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

With no annotations, the description must carry the behavioral burden. It usefully discloses that this is a balance-funded purchase and that the result may be asynchronous (activation code/QR 'when ready', else poll). It omits irreversibility, cost deduction semantics, and auth/permission requirements for a money-spending mutation.

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?

Two tight sentences with the core action front-loaded and the async/polling caveat immediately after. Every clause earns its place.

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 valuably explains what comes back (activation code and QR) and the polling escape hatch. That covers the main unknowns for a 3-param purchase tool; only permission/cost behavior is left unstated.

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 days, quantity, and packageCode are already fully documented in the schema. The description adds no parameter-level detail beyond 'from the balance', 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 ('Buy') and resource ('an eSIM plan'), plus the funding source ('from the balance'), which cleanly separates it from order_number, order_virtual_number, and quote_esim among the siblings. An agent can identify the operation without opening the 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?

It gives clear context for the normal path and names an explicit fallback: poll get_esim_order when the activation code/QR is not yet ready. It does not mention prerequisites such as quote_esim or balance checks, so it stops short of full when/when-not guidance.

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