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

Annotations only declare non-readonly, non-idempotent, non-destructive; the description layers on material behavior the agent could not infer: the 402-with-price negotiation, the 30-day non-renewing term, the `compra` mark that prevents duplicate credit debits, x402 nonce single-settlement, and the instruction to keep the receipt after a delivery error. That is exactly the kind of charge/retry semantics an agent needs before spending money.

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?

The core (which tiers, what they cost in pages) is front-loaded, but the text repeats itself almost verbatim: "Each purchase lasts 30 days, with no automatic renewal" appears twice and the 402-with-price statement appears twice. That duplication bloats a one-parameter tool and dilutes the genuinely useful payment details.

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 single-enum payment tool with no output schema, the description covers price negotiation, payment channels, ownership/claim flow and retry safety, which is nearly everything needed. The gap is that it never says what a successful purchase returns (receipt shape, new page quota), leaving the agent to discover the success payload at runtime.

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 enum already documents starter/pro/agency with page counts, so the schema carries the burden. The description merely restates the same tier-to-page mapping verbatim rather than adding constraints or defaults, 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+resource ("Buy a plan") and enumerates the three purchasable tiers, so an agent can separate it from list_plans, cancel_plan, pricing and billing without opening a schema. It never names those siblings explicitly, which is the only thing keeping it from a 5.

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?

Gives concrete usage context: pay via prepaid credit or x402, unauthenticated purchases are owned by the verified paying wallet, and the plan can be claimed after account sign-in. It does not state when the tool should not be used or route the agent to list_plans/cancel_plan as alternatives, so no exclusions are present.

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