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

The description adds significant behavioral context beyond the readOnlyHint and other annotations by listing the exact returned fields (e.g., 'name, call sign, IMO number, ship type...' and the AIS position details). It also discloses the regional limitation. However, it does not specify behavior when an MMSI is not found or is outside coverage, which could be a useful addition.

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 four sentences with a clear front-loaded purpose. Each sentence earns its place, though the enumeration of return fields is somewhat lengthy. Still, it is well-structured and not wasteful, earning a solid score.

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 single-parameter lookup tool with no output schema, the description is remarkably complete. It explains what the tool does, the exact return contents, the input sourcing flow, and an example. The only missing piece is explicit error behavior, but that is minor given the otherwise comprehensive coverage.

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% with the mmsi parameter already well-described as a 9-digit number. The description adds an example and suggests using ais_vessels_near to obtain the MMSI, but this doesn't fundamentally alter parameter semantics. Baseline 3 is appropriate because the schema does the heavy lifting.

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 clearly states the tool's function: 'Look up one specific ship by its MMSI number and get its registered details plus its latest AIS position.' It distinguishes from sibling tools by emphasizing 'one specific ship' and even names ais_vessels_near as the source for obtaining an MMSI, making the purpose unambiguous.

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?

Usage guidance is explicit: it provides example queries like 'where is MMSI 230052800' and instructs to 'Get an MMSI from ais_vessels_near first.' The regional coverage note ('COVERAGE IS REGIONAL') also sets expectations about when the tool will succeed, effectively giving alternation and precondition guidance.

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