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

Annotations already declare readOnlyHint=true and destructiveHint=false, and the description reinforces that with "reads or computes only and changes nothing." It adds genuinely non-structured context: per-call pricing ($0.01 via x402 USDC or card key), the first 25 free, failed calls free, and that repeats are billed — which is the practical meaning behind idempotentHint=false. Return format is not described, but a conversion tool's output is self-evident.

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?

Front-loaded with the supported categories, then example units, then usage, then pricing. Every sentence carries information, though the trailing pricing/auth sentence is dense and bundles cost, auth method, free tier, and refund policy into one run-on.

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 three-parameter, fully-required, deterministic tool with no output schema, the description covers supported domains, unit syntax, an example call, and the payment model. An agent has everything needed to invoke it correctly and to know it will be billed.

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 100%, so the baseline is 3, but the description earns above baseline by giving the accepted unit token vocabulary ("km, mi, lb, kg, floz, ml, mph, kn, acre, gib, kwh, psi") and a concrete example mapping value=10, from=km, to=mi. That tells the agent the expected casing/abbreviation form, which the schema's "Unit to convert from/to" strings do not.

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?

States a specific verb+resource ("Convert between units") and enumerates the supported measurement families — length, mass, volume, temperature, speed, area, data size, time, energy, pressure — plus concrete example unit tokens (km, mi, floz, gib, psi). No sibling tool in the list overlaps, so an agent can select it unambiguously.

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?

"Use when exact conversions matter" gives a positive selection condition, and the worked example shows the call shape. However, there is no exclusion guidance — nothing about when to prefer math_evaluate for arithmetic, and no stated limits on supported units beyond the examples.

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