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

Annotations already cover read-only, open-world, and non-idempotent hints, but the description adds meaningful behavior: the x402 payment gate, that failure returns isError with a PaymentRequired object and a fresh invoiceId (consistent with idempotentHint=false), and that unpaid calls are free. It omits any description of the successful return value despite there being no output schema.

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?

Two sentences, but the first is a dense pricing/network string that pushes the actual function to the middle. The payment retry detail earns its place; the repeated per-chain pricing (also in the title) does not need to lead.

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 small 4-parameter tool with no output schema, the payment flow is well covered, but the core contract is not: undefined 'op' values and no explanation of what a successful parse result looks like. An agent can pay correctly yet still guess at how to invoke the actual duration logic.

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 25%, so the description must compensate. It explains payment_signature and the _meta["x402/payment"] alternative well, but says nothing about the 'op' parameter (no enum, unclear legal values), 'start', or 'duration' formats, which are exactly the inputs an agent must construct correctly.

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 concrete verb+resource: "Parse an ISO 8601 duration or add it to a start instant." That distinguishes it from siblings like cron_next, timezone_dst, or unit_convert. It is clear, but the purpose is buried mid-sentence behind pricing boilerplate rather than stated first.

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 gives a useful payment retry protocol (first call unpaid returns PaymentRequired with a fresh invoiceId, retry with signed payload) and frames the scope as "solves one question." But it never says when to choose this over timezone_dst/cron_next/get_business_day, nor how to choose between the parse and add modes.

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