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

No annotations are present, so the description carries the full burden of behavioral disclosure. It discloses nothing: no return value, no units, no side effects, no assumptions. An agent cannot predict what a call to this tool will yield.

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 short, but it is not effectively concise—it merely repeats the tool name in a label-value format. No content earns its place, and the space is wasted on redundancy rather than 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 zero-parameter tool with no output schema, the description is the only source of meaning, and it provides none. An agent cannot infer what the tool returns, in what units, or why it exists as a distinct function.

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 and the schema is empty, so there are no parameter semantics to explain. Per the rubric, 0 params earns a baseline of 4; the description's failure to explain what the constant is or what value it returns is a contextual completeness problem, not a parameter schema problem.

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 is a pure tautology: 'Vacuum Permeability: Vacuum Permeability' restates the tool's name without adding any verb, resource, or function. It gives an agent no idea what the tool does, and it does not distinguish this constant from siblings like x402-vacuum-permittivity.

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 about when to use this tool versus any alternative. No context, no examples, no mention of related constants or conversions, and no indication of what problem it solves.

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