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

Annotations mark this as a non-readonly, non-idempotent, non-destructive mutation, and the description adds real value beyond them: stablecoin/x402 payment, the secret requirement, and the 72-hour forward-payment cap. The additive nature of 'extend by one, four or twenty-four hours' also aligns usefully with idempotentHint=false. It stops short of describing payment failure or whether the extension is applied before or after payment settles.

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 the action and scope, then cost model, then the hard limit. No filler.

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?

No output schema exists, so the description carries return-value burden, and it is largely adequate: it covers ownership, credentials, payment, and the 72-hour cap. It omits what a successful call yields (e.g. new expiry) and what happens on payment failure, which would fully close the gap.

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 each parameter already carries a description, so the schema does the heavy lifting. The description restates that the id and secret are needed but adds no format or constraint detail beyond what the schema and enum already provide; 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 and resource ('Extend an inbox you own by one, four or twenty-four hours') plus the key preservation guarantee ('keeping its address and its mail'). This distinguishes it cleanly from create_inbox and the other inbox siblings.

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?

Clear context: you must own the inbox, and you need its id and secret; the 72-hour ceiling tells the agent when the call will be rejected. No explicit 'when not to use' or named alternative beyond the passing reference to create_inbox for the payment model.

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