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

Annotations already declare non-readonly, idempotent, non-destructive, closed-world, so the safety profile is covered. The description still adds genuine behavior beyond them: adding a person already on the list with that name is a no-op that returns the existing person_id. Missing are failure modes, such as what happens if list_id is unknown or not owned by the caller, which is notable given no output schema.

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?

Two sentences, front-loaded with the primary action and the optional parameter, then a second sentence dedicated to the dedupe edge case. No filler; slightly terse on prerequisites but every sentence earns its place.

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 three-parameter mutation with no output schema, the description covers the core action and the one non-obvious return path (existing person_id). It would be stronger with a line on invalid/unauthorized list_id and on how the caller learns the newly created person_id.

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 only 33% – just budget ('Budget in US dollars') is documented – so name and list_id rely on the description, which adds that the list must be one of the caller's 'current' gift lists and that name is the person's name. It does not clarify the UUID format constraint, name uniqueness rules beyond the dedupe behavior, or whether budget is per-person currency-agnostic.

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 and resource: 'Adds a person the signed-in person is buying for to one of their current gift lists, optionally with a budget.' It scopes the operation to the caller's own lists, which distinguishes it from wishlist/group tools in the sibling set, though it never names an alternative explicitly.

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 implied rather than stated: the agent can infer this is for adding a recipient to an existing list, but there is no explicit when-to-use vs when-not guidance, no note about prerequisites (e.g., needing an existing list_id from get_my_gift_lists), and no mention of a sibling that adds gifts rather than people.

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