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

With no annotations, the description carries the full burden, and it does disclose real behavioral facts: the per-month price, the 1–12 range, and the two payment rails (balance or x402). It does not say whether the purchase stacks onto an existing subscription, what happens on insufficient balance, or whether the call is idempotent — material gaps 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?

A single sentence that front-loads the action, range, and price with zero filler. Every clause carries information an agent needs before calling.

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 payment mutation with no annotations and no output schema, the definition covers cost and scope but omits success/failure behavior and any prerequisite state (registered mailbox, sufficient balance). Adequate to invoke, thin for predicting the outcome.

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 only 50% and the 'months' parameter has no schema description beyond min/max, so the description's '1–12 mailbox months (15 € each)' is what makes that parameter meaningful — it supplies unit and cost. The 'api_key' fallback is already fully explained in the schema, so no further detail is needed there.

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 specific verb and resource — prepaying mailbox months — with concrete bounds (1–12) and unit price (15 € each). It is clearly a spend/prepay action, distinguishable from read-oriented siblings like get_subscription or get_balance, though it never explicitly contrasts itself with pay_item or top_up.

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?

Usage is only implied: an agent can infer this is the tool for extending mailbox time. There is no explicit 'use this instead of top_up/pay_item when...' guidance, and no stated preconditions such as needing a registered mailbox or a funded balance.

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