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

The description discloses several behavioral details beyond the annotations: the installed SIM keeps working, no new QR code is issued, the tool builds a cart and returns a checkout URL, and the user completes payment on the website. Annotations indicate it's not read-only and not destructive, and the description aligns with and adds context to these hints.

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 concise, consisting of two sentences plus two brief hints. It is front-loaded with the primary action and immediately adds distinguishing context. Every sentence serves a purpose without redundancy.

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 adequately covers the return value (a checkout URL) and explains the side effects (no new QR, SIM continues working). It also clarifies the payment flow. For a tool of this complexity, the description is complete and leaves no major gaps.

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?

The input schema has 100% description coverage, so each parameter already has a clear explanation. The description echoes the schema for iccid and package_code sources and mentions the default for country. While helpful, it adds little semantic value beyond what the schema already provides, earning a baseline score of 3.

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 clearly states the specific action: recharging an eSIM with more data, building a cart, and returning a checkout URL. It explicitly distinguishes itself from the sibling tool 'create_checkout' by specifying when to use this tool instead, fulfilling the specific verb+resource+scope criteria.

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 description provides clear guidance on when to use this tool ('Use this instead of create_checkout when the traveller already has one of our eSIMs'), states what it does not do ('Does NOT charge'), and tells the agent where to obtain the required parameters ('Get iccid from my_esims and package_code from list_esim_topups').

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