing about what happens for an ineligible or unknown ICCID.

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?

A single sentence with no filler, and the eligibility-check purpose is front-loaded ahead of the plan listing.

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 one-parameter, read-only lookup whose annotations cover the safety profile and whose schema covers the parameter, the description is nearly sufficient. The minor gap is that it doesn't say what the eligibility result looks like or what is returned when top-up is not possible.

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 coverage is 100% and the single iccid parameter is fully documented in the schema, including length constraints and where to find the value. The description only alludes to it via 'installed eSIM' and adds no format or validation detail beyond the schema, so baseline 3 applies.

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 compound verb (check eligibility + list plans) against a specific resource (top-ups for an installed Cypher eSIM), which an agent can clearly separate from list_plans or create_topup. It stops short of naming those siblings, so the differentiation is implied rather than explicit.

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?

The 'check whether ... can be topped up' framing implies this is the read step preceding create_topup, but the description never says so or names any alternative. An agent must infer when to call this versus create_topup or list_plans.

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