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 two real traits: the index is zero-based and non-Fibonacci inputs yield null rather than an error. The billing constraint ('5 free trial calls per signed wallet') is also useful non-annotation context. However, it says nothing about input format, size limits, or negative-number handling, so the mutation/edge-case picture is incomplete.

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?

A single front-loaded sentence, wasted only on restating the tool name as a 'Fibonacci Index:' prefix and on promotional trial-call boilerplate. The key behavior (null for non-Fibonacci) lands early and there is no padding beyond that.

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 read-only math tool with no output schema, the essential contract is mostly covered: what goes in, what comes back, and the null edge case. What is missing is the string-encoding expectation and confirmation of the return type, which an agent needs to call it correctly on large inputs.

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?

One parameter with 0% schema description coverage, so the description must compensate and largely does not. 'Given in value' only restates the parameter name; it never explains why the Fibonacci number is typed as a string (presumably for arbitrary precision) or what notation to use. The schema also marks the parameter non-required, which is unexplained.

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+resource combination: it computes the zero-based index of a Fibonacci number from a supplied value. This is clearly distinct from sibling tools like x402-is-fibonacci (membership test), x402-fibonacci-n (nth term), and x402-fibonacci-series. It does not name those siblings, so it falls short of the 5 bar.

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: you have a value and want its position in the sequence. The 'returning null when it is not one' clause hints at graceful handling but is not framed as when-to-use guidance. No alternatives are named (e.g. use is-fibonacci to merely test membership), so an agent must infer the routing.

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