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

Annotations (readOnlyHint, openWorldHint, idempotentHint) are confirmed by the description: 'Informational only' and 'Forecasts are rebuilt per request... answer changes minute to minute.' It adds error handling behavior and the dynamic nature, going beyond 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, all essential: first sentence gives purpose and output, second explains dynamic nature, third provides error handling and prerequisites. No wasted words, front-loaded with key info.

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?

Given one parameter, no output schema, and rich annotations, the description covers purpose, behavior, parameter handling, safety warning, and error conditions. It is fully adequate for an agent to use the tool 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 coverage is 100% (one parameter 'crossing'), so baseline 3. The description adds behavior guidance: ambiguous names return candidates, unknown names require calling list_crossings first. This adds value beyond the schema description.

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 what the tool does: 'Live barrier status, the next-30-minute closure timeline and approaching trains for ONE crossing, as markdown.' It distinguishes itself from siblings (busiest_today, crossings_near, list_crossings) by focusing on a single crossing's live status and mentioning list_crossings as a prerequisite for unknown names.

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 description provides when to use (for live status of one crossing) and includes a safety warning ('Informational only — never a substitute...'). It also explains behavior for ambiguous/unknown names. While it doesn't explicitly compare to alternatives, the scope is clear.

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