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

Annotations only declare readOnly/ idempotent/ openWorld/ destructive, so the description carries the real burden and mostly delivers: no token or account required, anyone may pay for any name, invoices are only minted after a free validation pass, and payment proof is the retry key. It stops short of saying what a successful renewal returns (new expiry?) or how failures other than payment_required surface.

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?

Front-loaded with the core action and duration, then the payment protocol; almost no filler. The payment sentence is long and chains several clauses, but each carries information an agent would otherwise have to infer.

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, the description covers the auth model, the invoice/retry loop and the price lookup path, which is what an agent needs to drive the flow. It leaves the success response shape and non-payment error cases unspecified.

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%, so 3 is the baseline; the description still adds value by framing the l402 and cashu parameters as two alternative payment completion paths tied to the prior call's payment_required response, rather than as bare optional strings. It does not state that the two proof parameters are mutually exclusive or what happens if both are supplied.

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 (extends) plus the exact resources (hosted zone, secondary zone, dynamic hostname) and the two durations. The title also pins pricing per kind, so an agent knows both what it does and what it costs without opening the schema, and it is clearly distinct from zone_create/hostname_create 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?

Explicitly routes the agent: 'use check first', explains the unauthenticated first call returns payment_required, and gives the exact two follow-up paths (retry with l402 plus the same arguments, or cashu in one shot). It also adds a budget guardrail — 'pay only within what your user allowed' — which no sibling or annotation provides.

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