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

Adds behavior beyond the annotations: no account required, the eSIM is emailed after payment, and there is no in-chat order tracking or discount field. The readOnlyHint=true is consistent here because the purchase itself occurs on an external page, not in the chat session.

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-loads the core action and keeps to three tight sentences. The payment-method parenthetical is mildly extraneous but still informative rather than filler.

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?

Covers prerequisites (no account), post-payment behavior (eSIM emailed), and limitations, which is what an agent needs for a handoff tool. Return-value description partially duplicates the existing output schema, so it is slightly redundant.

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 both parameters (product_id, language) are documented in the schema, including the language/currency effect. The description adds no parameter-level detail beyond the schema, 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?

States a specific verb and outcome — hand the customer to the e-eSIM shop to buy one plan and return a buy_url. It is plainly distinguishable from get_plan_details and search_esim_plans, which do not hand off to payment.

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?

Gives usable context (buy one plan, no account needed) and negative constraints (no in-chat order tracking, no discount-code field) that prevent misuse. It stops short of explicitly naming when to prefer this over the sibling plan-lookup tools, leaving that sequencing to inference.

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