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

Annotations already declare readOnlyHint=true, idempotenteHint=true, and destructiveHint=false, so the safety profile is covered. The description adds useful behavioral context beyond annotations: it accepts country-specific identifier formats and explicitly states 'Returns the matched charity or null,' which is important for agents deciding how to handle a not-found result.

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 description is two sentences with no filler: the first sentence front-loads the action and accepted identifier types, and the second provides usage guidance. Every clause contributes essential 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?

For a simple read-only tool with two clearly described parameters and no output schema, the description covers the important operational details: what identifier to pass per country, what the result will be (charity or null), and when to use the tool. Nothing critical for correct invocation 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 description coverage is 100%, so baseline is 3. The description adds meaningful context by enumerating concrete identifiers per country (EIN, Charity Commission number, RSIN, AHU/Kemenkumham) and clarifying that id_value can be a generic registration_number, which helps agents correctly map user input to the expected parameter.

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?

The description states a specific verb ('Look up'), a clear resource ('a charity'), and the precise scope (by official registry identifier, with country-specific examples). It distinguishes itself from siblings like search_charities and compare_charities by emphasizing exact registry verification and returning null when no match exists.

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?

The description explicitly states when to use the tool: 'Use this when the user provides a registry number and wants to confirm what's registered under it.' It does not explicitly name alternatives or exclusion conditions, but the context makes it clear this is for exact registry lookups, not fuzzy search or comparison.

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