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

Annotations already declare readOnlyHint=true, openWorldHint=true, idempotentHint=true and destructiveHint=false, and the description is consistent with that: it returns a buy_url rather than creating an order in chat, so nothing is contradicted. It adds genuinely useful behavioral context the annotations cannot express — supported payment methods, optional discount code, no account required, eSIM emailed after payment, and explicitly that there is no in-chat order tracking. A small gap: it does not say whether the URL expires or whether repeated calls produce the same link, which matters for an idempotent-looking handoff.

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?

Purpose and return value are front-loaded in the first sentence, and the closing constraints (no account, eSIM emailed, no in-chat tracking) each earn their place. The long enumeration of payment methods and local schemes is useful for answering customer questions but is the least essential part of the passage and makes it heavier than needed.

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, the description need not document every field, yet it still surfaces the one an agent must act on (buy_url) plus the price context. Combined with annotations and a fully documented schema, an agent has everything needed to route the customer, set expectations about account and delivery, and avoid promising order tracking.

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 the schema already explains days, bucket, currency, language and product_id in detail; the baseline of 3 applies. The description only reinforces the pricing context ("price in the customer's bucket or currency") without adding new syntax or precedence rules beyond what the schema states.

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?

"Hand the customer to the ibiPoint shop to buy one plan" states a specific verb (hand off to purchase) and resource, and the follow-on sentence names the artifact returned (buy_url plus plan name and price). It implicitly separates this from the research-oriented siblings (search_esim_plans, get_plan_details, recommend_plan) by making clear it is the terminal purchase step, but it never names an alternative explicitly, so it stops short of full sibling differentiation.

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 imperative "Hand the customer to the shop to buy one plan" plus "No account needed" and "There is no in-chat order tracking" gives clear when-to-use and when-not-to-promise context. It does not explicitly say to call recommend_plan or get_plan_details first, so the decision boundary with those siblings is left implicit.

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