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

Annotations declare a non-read-only, idempotent, non-destructive open-world call, and the description adds real context beyond them: payment is taken now via MB WAY/Multibanco/card, the buyer receives the voucher by email, the code is valid 12 months, and unused balance persists. It does not explain duplicate-purchase or failure behavior, which matters for a payment flow, keeping it out of the top band.

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?

Roughly 90 words in a single paragraph, with the core purpose front-loaded before the price/delivery detail. Dense but nearly every clause carries decision-relevant information; the parenthetical price list and delivery flow make it slightly run-on.

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 closes the loop by describing what the buyer gets (emailed voucher, printable/forwardable, 12-month code, balance carryover) and what must be confirmed before calling. For a nine-parameter payment tool, an agent has everything needed to invoke it correctly.

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 78%, but the description adds meaning the schema lacks: it maps prices to each package enum value (base 35€, completo 150€, equipa 250€, familia 200€, natal 159€) and states the exact permitted amounts (50/100/150/200/250), which corrects the schema's nonsensical ±9007199254740991 bounds on amount_eur. It also notes the phone requirement for MB WAY.

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?

The description opens with a specific verb and resource ('Sells a digital gift voucher for the studio') and immediately scopes it to two purchase modes: a named session package or a fixed amount. The selling verb cleanly separates it from siblings like check_gift_voucher and cancel_studio_booking, which do not transact.

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 an explicit precondition: 'Only call after the buyer confirms what to buy, their name and email, and the payment method,' which maps directly to the three required parameters. It does not, however, name the alternative sibling (check_gift_voucher) for the status-checking use case, so the exclusion is not fully closed.

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