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

Annotations already declare read-only, idempotent, non-destructive, but the description adds valuable behavior beyond annotations: 'Verified source: UK Charity Commission OGL v3', '24-hour cache', and the returned fields (registration status, income, expenditure, activities, trustee count). It also discloses the feedback fallback behavior if results do not serve the user.

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?

The core description is front-loaded and efficient, with the main action, scope, and routing in the first two sentences. Slight deduction for the appended report_feedback boilerplate, which is long and not strictly needed for correct tool invocation, though it remains clearly separated.

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?

Coverage is complete for a tool with one parameter and rich annotations plus an output schema. The description adds jurisdiction, source trust, cache behavior, expected return contents, and explicit sibling routing. No important aspect of using the tool correctly 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% (baseline 3), but the description enriches the parameter with format details: 'UK registered charity number (7 digits, e.g. 1234567) or full/partial organisation name.' This is a meaningful addition beyond the schema's 'number e.g. 1089464 or name substring.'

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?

Opens with a specific verb-resource pair and scope: 'Fetch UK registered charity details by charity number or organisation name.' It clearly distinguishes itself from sibling tools by saying 'UK only' and naming the US alternatives, so an agent can route correctly without opening schemas.

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?

Gives explicit when-to-use guidance: 'Use this for UK charities' and explicitly names alternatives ('Use nonprofit_fetch_nonprofit_by_ein or nonprofit_search_nonprofits_by_name for US nonprofits'). Also states 'UK only,' removing any ambiguity about jurisdiction.

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