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

No annotations are supplied, so the description carries the full load. It does disclose a meaningful behavioral trait: a 5-call free trial gated on a signed wallet, telling the agent that payment/auth context exists. It says nothing about output format, precision, rounding, or what happens on invalid input.

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?

Two compact clauses with the conversion stated first and the billing note second. Nothing is padded, though the emoji and trial-call sentence are marketing rather than functional 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 trivial single-value conversion, the direction is the essential information and it is present. However, with no annotations, no output schema, and an undocumented string parameter, the definition should at minimum state the expected input form and that the return is a numeric result.

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 is named "value" with type string and 0% schema description coverage, so the schema tells the agent nothing. The description does not compensate at all: it does not say the value must be numeric, whether it is a string or number, accepted units, or ranges.

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?

"Convert mps to mph" states a specific verb and a specific conversion pair, so an agent can tell it apart from the reverse sibling x402-mph-to-mps. The acronyms are never expanded (meters per second vs miles per second is left to inference), which keeps it just short of a 5.

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 guidance on when to use this versus x402-mph-to-mps, x402-mps-to-kph, x402-speed-convert, or x402-units-convert. The only extra sentence concerns billing eligibility, not usage conditions or prerequisites.

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