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

Annotations already cover read-only, idempotent, non-destructive, open-world behavior, so the description's added value is the jurisdictional limit (England and Wales only), the temporal depth of the data (five years of income history), and an explicit 2-credit cost that lets an agent reason about budget. It does not disclose behavior for an unknown or non-existent charity number, but the operational context it does add is genuinely useful.

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?

One sentence, front-loaded with the identity of the resource and scope, then a colon-delimited inventory of return contents and cost. Zero filler; every clause carries information an agent would otherwise have to infer.

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 carries the burden of describing the response, and it does so by enumerating the profile sections an agent will receive. For a rich, multi-section profile this is close to sufficient; the remaining gaps are failure behavior for invalid/unknown charity numbers and routing against the sibling charity tools.

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% for the single parameter and already gives the type, the 'required' note and an example ('202918'); the description only adds that it is a charity number scoped to England and Wales. That is marginal added meaning, so the baseline of 3 for a fully documented schema is correct.

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?

Specific verb+resource with explicit scope: 'Full profile of a charity in England and Wales by charity number', followed by an enumerated payload (status, activities, address/contacts, income/spending, staff, volunteers, trustees, five-year income history). An agent immediately knows this is the deep-detail endpoint versus a lighter lookup. It does not name or distinguish itself from sibling charity_lookup or charity_search, so it stops short of a 5.

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: the word 'Full' and the '(cost: 2 credits)' tag hint that this is the expensive, detailed option and that cheaper siblings exist, but no when-to-use/when-not-to-use rule or explicit alternative is stated. An agent must infer that charity_search is for discovery and this is for a known charity number.

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