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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds useful behavioral context: it returns live coordinates, speed, course, heading, navigation status, and raw AIS destination text, and it requires exactly one identifier. It also clarifies that the destination text is raw AIS text, not a recognized port, which is a meaningful behavioral nuance. It doesn't mention rate limits or error behavior, but the annotations plus the explicit identifier constraint provide solid transparency.

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 description is two sentences, front-loaded with the core purpose and identifier constraint, then the alternative tool routing. Every sentence earns its place: the first defines what the tool returns and how to identify the vessel, the second tells the agent when to use the sibling instead. No wasted words.

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 read-only lookup tool with a rich output schema, the description covers the purpose, identifier constraints, return contents, and the alternative for richer data. The output schema exists, so return values don't need to be spelled out. The only minor gap is error behavior (e.g., what happens if no vessel matches), but that's not essential for a read-only lookup with a clear identifier constraint.

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%, so the schema already documents all three parameters. The description adds value by stating the 'exactly one of' constraint and the digit counts (9 for MMSI, 7 for IMO), which reinforces the schema's examples and helps the agent pick the right identifier format. It doesn't add much beyond that, but the baseline is 3 and the description does add meaningful constraint context.

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 a specific verb ('Return'), a specific resource ('current position and basic information for a single vessel'), and the exact identifier options (MMSI, IMO, UUID). It also distinguishes itself from get_vessel_pro by listing what the pro version adds (destination/origin port, ATD, ETA, draught). This is a clear, specific purpose that an agent can act on.

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?

The description explicitly says 'This is the default tool for vessel lookups' and names the alternative (get_vessel_pro) with the exact conditions for choosing it: when you need recognized destination/origin port, ATD, reliable ETA, or draught. This is explicit when-to-use and when-not-to-use guidance.

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