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

Annotations mark this non-readOnly and non-destructive; the description adds real context beyond them — the cart/checkout-URL flow, that no charge occurs and payment happens externally, that no API key is needed, and that signed-in orders are auto-linked. It does not state auth requirements or URL expiry, which keeps it from a 5.

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 key behavior (returns a URL, does not charge) is front-loaded and most sentences carry distinct information. It runs slightly long with overlapping reassurances ('no crypto wallet and no API key needed') and the signed-in note, but nothing is truly wasteful.

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 compensates by describing the return value (checkout URL), the payment flow, guest-vs-signed-in behavior, and the unlimited-plan parameter requirement. An agent has enough to invoke it correctly end to end.

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%, so the schema already documents all five parameters including period_num's unlimited-plan rule. The description echoes the period_num requirement but adds no syntax or format detail beyond the structured fields, 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 verb and resource ('builds a cart and returns a checkout URL'), and its scope is distinguishable from siblings like search_esim_plans and get_order_status. It explicitly names the plan-search sibling as the source of variant_id, so an agent can place it in the workflow without opening another 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?

'Use once a plan is picked' gives clear triggering context and the description routes the agent from search_esim_plans. It also states the boundary 'Does NOT charge', but does not spell out when an alternative (e.g. topup_esim for existing eSIMs) should be preferred instead.

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