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

Annotations only declare readOnlyHint and openWorldHint; the description adds substantial non-obvious behavior: transit counts are platforms, names groups platforms, zero means unmapped-not-absent, null count means source not loaded, null name means un-named source. It also states limits (nothing about travel times/frequency), cost, and per-source licensing.

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?

It is long but a single front-loaded paragraph where most sentences carry load-bearing semantics or licensing. The one redundancy is re-listing all fourteen categories that the schema enum already enumerates verbatim, which costs some compactness.

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 explicitly explains the return shape (counts and names, distances under withinKm) and the meaning of null/zero values. For a multi-source, three-parameter tool this is complete enough to call 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% so the baseline is 3, but the description adds meaning beyond the schema: withinKm looks beyond the boundary (straight-line) and returns nearest-first per category with distances, and names groups platforms by stop name. The categories default to all fourteen is also clarified.

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?

Opens with a precise scope statement: what is physically inside one Finnish postal area, with specific categories and named sources (Digitransit, PTV, OpenStreetMap, LIPAS). This clearly distinguishes it from siblings like get_area_report, get_place_stats and search_places.

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 concrete triggering questions ("what is there in Tikkurila?", "is there a school in 99980?") and the condition that selects the withinKm behavior. It also routes city-name queries correctly ("a city name returns its areas to choose from"). It stops short of explicitly naming alternative sibling tools, so not a full 5.

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