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

Annotations already declare readOnlyHint, idempotentHint, and non-destructive, so the safety profile needs no restating. The description adds real behavioral context beyond them: the exact exclusion set (first-top-up, trial, free promotional groups), the CNY pricing restriction, the fact that low or unmeasured uptime groups are dropped, and that results carry dataAsOf plus 'the only wording the result supports for a price claim' — a useful anti-overclaiming constraint. Missing are any notes on empty-result behavior or rate limits, which keeps it at a solid 3 rather than higher.

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 core action and the filter conditions, then a return-shape sentence, then sibling routing — a logical order with no filler sentences. It is dense to the point of being clause-heavy, packing filters, exclusions, return fields, and caveats into three long sentences, which costs a little readability but nothing is gratuitous.

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 read tool with a full input schema, full annotations, and an output schema, the definition covers everything an agent needs: filter semantics, the single-candidate limitation, downstream verification steps, and the evidentiary caveat about price wording. Return values are largely deferred to the output schema, which is appropriate.

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 description coverage is 100%, so all three parameters are already well documented, establishing a baseline of 3. The description reinforces but does not extend the schema: its uptime guidance ('pass the user's number when they name an uptime requirement') and default-90 note are already present in the minUptime24hPercent schema text, and the model and locale parameters receive no added meaning in the description. Adequate but redundant rather than additive.

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?

The opening states a precise verb, resource, and scope: 'Find the relay with the lowest observed input price for one model,' immediately qualified with the filtering conditions (CNY-priced, uptime floor, exclusion of first-top-up/trial/free promo groups). It also explicitly distinguishes itself from ranking siblings by noting it returns 'a single filtered candidate, not an overall ranking.' An agent can tell this apart from get_model_board without opening either 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?

It names concrete when-to-use conditions ('pass the user's number when they name an uptime requirement') and explicitly routes to alternatives: get_model_board for comparing several relays, get_station for purity checks before recommending. Both the trigger and the exclusions are stated rather than inferred.

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