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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint, yet the description adds genuinely new operational facts: the today-default, per-plan lookback limits, a 1-credit-per-call cost and an explicit paid-plan prerequisite with a pricing link. These are exactly the behavioral costs and constraints an agent needs before invoking the tool.

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 purpose leads in the first sentence, followed by retention, cost and plan prerequisites in telegraphic but readable order. Nothing is redundant, though the pricing URL and plan enumeration make it denser than strictly necessary for selection.

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?

For a two-parameter read tool with annotations and no output schema, the description covers default behavior, retention, cost and access prerequisites, and it sketches the returned content (arrival and departure times). It does not mention result volume or pagination, which for a busy port on a single day could matter.

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 UN/LOCODE-or-name formatting and the YYYY-MM-DD pattern are already documented in the schema. The description adds only that the day is UTC and defaults to today, which is useful but marginal. Baseline 3 applies when the schema carries the parameter burden.

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 gives a concrete verb and resource: seagoing vessels alongside a specific port on one UTC day, plus when each arrived and departed. That is specific enough to separate it from generic port listings, but it never distinguishes itself from close siblings like port_vessels or vessel_port_calls, so a 4 rather than a 5.

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 clearly states the operating context: date defaults to today, history is limited to 90 days on Plus and 365 on Pro/Scale, and a paid Data API or Watch plan is required. What is missing is any when-to-use/when-not guidance relative to alternative call-listing tools.

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