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

Beyond the annotations (readOnly, idempotent, non-destructive), the description adds substantial behaviour: a one-hour recency window, a hard 50 nm cap, nearest-first ordering, and a documented privacy exclusion for private vessels with a private_vessels count returned. That privacy rule is a non-obvious constraint that would otherwise cause silent under-reporting.

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 scope, then the return shape, then the alternative, then the caveat — a sensible order. The parenthetical field enumeration is long but earns its place because there is no output schema; it is still dense enough that a skim could miss the privacy clause.

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 compensates by listing the compact row fields end to end, and it discloses both the truncation cap and the private-vessel exclusion count. An agent has everything needed to call this correctly and interpret the response.

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 lat/lon/mmsi/limit/radius_nm are all documented with the centre-or-ship choice and defaults. The description's radius cap restates the schema maximum rather than adding syntax or tie-breaking detail, so it stays at the baseline 3.

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 resource (ships) with precise scope: heard in the last hour, within a radius, centred on a point or another ship, nearest first. The radius framing cleanly distinguishes it from area-based siblings like vessels_in_area, and the get_vessel pointer handles the single-ship case.

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 routes the agent to get_vessel when full details of one ship are needed, and the radius framing implies a proximity use case. It does not, however, contrast itself with the other nearby siblings (vessels_in_area, search_vessels, lookup_vessels), so the agent must infer that boundary.

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