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

No annotations exist, so the description carries the full behavioral burden, and for a stateless unit conversion the main traits are that it is a pure deterministic mapping with no side effects - which is implied but never stated. The warning that 'A US 6 is a UK M, not a UK 6' is a genuinely useful anti-error disclosure that adds value beyond the schema.

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?

Two short sentences, zero waste. The core capability is front-loaded and the second sentence earns its place by preempting a common US/UK confusion.

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 no output schema and no annotations, the description should describe what comes back (a converted value in the target system, its unit/format, and behavior for unmappable input). It never addresses the return shape or error cases, leaving a gap for a tool whose output the agent must interpret.

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?

At 50% schema coverage, the description compensates by explaining what the enum values mean: mm is specifically the 'inner diameter in mm' and UK/EU are named sizing systems, with the US 6 = UK M example implicitly conveying that UK sizes are letters, not numbers. It adds real meaning beyond the bare enum list, though the 'from' default and value formatting beyond the one example remain unstated.

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?

States a specific verb ('Convert') plus resource ('a ring size') and enumerates the exact unit systems it bridges (US, UK, EU ISO 8653, inner diameter mm). This is clearly distinguishable from siblings like carat_to_mm or necklace_length, which deal with different jewelry attributes.

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 is implied by the narrow domain - any agent needing a ring-size translation will know to use it - but there is no explicit when/when-not guidance, no mention of the default from='us' behavior at call time, and no alternatives named. The second sentence warns about a mapping gotcha rather than routing the agent.

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