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

Annotations already cover safety (readOnly, idempotent, non-destructive, openWorld), so the bar is lower. The description adds genuinely useful non-structured context: the free daily allowance and the $0.003 USDC per-call x402 charge, which matters for an agent deciding whether to call. It does not disclose default radius behaviour or result ordering, but the pricing disclosure is substantive.

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 tight sentences with zero filler, front-loading what the tool returns before the cost note. Every clause earns its place.

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?

An output schema exists, so return-value detail is unnecessary, and annotations carry the safety profile. The description covers location flexibility and cost, but for a tool whose name collides with uk-flood__rainfall-near it lacks any disambiguation, which is the one real gap.

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 each of the four parameters (lat, lng, postcode, radius_km) documented, so the baseline is 3. The description only restates the either/or nature of postcode vs point that the schema already conveys, adding no new format or constraint detail.

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 (nearest rain gauges) and output (latest rainfall totals) plus accepted location inputs, so the agent knows exactly what it returns. However, it does nothing to distinguish itself from the near-identical sibling uk-flood__rainfall-near, leaving ambiguity about which provider's rainfall data to use.

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?

No when-to-use guidance, no prerequisites, and no mention of when to prefer the sibling uk-flood__rainfall-near or uk-rivers__stations-near. The only contextual guidance is commercial (free tier / per-call price), which is not selection guidance.

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