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

With no annotations provided, the description carries the full burden of behavioral disclosure. It states the value is a 'constant,' implying determinism and no side effects, but critically omits the units (kilometers? meters? miles?) and return format — an essential detail for a physical constant whose value differs by 1000x depending on unit convention.

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?

The description is a compact one-liner appropriate for a zero-parameter constant-retrieval tool. The 'Earth Radius:' prefix is slightly redundant with the tool name, but the whole string contains no wasteful filler.

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 such a simple tool, the description is nearly sufficient: no parameters, no annotations needed for safety, no output schema expected. However, the missing unit specification is a real completeness gap — an agent cannot correctly consume the returned value without knowing whether it is 6,371 km or 6,371,000 m.

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 has zero parameters, so the baseline of 4 applies and there is nothing for the description to document. The empty schema and 100% coverage leave no ambiguity about invocation requirements.

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 names a specific resource (the Earth radius constant) and clarifies it is a fixed value rather than a calculation. It is mildly tautological with the tool name, but the word 'constant' does distinguish it from geometry tools that compute radius from inputs.

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?

No guidance is given on when to use this tool versus alternatives, and among the hundreds of siblings (x402-earth-gravity, x402-earth-mass, x402-sphere-radius-from-volume) the intended selection context is left entirely to inference. The usage is only implied by the name itself.

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