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

The annotations already declare readOnly/destructive=false, so safety is covered. The description adds real behavioral context beyond them: what the results contain, the empty-list outcome for non-rechargeable or carrier-changed eSIMs, and the auth prerequisite (Cotlet API key with access to this app). It doesn't cover pagination or ordering, but it is well above the annotation baseline.

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?

Front-loaded with the purpose, then the return contents, the edge case, and the follow-on tool in a logical order. Slightly redundant ('Top-up packages available...' and 'Returns the top-up packages Airalo offers for one eSIM' say largely the same thing) and the 'Airalo:' prefix repeats the title, but overall tight.

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 no output schema, the description carries the return-value burden and does so (data, validity, price), plus it discloses the auth requirement and the non-rechargeable edge case. Nothing critical is missing for calling this simple one-parameter read.

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 esim_id parameter is already documented as coming from list_esims. The description restates 'for one eSIM' but adds no format or sourcing detail beyond what the schema provides, so the 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?

The description states a specific verb and resource ('Top-up packages available for a rechargeable eSIM') and lists the fields returned (data, validity, price), so the agent can tell what it does. It names top_up_esim as the consumer of the results but does not distinguish it from sibling list/search tools like airalo_search_packages, so it stops just short of a 5.

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 the downstream workflow explicitly ('Use package_id with top_up_esim') and an important edge-case condition: the list is empty for a non-rechargeable eSIM or one whose carrier changed, in which case a new eSIM is needed. It doesn't contrast with alternatives such as search_packages, but the when-to-use context is clear.

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