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

With no annotations at all, the description does real work: it discloses the return payload (pack list plus checkout_url), the payment channel (Polar, card), the asynchronous side effect (credits land automatically once paid), and that invoking it costs nothing. It does not state the auth prerequisite (implicitly 'la key actual') or what happens on repeated calls / unpaid sessions, which keeps it short of a 5.

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 what is returned, then the agent's required action, then the post-payment behavior. Every clause carries information an agent needs; nothing is padding.

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?

For a two-optional-parameter tool with no output schema, the description covers the essentials: what comes back, what to do with it, and the downstream effect. Gaps are minor — shape of the pack objects and any auth/error expectations are unstated — but an agent can call it correctly from this text.

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 the schema already explains both parameters, including the 'pack más chico que la cubre' rule for credits. The description references 'packs de créditos' conceptually but adds no syntax, precedence, or interaction rules for choosing between credits and product_id, so the baseline of 3 is right.

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 names a concrete verb and resource set: it 'Devuelve los packs de créditos disponibles y una checkout_url' for the current key, clarifying that despite the name it does not itself perform a purchase. The payment provider (Polar, tarjeta) and the object returned are both specified, so an agent knows exactly what it gets back and can distinguish it from every sibling (validation/execution utilities).

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 a clear usage instruction — 'Mostrá la URL al usuario' — and closes the loop on what happens after the human pays ('los créditos se acreditan solos'), plus 'Sin costo' signals the call is free to invoke. What it lacks is explicit when-to-use framing against any alternative flow (e.g. what to do if the user has no card or wants to check balance), and the sibling catalog offers no substitute.

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