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

Annotations already cover read-only, idempotent and no-destruction semantics, so the description's job is to add cost and access constraints — which it does: 5 credits per call, paid plan requirement with a pricing link, and tier-dependent lookback of 90/365 days that overrides the schema's flat 365 maximum. It stops short of describing response shape or error behavior, which is a minor gap given no output schema.

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 core summary and no filler; the metric list is dense but each item tells the agent what will come back. The credit and plan sentences are terse and necessary given the paywall.

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 return-value burden and largely meets it by enumerating the summary's components. Remaining gaps are units for distance/speeds and how 'not heard' is determined, but an agent has enough to decide and invoke 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% and both parameters are documented, so baseline is 3; the description earns above that by adding the default window (30 days) and the plan-dependent effective ceiling on 'days' that the schema cannot express. It does not clarify units for distance or speed, but those are output rather than parameter concerns.

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 and outcome: an aggregated activity summary enumerating hours under way, in port, stopped, distance, speeds, calls and top ports. That enumeration distinguishes it from raw-position or track tools, though it never names a sibling to route against explicitly.

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 prerequisites clearly (paid Data API/Watch plan, 5 credits, lookback caps by tier) but not when-to-use guidance versus alternatives such as vessel_track or vessel_port_calls. An agent can infer this is the aggregate-summary option, but must reason it out rather than being told.

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