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

Annotations already declare readOnly/idempotent/openWorld/no-destructive, so the safety profile is covered. The description adds genuinely non-obvious behavior: the call is refused without a BYO Charity Commission key and Pipeworx does not front one. Return-format detail is not needed since the field list is enumerated.

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 the action and the field inventory, then the source-authority note, then the key requirement. Slightly redundant in mentioning the API key that the schema also explains, but every sentence carries information.

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, the description compensates by enumerating the returned fields in detail, and it flags the prerequisite (API key) that would otherwise cause silent failure. An agent has everything needed to call it correctly and to know what it will get back.

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 description coverage is 100%, so the schema already documents charity_number, subsidiary_number and _apiKey. The description repeats the _apiKey requirement and adds an example value, but adds no syntax or format meaning beyond the schema — baseline 3 applies.

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 and resource ('Fetch the full UK Register of Charities entry for one charity number') and enumerates the returned fields (names, status, dates, objects, contact, income/expenditure, regulators). This clearly separates it from ukcharity_search by scoping to a single number, though it never explicitly names the overlapping siblings ukcharity_financials or ukcharity_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?

Useful operational guidance is present — the source is authoritative, an API key is required, and a worked example is given. However, there is no when-to-use/when-not guidance relative to the other ukcharity_* tools that return overlapping subsets of the same data.

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