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

Beyond the readOnly/idempotent annotations, it discloses meaningful behavioral context: the response includes 'how long ago it was heard' (freshness), a 40-minute track window, and 'the data sources to credit'. It also flags two substantive caveats — flag is the radio-licence flag from the MMSI and may differ from the registry flag, and dwt_estimate is derived from AIS dimensions and is null for tugs and pleasure craft.

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 the returned-field inventory, then caveats, then the alternative. It is dense and long, but with no output schema each clause is doing real work; only the field enumeration borders on sprawling.

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?

There is no output schema, and the description compensates fully by describing return contents, freshness semantics, provenance, and the limits of two derived fields, plus a routing note. An agent has everything needed to call it and interpret the result.

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% for the single mmsi parameter, so the schema already carries the burden and baseline is 3. The description adds some meaning by explaining what the MMSI encodes (radio licence flag) and what is derived from it, but it adds nothing about the parameter's format or constraints.

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?

The description enumerates exactly what 'get_vessel' returns for a single ship: identity, position, motion, navigational status, destination, ETA, freshness, a 40-minute track, and provenance. It is unmistakably a single-vessel detail lookup, and it explicitly separates itself from lookup_vessels for archive/sanctions data.

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?

It gives a concrete routing rule: 'For archive records and sanctions flags, use lookup_vessels (API key)', implying this tool serves live/current snapshots. It does not distinguish against other close siblings like vessel_track or vessels_near, 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.