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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint as true. The description adds valuable behavioral context by explaining the distance calculation is straight-line great circle (not road/rail) and describes the response format. No contradictions with annotations.

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 sentences, no wasted words. First sentence gives purpose and output summary, second clarifies distance metric, third clarifies use case. Information is front-loaded and every sentence 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?

No output schema, but description covers what each result line contains. Parameters are fully documented in schema. Annotations cover safety. Missing details about error handling or timeouts, but given the tool's simplicity, it is fairly complete.

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 100%, with well-described parameters (lat, lon, limit). The description adds extra meaning: distance is in MILES and status URL ends in .md, which are not in schema. However, since schema already covers parameters well, the added value is moderate.

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 clearly states the tool returns the nearest listed level crossings to a point, sorted closest first, and specifies the output fields (distance in miles, name, type, region, status URL). It effectively distinguishes from sibling tools like busiest_today and crossing_status by focusing on proximity.

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?

Explicitly tells when to use ('which crossings are near this place') and when not to use ('NOT distance by road or along the railway'). It also contrasts with a potential alternative ('not which will this train reach next'). Provides clear, actionable guidance.

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