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

Annotations already declare it read-only, idempotent and non-destructive, and the description layers on genuinely useful context beyond them: precision (12 significant digits), failure behavior (cross-category mix returns 400 and is never billed), and the payment model (0.005 USDC via x402 on Solana/Base/Polygon/Arbitrum/Arc). Cost and error semantics are never derivable from annotations alone.

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?

Three tight sentences with no filler; the usage trigger is front-loaded, then output precision, then error and cost semantics. Every clause carries information an agent can act 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?

With an output schema present, the description need not restate return shape, and it still summarizes the return (converted value to 12 significant digits). For a 3-parameter, read-only, paid tool, the description covers invocation, failure mode, and billing, leaving nothing material unstated.

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% and each of value/from/to is documented in the schema, so the baseline is 3. The description adds only indirect parameter context (the set of valid unit categories and the category-mixing constraint) and gives no syntax or format detail beyond what the schema states.

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 operation (convert a value between units) and enumerates the covered categories (length, mass, volume, time, data, speed, area, pressure, energy, temperature), which cleanly separates it from siblings like get_fx_rates (currency) or get_base_gas (chain metrics). An agent can identify this as the unit-conversion tool without opening any schema.

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 an agent needs an exact unit conversion" gives an explicit trigger condition, and "Mixing categories is a 400" communicates a clear exclusion/constraint. It stops short of naming an alternative tool for edge cases (e.g. FX/currency conversion), so it is clear context rather than full when/when-not routing.

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