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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is fully covered. The description adds genuinely useful context the annotations cannot convey: a concrete pricing model (free within daily allowance, then $0.001 USDC per call via x402), which materially affects invocation decisions.

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 waste: the first front-loads what the tool returns, the second delivers the cost model. 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 values need not be explained, and annotations carry the safety profile while the description covers cost and field scope. The only gap is the absent when-to-use/routing guidance against sibling 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?

With a single parameter at 100% schema coverage, the schema already documents 'id' including its provenance. The description adds no format, syntax, or derivation detail beyond implying the id identifies 'one station', so the baseline 3 is appropriate.

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 description names the resource ('One station') and enumerates the returned attributes (river, location, catchment area, date opened, measures), which lets an agent distinguish this singular-detail tool from plural siblings like uk-rivers__stations-near and uk-rivers__search. It stops short of an explicit verb and never directly contrasts those siblings, so it is a bit short of a 5.

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?

There is no when-to-use guidance, no mention of when to prefer this over uk-rivers__latest or uk-rivers__readings, and no stated prerequisites. The only routing hint ('from stations-near or search') lives in the schema, not the description, so the description itself provides no usage guidance.

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