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

With no annotations at all, the description carries the full burden, and it does disclose one genuinely useful behavioral trait: a metered free trial of five calls gated on a signed wallet, which tells the agent about cost/auth. However it never states what the call returns, whether the result is a constant or computed, or in what units, leaving the core behavior opaque.

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 with no filler, and the pricing note is placed where it is quickly seen. Brevity is not the problem — the brevity comes at the cost of substance, but the structure itself is clean and front-loaded.

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-lookup tool with no output schema and no annotations, the definition is only barely sufficient. The agent can invoke it but cannot predict the returned value or units, which for a physics constant is exactly the information a caller needs. The billing note is a plus but does not fill that gap.

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. Baseline 4 applies; the description neither adds nor loses anything here.

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 only restates the name: 'Light Speed: Speed of light.' It names a resource (the speed-of-light constant) but uses no verb and never says what the tool actually does with it — return the value, convert it, or something else. An agent must guess, and it is not distinguished from nearby siblings such as x402-speed, x402-speed-convert, or x402-velocity.

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, and no conditions for invocation. The only contextual hint is the '5 free trial calls per signed wallet' note, which speaks to billing rather than to when this tool should be chosen over the many speed-related siblings.

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