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 one real behavioral constraint: a signed wallet is required and only 5 free trial calls are available, which tells the agent to expect an auth/paywall step. It says nothing about the response shape, rounding, error behavior, or what happens when the trial is exhausted.

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 sentence is short and front-loaded, leading with the resource name and the computed quantity. The appended emoji promo for free trial calls is extraneous but brief.

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 simple two-parameter calculator with no output schema and no annotations, the description is adequate: it names the inputs and the result. It stops short of specifying unit conventions for the rate, which is the most likely source of misinvocation in a perpetuity calculation.

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 0% and both parameters are untyped strings with no descriptions, so the description is the only source of meaning. It usefully clarifies that 'payment' is the periodic payment and 'rate' is an annual rate expressed in percent, covering both params. It still leaves the string format ambiguous (e.g. whether to pass "5" or "0.05").

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: returns the present value of a perpetuity. An agent can tell what it computes without opening the schema. However, it does not distinguish itself from close siblings such as x402-present-value or x402-annuity-pv, so the agent gets no help choosing between them.

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?

There is no when-to-use or when-not-to-use guidance, and no mention of the alternative tools (e.g. annuity-pv, present-value, growing perpetuity) that overlap with this one. The only usage signal is implicit in the formula description.

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