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

No annotations exist, so the description carries the full burden; it does add real context: data source (Charity Commission E&W), jurisdiction limit implied by E&W, OGL v3 licence, and a price of $0.05 USDC. It does not disclose behavior on ambiguous name matches, which argument wins if both name and number are supplied, or what a missing charity returns.

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 resource and returned fields in one sentence, followed by lookup mode, licence, example, and pricing. It is dense and earns most of its words, though the pricing/upsell material appearing twice ($0.05 USDC here, $0.25 for the full version) is slightly redundant.

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, listing the returned fields is genuinely useful, and the sourcing/licence notes round it out. Against three fully undocumented parameters and no annotations, however, the missing suffix semantics and unclear multi-argument behavior leave gaps an agent could stumble on.

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 0% and nothing is required, so the description must explain all three params. It explains 'number' (with example {number:"1089464"}) and 'name' as lookup modes, but 'suffix' is never mentioned and there is no statement of precedence or mutual exclusivity between name and 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?

Names a specific verb and resource ('cheap UK charity snapshot') and enumerates the returned fields (status, registration/removal, income/spending, trustees, website, activities). It also names the sibling it is the lighter version of, so an agent can distinguish uk_charity from uk_charity_full without opening 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?

'Use this for a cheap UK charity snapshot' plus the explicit pointer to /uk-charity-full for 'Trustees, finances, history' gives a clear selection rule between the two siblings. The guidance is framed as price/upsell rather than data completeness, so it stops short of saying exactly when the cheap snapshot is insufficient.

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