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

With no annotations, the description must carry the full burden. It usefully discloses that inputs may be decimal strings and that results beyond 2^53-1 come back as a decimal string, but it says nothing about error behavior (modulus zero, negative exponent), whether all three inputs are mandatory, or what happens on invalid input.

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 sentences, front-loaded with the operation and formula, followed by the input/output format note. The free-trial marketing clause is the only element that doesn't serve task execution.

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 three-argument arithmetic tool with no output schema, the description covers what the operation is, the accepted input representation, and the output representation. The main gap is the lack of requiredness information, since the schema marks zero required parameters while the prose implies all three are needed.

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% — three bare string properties with no descriptions and no required list. The description compensates by stating the parameters are integers and that decimal strings are accepted, which is real added meaning, though it doesn't clarify requiredness or the semantics of each field beyond the formula.

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?

"Calculate modular exponentiation (base^exponent mod modulus) exactly" gives a precise verb, resource, and even the formula, so an agent knows exactly the computation performed despite the terse tool name. It does not name any sibling (e.g., x402-power, x402-multiplicative-inverse) to differentiate scope, which keeps it below 5.

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 statement of when to use this over alternatives, no prerequisites, and no exclusions. Usage is only inferable from the math formula itself; the only added guidance is the pricing note about free trial calls.

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