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

With no annotations, the description carries the full behavioral burden. It implies a read-only boolean check and discloses a real constraint not present in structured data (5 free trial calls per signed wallet). However, it says nothing about the return shape (bare boolean vs. which digit failed) or edge-case handling for negatives, zero, or decimals.

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 clauses with the definition front-loaded; the only waste is the redundant 'Is Self Dividing:' lead-in that restates the name and the trailing emoji promo line.

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 predicate with no output schema and no annotations, the definition covers what is computed but omits the return contract and input format. An agent knows the semantics but not exactly what it will get back, which is a meaningful gap for a check tool.

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?

Schema coverage is 0% for the single 'value' parameter, so the description must compensate. It does clarify that value is the number whose decimal digits are inspected, but never states the expected format (string of digits, integer, sign handling), leaving a gap for a 0%-documented param.

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 the exact predicate and defines it operationally: 'every decimal digit of value is nonzero and divides value evenly'. That is a specific, self-distinguishing check among the many x402-is-* siblings. It stops short of explicitly contrasting itself with neighbors like is-harshad or is-prime, but the definition itself is unambiguous.

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 prerequisites, and no mention of any alternative tool. The only usage-adjacent text is the pricing note ('5 free trial calls per signed wallet'), which is commercial rather than routing guidance.

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