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

Despite being a paid mutation (readOnlyHint=false), the description adds safety-relevant context the annotations don't carry: 'The object is untouched; only its expiry moves' confirms non-destructiveness, and it discloses the payment-before-action behavior, auth requirements ('No account, no API key'), and the receipt location in _meta['x402/payment-response']. This is substantial disclosure beyond the structured fields.

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 action and payment flow are front-loaded and efficiently explained, and the x402 two-phase detail earns its place. However, the trailing feedback paragraph ('Complaints, questions, quote requests... every report is read and analysed daily') is roughly a quarter of the description and is irrelevant to selecting or invoking the tool.

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, two-phase tool with no output schema, the description covers the full call sequence, where the price and receipt appear, the return values ('Returns the new and previous expiry'), and the auth prerequisites. It omits failure modes (unknown id, rejected or wrong-amount payment), but everything essential to driving the flow is present.

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 both parameters are well documented in the schema itself (id as the object id returned at purchase; plan as an enum setting size and price), so the baseline of 3 applies. The description adds only marginal color — 'buying it another plan term' frames plan as a renewal term — but does not carry meaning the schema lacks.

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 states a specific verb and resource — extending the expiry of 'an object you own' by buying 'another plan term' — and pins down the exact effect: 'only its expiry moves.' This distinguishes it from storage_buy (acquiring a new object) and storage_replace (changing content), so an agent can separate it from siblings without opening their schemas.

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?

The two-call x402 sequence is explicit: call once without payment to receive the price as a PaymentRequired, then call again with the signed payment in _meta['x402/payment']. That is strong invocation guidance, but the description never names alternative tools or states when not to use it, so it stops short of a 5.

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