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 the payment/auth model (five free calls per signed wallet), which is genuinely useful, but it says nothing about units (kg, u, eV/c²), precision, or the shape of the returned value — the details that matter most for a physics constant.

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, front-loaded sentences with no filler. The emoji is decorative but the free-trial clause carries real information, so the brevity is not costing the agent much.

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 zero-parameter constant tool with no output schema, the description still leaves the return units and precision unspecified, which is the key ambiguity an agent faces. It is adequate given the tool's triviality but not complete.

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 schema has zero parameters, so there is nothing for the description to disambiguate; baseline 4 applies. The description correctly implies a no-argument constant lookup.

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 identifies the resource ('Mass of proton') but merely restates the tool name's title, giving no verb like 'returns' and no distinction from close siblings such as x402-electron-mass, x402-neutron-mass, or x402-earth-mass. An agent can infer the topic, but the phrasing is tautological rather than a specific action statement.

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 indication of when to use this constant versus the other mass tools in the catalog, no prerequisites, and no note that it takes no arguments. The free-trial note is about pricing, not about usage conditions.

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