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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is covered elsewhere. The description adds a little useful behavior (it returns "our name" per number for confirmation), but says nothing about invalid/unknown numbers, partial results, or batch limits.

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, front-loaded with the action and ending with the motivating outcome. Mildly redundant in the second sentence, but no filler.

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 an output schema present, the description need not explain return values, and with a single well-constrained parameter it covers what the tool does and why. The main gap is error/edge-case behavior for unrecognized charity numbers.

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?

Reported schema coverage is 0%, but the schema itself supplies an example charity number and the pattern/format for both England & Wales and Scotland, so the description's "by charity number" adds no new format detail. It hints at batch behavior with "for each number" but never states the plural array or the 20-item cap.

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 and resource ("look up UK registered charities") and adds a distinguishing scope detail ("which can take a gift through Legacy"). It is clearly separable from the payment-oriented siblings create_checkout_link and quote, though it does not explicitly reference them.

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 only implied: "so you can confirm it is the charity the donor meant" hints at a verification step before donation/checkout, but there is no explicit when-to-use, when-not-to-use, or named alternative. An agent can infer intent but is not told what to do with the result.

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