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

No annotations are supplied, so the description carries the full burden. It does disclose a billing behavior ('5 free trial calls per signed wallet'), which is useful non-obvious context, but it omits the units and precision of the returned constant and gives no indication of the response shape, which matters for a zero-parameter constant lookup.

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?

Very short and front-loaded, with the resource stated first and the billing note second. The only waste is that the leading clause duplicates the tool name instead of adding information.

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 simple zero-argument constant lookup with no output schema, the description is close to adequate, but a physical constant tool's most important omission is the unit of the returned value (kg, amu, MeV/c²), which an agent cannot infer from any structured field here.

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 nothing for the description to disambiguate; the baseline for a parameterless tool applies. No parameter-related information is missing.

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 physical resource (neutron mass) but 'Mass of neutron' essentially restates the tool name rather than adding scope. It does not distinguish this from the many physics-constant siblings such as x402-proton-mass, x402-electron-mass, or x402-earth-mass, nor does it state what value/unit is produced.

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 mention of alternatives (e.g., use x402-proton-mass for proton mass), and no indication that the tool takes no arguments. The agent must infer usage entirely from the name.

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