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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so safety is covered. The description goes well beyond them by disclosing data provenance and caveats: flag derived from MMSI (radio-licence flag, not always registry flag), dwt_estimate being an AIS-derived approximation with explicit null cases, decoded port codes and crew-supplied ETA. That is genuine interpretative context the schema cannot carry.

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?

The most important framing ('Everything known about one ship now') is front-loaded, and every clause is substantively useful given there is no output schema. The middle sentence is a dense comma-list that is slightly hard to scan, but nothing is filler.

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 carries the full return-value burden and discharges it well, enumerating identity, position, motion, destination, track window, sources and field caveats. The gaps are usage routing against siblings and error behaviour on an unknown MMSI, which keeps it short of a 5.

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% for the single 'mmsi' parameter (9-digit, bounded 1–999999999), so the schema already does the heavy lifting. The description adds only indirect meaning via the MMSI-derived flag note, which explains a returned field rather than the input format.

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 specific verb+resource+scope — 'Everything known about one ship now' — which is far more informative than the tautological title. However, it does not distinguish itself from siblings it clearly overlaps with: 'track over the last 40 minutes' collides with vessel_track, and the single-ship framing is never contrasted against search_vessels or vessels_near.

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?

There is no explicit when-to-use, when-not-to-use, or named alternative anywhere in the description. An agent must infer from the MMSI parameter that this is the single-vessel lookup, but the description never routes it against search_vessels, vessel_track or vessels_near, which are the obvious competing calls.

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