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

With zero annotations the description carries the full burden, and it does disclose the output shape (consecutive percentage changes) and a billing constraint (5 free trial calls per signed wallet). It says nothing about what happens with single-element series, negative/zero prices, or what error/return occurs after 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?

Three short clauses, front-loaded with the verb and resource, then input needs, then output behaviour, then the billing note. Nothing is padded and no sentence is redundant.

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 1-parameter, annotation-free tool with no output schema, the description covers purpose, output shape, and cost, which is most of what an agent needs. It still omits the input encoding and any edge-case behaviour, which are the two things most likely to cause a failed call.

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 the single parameter is typed as a plain string with no description, so the definition must compensate. It does explain that the string represents a price series array, which is useful, but it leaves the encoding ambiguous (comma-separated vs JSON literal) and arguably conflicts with the declared 'string' type by calling it an 'array'.

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 gives a specific verb and resource: 'Calculate period-over-period returns of a price series', plus the exact output semantics ('percentage changes between consecutive items'). It does not differentiate itself from near-neighbours in the sibling list such as x402-log-returns, x402-percentage-change, or x402-percent-change-array, so an agent must infer which one to pick.

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 only implied by 'Provide values array'; there is no statement of when this tool is preferred over percentage-change or log-returns variants, nor any preconditions about series length or ordering. The only explicit guidance is the free-trial note.

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