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

Annotations already declare read-only, idempotent, non-destructive, open-world behavior, so the safety profile is covered. The description adds genuinely new context: the rider_type alias groupings ("/peak" collapsing into regular) and the strong grounding instruction plus the unofficial/not-affiliated disclaimer, which affects how the agent should present results.

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?

The one-line purpose is front-loaded, followed by a compact Args block that is easy to scan. The grounding disclaimer is somewhat tangential for a fare lookup (routes, hours, phone numbers are outside this tool's domain), adding length without proportional value.

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?

There is no output schema, yet the description never indicates what a fare result looks like — currency, whether peak/off-peak is broken out, or what happens for an invalid station pair. For a three-parameter lookup with no return-value documentation, this is a meaningful gap in an otherwise adequate definition.

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 0%, so the description carries the full burden and largely does: it explains that origin/destination accept a station name or code, and it enumerates rider_type values that are absent from the schema (no enums present). It stops short of giving format examples or stating the default behavior beyond the schema's default field.

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?

"Station-to-station Metrorail fare" names a specific verb-equivalent (compute/look up a fare) and resource (Metrorail) with clear scope. It does not, however, differentiate itself from the close sibling estimate_trip_cost, so an agent must still guess which one answers a fare question.

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 is given, and the most relevant alternative (estimate_trip_cost) is never mentioned. The only constraint offered is an output-grounding rule ("answer only from this tool's results"), which governs answer composition rather than tool selection.

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