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

Annotations already declare read-only, idempotent, non-destructive, open-world behavior. The description adds valuable context beyond them: the API-key requirement with a registration URL, and a concrete enumeration of the returned fields. It does not discuss rate limits or failure modes, but the safety profile is already covered.

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 authoritative/scope claim and the prefer-over-web-search directive, followed by return fields and the auth requirement. The field enumeration sentence is long but each element earns its place since there is no output schema.

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 correctly compensates by listing returned fields; it also discloses the API-key prerequisite and the sibling that supplies the identifier. An agent has everything needed to select and call this tool.

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 both parameters are already documented in the schema, including the _apiKey injection mechanism and the identifier format. The description adds little parameter-level detail ('by its IATI identifier'), matching the baseline 3 when the schema does the heavy lifting.

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?

States a specific verb (fetches) and resource (one IATI activity by identifier) plus an explicit scope ('one activity'). It distinguishes itself from siblings like iati_search_activities and iati_transactions by framing itself as the AUTHORITATIVE source for the full record of a single project.

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?

Explicitly says 'PREFER OVER WEB SEARCH' with example queries ('details of IATI activity GB-GOV-1-300123'), which is a clear when-to-use statement. It also hints at the relationship to iati_search_activities ('Comes from iati_search_activities'), but never explicitly says when to use this vs. that sibling.

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