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

No annotations are supplied, so the description carries the full disclosure burden. The only behavioral detail beyond the name is the pricing note ('5 free trial calls per signed wallet'), which is useful but says nothing about determinism, input format, return shape, or whether the call is a pure computation.

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?

It is short and front-loaded, but the first clause is pure tautology and the space saved is not spent on anything informative. Brevity here comes at the cost of specification rather than through efficient editing.

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?

With four undocumented parameters, no output schema, and no annotations, this definition leaves every substantive question unanswered. For a computation tool whose entire value is the input contract, the description is effectively empty.

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 four string parameters, and the schema is genuinely ambiguous: it offers both 'a'/'p' and 'actual'/'predicted' as aliases. The description resolves none of this — no array syntax, delimiter, or ordering convention is given, leaving an agent unable to construct a valid call.

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 restates the tool name almost verbatim: "Mean Absolute Error: Mean of absolute error." It never says what is being computed from what inputs, nor how this differs from the many sibling metric tools (x402-mean-squared-error, x402-root-mean-squared-error, x402-mean-abs-deviation).

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 guidance, no prerequisites, and no mention of alternatives. An agent choosing among correlated error and deviation metrics gets no signal about which one fits a given task.

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