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

Annotations already cover read-only/idempotent safety, and the description adds real context beyond them: the reply is scoped to 'the active release', compact mode shrinks the payload, and factor metrics are included alongside area metrics. It stops short of describing pagination or size limits for the full (non-compact) response.

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?

Front-loaded with what is returned, followed by the calling sequence and then the compact caveat — every sentence carries information. The parenthetical field lists and the trailing 'Sources carry licence and methodology links' make it a dense block rather than cleanly separated beats, but nothing is padded.

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 an output schema exists, the description need not explain return shapes, yet it still flags the catalogue's structure (area vs. factor metrics), release scoping, and provenance/licence caveats. An agent has everything needed to call it and use the result to drive get_area_metric and rank_areas_by_metric.

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%, so the baseline is 3; the description still adds purpose for the one flag, explaining that compact=true returns 'only id, unit, direction and defaultPeriod' for callers who 'just need valid metric_id/period values'. That is a use-case framing the schema's own one-line description does not fully supply.

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?

Specific verb and resource ('List every area metric the active release serves'), then enumerates the returned fields (id, unit, direction, periods, legend, provenance, caveats) plus the 'factor' kind. It also names the siblings that consume this data (get_area_metric, rank_areas_by_metric), so an agent can tell the catalog tool apart from the query tools at a glance.

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 states the ordering rule: 'Call first to learn valid metric_id/period values for the other area tools,' which tells the agent both when to use it and which alternatives it feeds. The compact=true variant adds a second decision point (cheap discovery vs. full metadata).

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