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

The annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds non-obvious operational context the annotations lack: the free tier of 10 calls/day, then $0.001/call via x402 in params._meta['x402/payment'] or an X-API-Key, plus the serving host. Missing detail includes what happens on payment failure or what error looks like.

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?

The functional purpose is front-loaded in the first sentence and the pricing/billing sentence comes second, which is the right order. The billing sentence is long but contains concrete, non-redundant facts; no filler.

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?

Because an output schema exists, return values need not be explained. However, the tool declares zero required parameters, and the description never says that at least one of apr/apy is needed to produce a conversion, so the agent cannot tell whether a parameterless call is valid or what would happen.

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% with per-parameter descriptions and examples, so the schema is doing the heavy lifting. The description adds only the semantic tie between 'N days' and `days`, and 'optional performance fee' and `feePercent`, which is helpful but marginal. It does not clarify that all six parameters are optional or which combination is required.

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 specific verbs and resources: 'Convert APR to APY (or back)', 'project earnings over N days', 'doubling time'. The scope is clear from the opening phrase, though 'doubling time' is mentioned without a corresponding input, and the description does not distinguish itself from near-neighbors such as currency_convert.

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?

Usage is only implied through the opening phrase 'Convert APR to APY (or back)': the agent can infer that apr and apy are alternative inputs, but nothing says whether earnings projection requires principal and days, or what the tool does differently from related calculators. There are no explicit when-not-to-use conditions or defaults.

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