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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely non-obvious behavior beyond that: it never fabricates figures and returns null when a value is not stored, which tells the agent how to interpret gaps rather than treating them as errors. It omits what happens when the slug itself is unknown (error vs. null), leaving one behavioral gap.

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 short clauses with zero filler: the return payload is front-loaded, the sibling routing follows, and the null/data-integrity caveat closes. Every sentence carries distinct information.

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?

With no output schema, the description usefully names the fields returned, and it clarifies the null semantics an agent needs to interpret the response. The only meaningful omission is behavior for an unrecognized slug, which matters for a required-key lookup tool.

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 the single slug parameter already has a description plus an example ("inca-trail"), so the schema carries the parameter semantics. The description adds no format, casing, or slug-resolution detail beyond that, making the baseline 3 correct.

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 ("Returns the full sourced trail record") and enumerates the returned fields (distance, grade, permits, geometry URL), which pins down scope precisely. It also implicitly separates itself from search_trails by being a single-record lookup keyed on slug.

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 an explicit routing rule: "Use for route facts; use get_permit for fee-only questions," naming the alternative and the condition that selects it. It does not cover other plausible alternatives such as get_geojson for geometry or search_trails for discovery, so the guidance is clear but not exhaustive.

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