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

With no annotations, the description carries the full burden and does well: it discloses the paid model ($0.005 USDC per call via x402), the payment location (_meta["x402/payment"]), and the free 3-row preview flag. It omits response shapes and error behavior, which keeps it short of a 5.

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?

Purpose is front-loaded, then the route menu, then the payment/preview note — no filler. The route enumeration is dense and list-like but each entry carries routing value, so it stays efficient.

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 multi-route, route-dispatched API with no output schema and a nested params object, the description covers the routes, their key parameters, cost, and preview mode. It stops short of describing return shapes or error handling, but the essential calling information is present.

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. The description does add per-route parameter listings (/convert takes value*, from*, to*; /search takes q, quantity_kind, limit, offset), which supplements the free-form 'params' object whose additionalProperties only say string — useful but mostly restating discoverable route docs rather than explaining semantics like format or required-vs-optional beyond asterisks.

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 resource and capability ('Unit Converter and Physical Constants') and distinguishes itself from the sibling API wrappers (airports, colors, exchanges) by naming the domain of 'unit maths'. The enumerated route list makes the tool's scope concrete without opening the 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?

The route list with per-route parameters gives clear guidance on which internal endpoint to select for a given need (search vs convert vs constants vs isotopes). It does not explicitly state when to prefer this tool over a sibling, but the self-evident domain split (units vs airports vs colors) makes that inferable.

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