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

Annotations already declare readOnlyHint=true and openWorldHint=true, and the description goes beyond them with two things annotations cannot convey: data-quality caveats ('the feed can be delayed or empty late at night') and the pricing/payment model ($0.002 USDC per successful call via x402, failed calls uncharged). That cost and reliability context is exactly what an agent needs before choosing this tool. It stops short of describing pagination or refresh cadence, so not a 5.

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?

Three tight sentences, front-loaded with what the tool returns, then the reliability caveat, then the pricing. Nothing is redundant with the schema or annotations, and every clause carries information an agent would otherwise have to guess.

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 the description covers scope, source, data-quality risk, and cost. For a two-parameter read-only lookup this is close to complete; only the absence of guidance on what to do when the feed returns empty keeps it from a 5.

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% and both parameters carry their own descriptions with examples and format hints, so the schema does the heavy lifting. The description only restates 'one station on one line' and adds no syntax, casing, or validity guidance 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?

States a specific verb and resource — realtime next-train arrivals for Hong Kong MTR — plus the scope constraint (one station, one line) and the upstream source (HK government open data feed). It also enumerates the returned fields (direction, destination platform, ttnt, scheduled time), which separates it from sibling realtime transit tools like travel_mbta_realtime or travel_septa_train_view.

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?

The 'one station on one line' phrasing implies the call granularity, so an agent understands it must call per station, but there is no explicit when-to-use/when-not guidance and no named alternative (e.g. a route-planning sibling) to route away to. Usage is inferable rather than stated.

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