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

With no annotations, the description carries the full behavioral burden. It does usefully disclose that the return type is a BigInt string above 50 and a plain number otherwise, plus the trial-call limit. However, it says nothing about permissions, error behavior for negative/huge n, or why the output type shifts.

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?

One front-loaded sentence with the core behavior first, followed by a short quota note. The emoji marketing tail is minor noise but does not obscure the main clause.

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 pure-computation tool with no output schema, the description covers the operation and the output-type rule, which is the key thing an agent needs. It falls short on the ambiguous three-parameter input surface, which is left unexplained.

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 description coverage is 0% across three params (n, input, value). The description only explains 'n'; 'input' and 'value' remain completely undocumented in both schema and prose, leaving the agent to guess whether they are aliases or alternatives.

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: 'Computes the nth Fibonacci number from n.' That is enough to separate it from list-producing siblings like x402-fibonacci-series or x402-fibonacci, though it never explicitly names an alternative. Clear purpose, no sibling differentiation.

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 statement of when to pick this tool over x402-fibonacci, x402-fibonacci-index, or x402-fibonacci-series. The only usage-adjacent sentence is a pricing note ('5 free trial calls per signed wallet'), which says nothing about selection criteria.

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