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

Annotations already establish read-only, idempotent, non-destructive behavior. The description adds meaningful behavioral context on top: accepted input ranges for each mode, the half-size format for UK letters, and the four output values returned. It does not cover invalid-input behavior, but this is a minor gap for a simple conversion 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 description is compact and front-loaded with the core purpose, followed by mode-specific input rules and a clear statement of outputs. Every sentence contributes useful information, and the source URL is a minor but harmless addition.

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 simple conversion tool with a full input schema and an output schema, the description covers everything needed: input modes, valid ranges, value formatting, and expected return fields. An agent can select and call this tool correctly without additional documentation.

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 adds valuable meaning beyond the schema: valid ranges for each mode, example half-size notation like 'N 1/2', and the semantic distinction between UK letters, US numbers, and mm circumference. This helps an agent construct valid values more accurately.

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 uses a specific verb and resource: 'Convert a ring size between UK, US and millimetre scales.' It clearly identifies the tool's domain and distinguishes it from the many other converter tools in the sibling list, such as shoe size or unit converters.

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 gives clear mode-specific usage guidance: what value formats are valid for 'uk', 'us', and 'mm' modes, including ranges and half-size notation. It does not explicitly name alternatives or state when not to use this tool, but the domain is specific enough that the intended context is clear.

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