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

With no annotations supplied, the description carries the full burden. It does disclose a real behavioral trait the schema cannot express — the metered x402 payment model with 5 free trial calls per signed wallet — which is valuable context for an agent. However, it says nothing about the return value's unit (km, miles, meters) or precision, which is the key behavioral unknown for a constant-lookup tool.

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 short fragments, front-loaded with the subject before the pricing note; nothing is padded or buried. The emoji and trial-call marketing add length without technical value, but the total is small.

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 trivially simple zero-param constant lookup with no output schema, the essential missing piece is what the returned number looks like (unit and approximate magnitude). The payment terms are covered, but the one fact an agent needs to actually use the result is not.

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 tool takes zero parameters, so there is no argument semantics to explain; the baseline for a no-param tool applies. Nothing in the description misleads about inputs.

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?

"Earth Radius: Earth radius constant" restates the tool name (x402-earth-radius) plus the word "constant"; it names no verb and no distinct scope. It does imply the tool returns the Earth radius rather than computing with it, but that is inferred from the name itself, not added by the description.

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 use this instead of near-identical siblings such as x402-earth-mass, x402-earth-gravity, or x402-astronomical-unit. The only extra sentence concerns payment, not selection guidance or prerequisites.

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