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

With no annotations, the description carries the full burden; it does disclose a real behavioral detail an agent could not otherwise infer — a paywall with '5 free trial calls per signed wallet.' It does not describe the return payload shape (e.g., whether the constant is returned as a number, string, or object), which matters since no output schema exists.

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 sentences, front-loaded with the identity of the constant and followed by the access constraint. The emoji is decorative but harmless; no sentence is padding.

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 trivial zero-argument constant-returning tool with no output schema, the description supplies the identity, the literal value, and a quota warning — enough to call it correctly. The only gap is the exact response format, a minor omission given the tool's simplicity.

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 no parameter semantics to explain; the baseline of 4 applies. The description adds nothing param-related but nothing is missing either.

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 gives a specific verb and resource ('Return Avogadro's constant') and even states the value (6.02214076 x 10^23) plus what it represents (constituent particles per mole). This clearly separates it from adjacent siblings such as x402-moles-from-mass, x402-molar-mass, or x402-period.

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?

The description implies the tool is a fixed scientific-constant lookup, but it never states when to reach for it versus related molar-mass/mole-conversion siblings, nor any exclusions. For a zero-parameter constant it is mostly self-evident, so this is adequate-but-implied rather than explicit.

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