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

Annotations already declare read-only, idempotent, open-world and non-destructive, so the description's added value is in disclosing the null-on-failure return and plan-dependent behavior: free accounts get answers 24 hours behind plus a 1 square degree box and 1,000 credits/month, while paid plans are live. That staleness and quota information is genuinely beyond the annotations. The 'box of at most 1 square degree' phrasing is opaque and should be clearer.

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 return semantics are front-loaded and the whole thing is short, but the second half is a dense jumble — 'Free. Free accounts too:' is redundant, and quota, staleness and coverage area are crammed into one semicolon-joined clause that is hard to parse.

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 one simple parameter and no output schema, the main open question is what the return actually looks like — a port code, a name, an object? The description says 'the port ... or null' but not its shape. Plan/limit behavior is covered, so the definition is adequate but leaves a real gap for an agent that must consume 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%, so the baseline is 3, but the description earns an extra point by supplying concrete format examples of what a destination string looks like, which the schema's 'as the vessel broadcasts it' alone does not convey.

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 states concretely what the tool returns: the port that an AIS destination string (e.g. "NL RTM", "ROTTERDAM PILOT") denotes, or null. The examples make the input domain unmistakable and separate it from a port search tool, though the phrasing is a noun fragment rather than a verb+resource statement and no sibling is named.

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 or when-not-to-use guidance, and no alternative is named (search_ports is the obvious candidate for name-based lookup vs. raw broadcast text). Only the implicit cue 'AIS destination text' and the 'or null' fallback hint at the intended call context.

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