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

Annotations already declare readOnly/idempotent/openWorld, so the safety profile is covered. The description adds genuinely new behavioral context beyond annotations: the source is the legal record (authority claim) and an auth requirement that a call without a BYO _apiKey is refused. Return shape is also summarized.

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?

Front-loaded with purpose and return fields, then authority, then the auth caveat and example. Mostly efficient, though the API-key requirement is stated in both prose and schema, a minor redundancy.

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 search tool with 100% schema coverage and no output schema, the description covers purpose, return fields, authority, and the notable auth prerequisite. Nothing critical for correct invocation is missing.

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 limit, query, _apiKey, and registered_only are all documented in the schema. The description only restates query via an example and repeats the _apiKey requirement already in the schema, adding little parameter-level detail beyond the baseline.

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 (Search) and resource (UK Register of Charities) and enumerates exactly what is returned (charity number, registration status, address, income). It also asserts authority and scope (England and Wales, Charity Commission register), letting an agent distinguish it from ukcharity_details/financials/trustees.

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?

Clearly establishes the context (name-based lookup against the authoritative register) and provides a working example call. It does not explicitly name sibling tools like ukcharity_details as the drill-down alternative or state when-not to use it, so it stops short of full routing guidance.

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