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

Goes well beyond the annotations (readOnly=false, idempotent=false) by disclosing the 30-day non-renewing term, the 402 price-negotiation response, accepted payment headers (X-Credito, PAYMENT-SIGNATURE v2, X-PAYMENT v1), wallet-based ownership for unauthenticated buyers, and replay protections (compra mark, single-settle nonce). This is exactly the context needed to invoke a paid mutation correctly and avoid double charges.

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 description duplicates content almost verbatim: 'Each purchase lasts 30 days, with no automatic renewal' appears twice, and the 402/payment explanation is repeated with slight rephrasing in consecutive blocks. The redundancy bloats the text and pushes key facts (replay protection, receipt retention) out of a front-loaded position.

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 paid mutation with no output schema, it covers pricing discovery, payment mechanics, ownership resolution, and error recovery — the important gaps are filled. The duplicated wording makes the payment/ownership sequence harder to parse than it needs to be, but nothing essential is missing.

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% and the single enum parameter is already fully documented in the schema. The description restates the same tier-to-monitor-mapping but adds no new syntax or semantics, so 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 tiers with their monitor counts, which cleanly separates it from siblings like list_plans, pricing, and cancel_plan. An agent knows exactly what this tool accomplishes.

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 lays out the calling flow — call, receive a 402 with the price, pay via prepaid credit or x402, then repeat — which tells the agent how to actually use the tool. It does not name alternatives (e.g., use list_plans to inspect tiers or cancel_plan to reverse), so there is no explicit when-not guidance.

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