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

Annotations already cover readOnly, idempotent, openWorld, and non-destructive, so the bar is lower. The description adds genuinely non-structured context: a metered pricing model (free within daily allowance, then $0.002 USDC per call via x402) and the default ordering. It stops short of describing pagination or truncation behavior, but the pricing disclosure is a real value-add.

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?

Two sentences, zero filler, front-loaded with scope and cadence before the pricing caveat. Every clause (cadence, granularity, entity scope, time window, ordering, cost) carries distinct information.

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, return-format explanation is unnecessary, and annotations cover the safety profile. Scope, window, ordering, and cost are all present; the only meaningful gap is the lack of sibling routing guidance against uk-rivers__latest and the historical station tools.

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 the schema already documents id, days, and type, including the 1–7 constraint. The description reinforces the 7-day cap and the one-type-per-station constraint but adds no syntax or format detail beyond the schema. Baseline 3 applies.

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?

Specifies the verb (retrieve readings), the resource (15-minute readings or daily means), the scope (one type at a station, up to 7 days), and the ordering (newest first). It is clear what the tool returns, though it never names the sibling it differs from (e.g., uk-rivers__latest for a single latest value), so differentiation is left to inference.

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?

"Up to 7 days" implies a time-range use case, but there is no explicit when-to-use guidance and no mention of alternatives such as uk-rivers__latest, uk-flood__station-history, or uk-rivers__station. An agent must infer the routing decision purely from the tool name.

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