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

No annotations are provided, so the description carries the full burden. It does disclose a genuinely useful behavioral trait the schema does not: a quota of 10 free calls/day. However, it says nothing about what is returned (line, direction, countdown), whether station names must match exactly, or failure behavior for unknown stations.

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 compact sentences with the core capability front-loaded and the quota constraint following. Every clause earns its place; nothing is padded.

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 simple two-parameter lookup with no output schema, the description covers the essentials (language support, rate limit) but omits the return shape and any hint about station-name matching, leaving an agent to guess how results come back.

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 50% – the 'lang' param is documented but 'station' has no schema description. The description compensates by stating the station can be given in English, Japanese or Korean, which is exactly the missing semantic detail for that parameter.

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 ('Next trains at a Seoul subway station') plus a real-time scope ('right now'), which is clear enough to distinguish it from the broader sibling seoul_now. It does not explicitly name a sibling to contrast against, so it falls 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?

The 'right now' phrasing implies a real-time lookup use case, but there is no explicit when-to-use or when-not-to-use guidance, nor any mention of the sibling tools (seoul_now, seoul_places) that an agent might confuse this with.

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