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

With no annotations, the description carries the burden, and it does disclose the key behavior: a zero-based index is returned, and null is returned when the value is not in the sequence. It also exposes a payment/rate constraint ('5 free trial calls per signed wallet'), which is genuine behavioral context. However it omits failure behavior for malformed input and any note on large-value handling.

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 semantics are front-loaded in a single dense sentence, with the pricing note trailing. Nothing is redundant, though the free-trial tag and emoji are boilerplate rather than description-specific content.

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 one-parameter lookup with no output schema, the description covers what is returned and the miss case, which is the essential contract. The remaining gap is how the value argument should be formatted, which matters somewhat given the undocumented string parameter.

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?

The single parameter has 0% schema description coverage, so the description must compensate. 'matching value' does establish that the parameter is the Lucas number to look up rather than a position, which is the crucial disambiguation versus x402-lucas-number. It does not explain that the parameter is a string or how big values / formatting should be 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?

The description names a specific operation (return the zero-based index) on a specific resource (a Lucas number matching a value), and states the null case, so it reads as the inverse of sibling tools like x402-lucas-number / x402-lucas-series. It does not explicitly name those siblings, but the verb+resource pairing is unambiguous enough to select 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?

Usage is implied rather than stated: an agent can infer it should call this when it has a Lucas value and wants its position. There is no explicit guidance about when to prefer this over x402-lucas-number or x402-fibonacci-index, and no stated input constraints, so selection still requires inference.

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