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

Annotations already declare readOnlyHint/openWorldHint, and the description adds genuinely useful behavior: the returned buy_url is where the customer taps Buy now, payment supports card/Apple Pay/Google Pay plus an optional discount code, no account is required, and the eSIM is emailed after payment. It does not restate the annotations, and the 'no in-chat order tracking' caveat sets accurate expectations about what the tool does not do.

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?

Three tight sentences, front-loaded with the handoff action, then the return value, then the post-payment behavior. No filler and every clause carries information an agent needs.

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 an output schema present, return values need not be described, yet it usefully summarizes them; the pre-payment and post-payment expectations are covered. The only real gap is that it never says where product_id comes from, which matters for a single-required-param handoff tool.

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 description coverage is 100%, so both parameters are already documented, including that language drives the payment page language/currency. The description adds only 'buy one plan' context for product_id and says nothing about the language parameter, 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?

Specific verb and resource: it hands the customer off to the shop to buy one plan and returns a buy_url, a plan name, and a price. The purchase/handoff framing is clearly distinct from get_plan_details and search_esim_plans, though the siblings are never named explicitly.

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?

Usage is implied by 'to buy one plan' and the note that there is no in-chat order tracking, which tells the agent this ends the in-chat flow. However, it never states when this should be called versus the sibling tools, nor that product_id must come from a prior search/details call.

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