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

With no annotations, the description carries the full burden and does well: it discloses read-only access, no key required, determinism ('same input, same answer'), the fare categories returned, and that a source URL is included. It lacks rate-limit or failure-mode notes, keeping it short of 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?

Two tight sentences, front-loaded with the core purpose and followed by return payload and operational traits. Every clause adds information; no filler.

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?

No output schema exists, so the description's enumeration of returned fare types (Octopus adult/student/JoyYou-sixty/child/elderly/PWD, single-journey adult, HKD, source URL) fills that gap. Combined with the no-key/read-only notes, an agent has everything needed to call it correctly.

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 are documented with examples in the schema; the description only restates that inputs may be a station name or MTR station id. Baseline 3 applies since the schema already carries the parameter 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 verb ('look up') and resource ('published MTR fare between two stations in Hong Kong'), plus the input format. Clearly distinguishable from the sibling tools log_trip, monthly_summary and subsidy_amount, which do not retrieve published fare tables.

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?

Gives clear context: fare lookup between two named/id stations, deterministic, no key. It does not name an alternative tool or state when-not to use it, but the siblings are functionally distinct enough that this is a minor gap.

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