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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered. The description adds genuinely useful behavior beyond that: the upstream source (Charity Commission annual returns for England and Wales) and the auth requirement that no key is fronted and a call without one is refused.

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?

Purpose, trigger examples and auth caveat are front-loaded in three tight sentences with no filler. The auth sentence is somewhat long, but every clause carries actionable information an agent needs before calling.

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 explicitly enumerates the return fields, so an agent knows what it gets back. Combined with the auth caveat, the scope (England and Wales) and the worked example, an agent has everything needed to call this correctly.

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 baseline is 3. The description reinforces usage with a literal example call using charity_number and repeats the BYO-key requirement, but adds no format or semantics beyond what the schema already documents for _apiKey, charity_number and subsidiary_number.

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?

It names a specific verb and resource ('Return a UK charity's reported financial history year by year') and enumerates the returned fields (gross income, expenditure, financial period). This clearly differentiates it from siblings like ukcharity_details and ukcharity_trustees without needing to open either schema.

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?

It supplies concrete trigger questions ('how has charity X's income changed', 'what did charity Y spend last year'), which is strong when-to-use context. It stops short of naming the sibling tools it is not (e.g. details vs trustees) or stating any exclusion condition.

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